From ea8d2901917cf8b4df23f030c79af416b39cc3ee Mon Sep 17 00:00:00 2001
From: mattijs <mattijs>
Date: Mon, 4 Feb 2013 10:17:37 +0000
Subject: [PATCH] ENH: communicators: initial version - extended Pstream API

---
 .../Test-parallel-communicators.C             |  63 ++---
 .../extrude/extrudeMesh/extrudeMesh.C         |   1 +
 .../decomposePar/domainDecomposition.C        |   2 +
 etc/bashrc                                    |   2 +-
 etc/cshrc                                     |   2 +-
 .../IOobjects/IOdictionary/IOdictionaryIO.C   |   5 +-
 src/OpenFOAM/db/IOstreams/Pstreams/IPstream.C |   2 +
 src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H |   1 +
 src/OpenFOAM/db/IOstreams/Pstreams/OPstream.C |   3 +-
 src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H |   1 +
 src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H  |  65 +++--
 .../db/IOstreams/Pstreams/PstreamBuffers.C    |  16 +-
 .../db/IOstreams/Pstreams/PstreamBuffers.H    |   3 +
 .../Pstreams/PstreamCombineReduceOps.H        |  42 ++-
 .../db/IOstreams/Pstreams/PstreamReduceOps.H  |  65 +++--
 .../db/IOstreams/Pstreams/UIPstream.C         |   4 +-
 .../db/IOstreams/Pstreams/UIPstream.H         |   6 +-
 .../db/IOstreams/Pstreams/UOPstream.C         |   9 +-
 .../db/IOstreams/Pstreams/UOPstream.H         |   6 +-
 src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C | 187 +++++++++++--
 src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H | 179 ++++++++++---
 .../IOstreams/Pstreams/combineGatherScatter.C | 250 ++++++++++++++----
 src/OpenFOAM/db/IOstreams/Pstreams/exchange.C |  27 +-
 .../db/IOstreams/Pstreams/gatherScatter.C     |  68 +++--
 .../db/IOstreams/Pstreams/gatherScatterList.C |  78 ++++--
 src/OpenFOAM/db/regIOobject/regIOobjectRead.C |   5 +-
 .../LUscalarMatrix/procLduInterface.C         |  10 +-
 .../LUscalarMatrix/procLduInterface.H         |   3 +-
 .../lduInterface/processorLduInterface.H      |   7 +-
 .../processorLduInterfaceTemplates.C          |  24 +-
 .../processorLduInterfaceField.H              |   3 +
 .../processorGAMGInterfaceField.H             |   6 +
 .../processorGAMGInterface.H                  |   8 +-
 .../ProcessorTopology/ProcessorTopology.C     |  16 +-
 .../ProcessorTopology/ProcessorTopology.H     |   4 +-
 .../polyMesh/globalMeshData/globalMeshData.C  |   2 +-
 .../mapPolyMesh/mapDistribute/mapDistribute.C |   6 +-
 .../constraint/processor/processorPolyPatch.C |  14 +
 .../constraint/processor/processorPolyPatch.H |  11 +
 .../processorCyclicPolyPatch.C                |   2 +
 .../processorCyclicPolyPatch.H                |   1 +
 src/Pstream/dummy/UIPread.C                   |  16 +-
 src/Pstream/dummy/UOPwrite.C                  |   6 +-
 src/Pstream/dummy/UPstream.C                  |  27 +-
 src/Pstream/mpi/PstreamGlobals.C              |  31 ++-
 src/Pstream/mpi/PstreamGlobals.H              |  11 +-
 src/Pstream/mpi/UIPread.C                     |  27 +-
 src/Pstream/mpi/UOPwrite.C                    |  21 +-
 src/Pstream/mpi/UPstream.C                    | 201 ++++++++++++--
 src/Pstream/mpi/allReduce.H                   |   3 +-
 src/Pstream/mpi/allReduceTemplates.C          |  39 +--
 .../fvMeshDistribute/fvMeshDistribute.C       |   2 +
 .../processor/processorFvPatchField.C         |  26 +-
 .../processor/processorFvPatchField.H         |   6 +
 .../processor/processorFvPatchScalarField.C   |  10 +-
 .../constraint/processor/processorFvPatch.H   |  10 +-
 56 files changed, 1250 insertions(+), 395 deletions(-)

diff --git a/applications/test/parallel-communicators/Test-parallel-communicators.C b/applications/test/parallel-communicators/Test-parallel-communicators.C
index e5c4b99648..2b9277fb6e 100644
--- a/applications/test/parallel-communicators/Test-parallel-communicators.C
+++ b/applications/test/parallel-communicators/Test-parallel-communicators.C
@@ -150,45 +150,48 @@ int main(int argc, char *argv[])
         top.append(i);
     }
 
-    Pout<< "bottom:" << bottom << endl;
-    Pout<< "top:" << top << endl;
+    //Pout<< "bottom:" << bottom << endl;
+    Pout<< "top             :" << top << endl;
 
 
     scalar localValue = 111*UPstream::myProcNo(UPstream::worldComm);
-    Pout<< "localValue:" << localValue << endl;
+    Pout<< "localValue      :" << localValue << endl;
 
-    if (Pstream::myProcNo(UPstream::worldComm) < n/2)
+
+    label comm = Pstream::allocateCommunicator
+    (
+        UPstream::worldComm,
+        top
+    );
+
+    Pout<< "allocated comm  :" << comm << endl;
+    Pout<< "comm myproc     :" << Pstream::myProcNo(comm)
+        << endl;
+
+
+    if (Pstream::myProcNo(comm) != -1)
     {
-        label comm = Pstream::allocateCommunicator
+        //scalar sum = sumReduce(comm, localValue);
+        //scalar sum = localValue;
+        //reduce
+        //(
+        //    UPstream::treeCommunication(comm),
+        //    sum,
+        //    sumOp<scalar>(),
+        //    Pstream::msgType(),
+        //    comm
+        //);
+        scalar sum = returnReduce
         (
-            UPstream::worldComm,
-            bottom
+            localValue,
+            sumOp<scalar>(),
+            Pstream::msgType(),
+            comm
         );
-
-        Pout<< "allocated bottom comm:" << comm << endl;
-        Pout<< "comm myproc          :" << Pstream::myProcNo(comm)
-            << endl;
-        scalar sum = sumReduce(comm, localValue);
-        Pout<< "sum                  :" << sum << endl;
-
-        Pstream::freeCommunicator(comm);
+        Pout<< "sum             :" << sum << endl;
     }
-    else
-    {
-        label comm = Pstream::allocateCommunicator
-        (
-            UPstream::worldComm,
-            top
-        );
 
-        Pout<< "allocated top comm:" << comm << endl;
-        Pout<< "comm myproc       :" << Pstream::myProcNo(comm)
-            << endl;
-        scalar sum = sumReduce(comm, localValue);
-        Pout<< "sum                  :" << sum << endl;
-
-        Pstream::freeCommunicator(comm);
-    }
+    Pstream::freeCommunicator(comm);
 
 
     Pout<< "End\n" << endl;
diff --git a/applications/utilities/mesh/generation/extrude/extrudeMesh/extrudeMesh.C b/applications/utilities/mesh/generation/extrude/extrudeMesh/extrudeMesh.C
index bafe305504..6e6c02546d 100644
--- a/applications/utilities/mesh/generation/extrude/extrudeMesh/extrudeMesh.C
+++ b/applications/utilities/mesh/generation/extrude/extrudeMesh/extrudeMesh.C
@@ -496,6 +496,7 @@ int main(int argc, char *argv[])
                         mesh.nFaces(),      // start
                         patchI,             // index
                         mesh.boundaryMesh(),// polyBoundaryMesh
+                        Pstream::worldComm, // communicator
                         Pstream::myProcNo(),// myProcNo
                         nbrProcI            // neighbProcNo
                     )
diff --git a/applications/utilities/parallelProcessing/decomposePar/domainDecomposition.C b/applications/utilities/parallelProcessing/decomposePar/domainDecomposition.C
index f5dabf6077..04e1c18685 100644
--- a/applications/utilities/parallelProcessing/decomposePar/domainDecomposition.C
+++ b/applications/utilities/parallelProcessing/decomposePar/domainDecomposition.C
@@ -448,6 +448,7 @@ bool Foam::domainDecomposition::writeDecomposition()
                             curStart,
                             nPatches,
                             procMesh.boundaryMesh(),
+                            Pstream::worldComm,
                             procI,
                             curNeighbourProcessors[procPatchI]
                         );
@@ -475,6 +476,7 @@ bool Foam::domainDecomposition::writeDecomposition()
                             curStart,
                             nPatches,
                             procMesh.boundaryMesh(),
+                            Pstream::worldComm,
                             procI,
                             curNeighbourProcessors[procPatchI],
                             referPatch,
diff --git a/etc/bashrc b/etc/bashrc
index f077631ac8..79cf4f47d8 100644
--- a/etc/bashrc
+++ b/etc/bashrc
@@ -32,7 +32,7 @@
 #------------------------------------------------------------------------------
 
 export WM_PROJECT=OpenFOAM
-export WM_PROJECT_VERSION=dev
+export WM_PROJECT_VERSION=dev.procAgglom
 
 ################################################################################
 # USER EDITABLE PART: Changes made here may be lost with the next upgrade
diff --git a/etc/cshrc b/etc/cshrc
index 6b5eb16632..6453413b38 100644
--- a/etc/cshrc
+++ b/etc/cshrc
@@ -31,7 +31,7 @@
 #------------------------------------------------------------------------------
 
 setenv WM_PROJECT OpenFOAM
-setenv WM_PROJECT_VERSION dev
+setenv WM_PROJECT_VERSION dev.procAgglom
 
 ################################################################################
 # USER EDITABLE PART: Changes made here may be lost with the next upgrade
diff --git a/src/OpenFOAM/db/IOobjects/IOdictionary/IOdictionaryIO.C b/src/OpenFOAM/db/IOobjects/IOdictionary/IOdictionaryIO.C
index e620b3b942..39dd109366 100644
--- a/src/OpenFOAM/db/IOobjects/IOdictionary/IOdictionaryIO.C
+++ b/src/OpenFOAM/db/IOobjects/IOdictionary/IOdictionaryIO.C
@@ -78,9 +78,10 @@ void Foam::IOdictionary::readFile(const bool masterOnly)
         (
             comms,
             const_cast<word&>(headerClassName()),
-            Pstream::msgType()
+            Pstream::msgType(),
+            Pstream::worldComm
         );
-        Pstream::scatter(comms, note(), Pstream::msgType());
+        Pstream::scatter(comms, note(), Pstream::msgType(), Pstream::worldComm);
 
         // Get my communication order
         const Pstream::commsStruct& myComm = comms[Pstream::myProcNo()];
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.C
index 1431585bfd..6871c3ff65 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.C
@@ -33,6 +33,7 @@ Foam::IPstream::IPstream
     const int fromProcNo,
     const label bufSize,
     const int tag,
+    const label comm,
     streamFormat format,
     versionNumber version
 )
@@ -45,6 +46,7 @@ Foam::IPstream::IPstream
         buf_,
         externalBufPosition_,
         tag,                        // tag
+        comm,
         false,                      // do not clear buf_ if at end
         format,
         version
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
index eb236a0c78..3a0047df1a 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
@@ -69,6 +69,7 @@ public:
             const int fromProcNo,
             const label bufSize = 0,
             const int tag = UPstream::msgType(),
+            const label comm = UPstream::worldComm,
             streamFormat format=BINARY,
             versionNumber version=currentVersion
         );
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.C
index 233fd10f6e..a9fccf4f1b 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.C
@@ -33,12 +33,13 @@ Foam::OPstream::OPstream
     const int toProcNo,
     const label bufSize,
     const int tag,
+    const label comm,
     streamFormat format,
     versionNumber version
 )
 :
     Pstream(commsType, bufSize),
-    UOPstream(commsType, toProcNo, buf_, tag, true, format, version)
+    UOPstream(commsType, toProcNo, buf_, tag, comm, true, format, version)
 {}
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
index edc8dce488..2abff736b4 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
@@ -66,6 +66,7 @@ public:
             const int toProcNo,
             const label bufSize = 0,
             const int tag = UPstream::msgType(),
+            const label comm = UPstream::worldComm,
             streamFormat format=BINARY,
             versionNumber version=currentVersion
         );
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
index 9f39306ccc..0fcd546582 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
@@ -98,7 +98,8 @@ public:
                 const List<commsStruct>& comms,
                 T& Value,
                 const BinaryOp& bop,
-                const int tag
+                const int tag,
+                const label comm
             );
 
             //- Like above but switches between linear/tree communication
@@ -107,7 +108,8 @@ public:
             (
                 T& Value,
                 const BinaryOp& bop,
-                const int tag = Pstream::msgType()
+                const int tag = Pstream::msgType(),
+                const label comm = Pstream::worldComm
             );
 
             //- Scatter data. Distribute without modification. Reverse of gather
@@ -116,12 +118,18 @@ public:
             (
                 const List<commsStruct>& comms,
                 T& Value,
-                const int tag
+                const int tag,
+                const label comm
             );
 
             //- Like above but switches between linear/tree communication
             template <class T>
-            static void scatter(T& Value, const int tag = Pstream::msgType());
+            static void scatter
+            (
+                T& Value,
+                const int tag = Pstream::msgType(),
+                const label comm = Pstream::worldComm
+            );
 
 
         // Combine variants. Inplace combine values from processors.
@@ -133,7 +141,8 @@ public:
                 const List<commsStruct>& comms,
                 T& Value,
                 const CombineOp& cop,
-                const int tag
+                const int tag,
+                const label comm
             );
 
             //- Like above but switches between linear/tree communication
@@ -142,7 +151,8 @@ public:
             (
                 T& Value,
                 const CombineOp& cop,
-                const int tag = Pstream::msgType()
+                const int tag = Pstream::msgType(),
+                const label comm = Pstream::worldComm
             );
 
             //- Scatter data. Reverse of combineGather
@@ -151,7 +161,8 @@ public:
             (
                 const List<commsStruct>& comms,
                 T& Value,
-                const int tag
+                const int tag,
+                const label comm
             );
 
             //- Like above but switches between linear/tree communication
@@ -159,7 +170,8 @@ public:
             static void combineScatter
             (
                 T& Value,
-                const int tag = Pstream::msgType()
+                const int tag = Pstream::msgType(),
+                const label comm = Pstream::worldComm
             );
 
         // Combine variants working on whole List at a time.
@@ -170,7 +182,8 @@ public:
                 const List<commsStruct>& comms,
                 List<T>& Value,
                 const CombineOp& cop,
-                const int tag
+                const int tag,
+                const label comm
             );
 
             //- Like above but switches between linear/tree communication
@@ -179,7 +192,8 @@ public:
             (
                 List<T>& Value,
                 const CombineOp& cop,
-                const int tag = Pstream::msgType()
+                const int tag = Pstream::msgType(),
+                const label comm = Pstream::worldComm
             );
 
             //- Scatter data. Reverse of combineGather
@@ -188,7 +202,8 @@ public:
             (
                 const List<commsStruct>& comms,
                 List<T>& Value,
-                const int tag
+                const int tag,
+                const label comm
             );
 
             //- Like above but switches between linear/tree communication
@@ -196,7 +211,8 @@ public:
             static void listCombineScatter
             (
                 List<T>& Value,
-                const int tag = Pstream::msgType()
+                const int tag = Pstream::msgType(),
+                const label comm = Pstream::worldComm
             );
 
         // Combine variants working on whole map at a time. Container needs to
@@ -208,7 +224,8 @@ public:
                 const List<commsStruct>& comms,
                 Container& Values,
                 const CombineOp& cop,
-                const int tag
+                const int tag,
+                const label comm
             );
 
             //- Like above but switches between linear/tree communication
@@ -217,7 +234,8 @@ public:
             (
                 Container& Values,
                 const CombineOp& cop,
-                const int tag = Pstream::msgType()
+                const int tag = Pstream::msgType(),
+                const label comm = UPstream::worldComm
             );
 
             //- Scatter data. Reverse of combineGather
@@ -226,7 +244,8 @@ public:
             (
                 const List<commsStruct>& comms,
                 Container& Values,
-                const int tag
+                const int tag,
+                const label comm
             );
 
             //- Like above but switches between linear/tree communication
@@ -234,7 +253,8 @@ public:
             static void mapCombineScatter
             (
                 Container& Values,
-                const int tag = Pstream::msgType()
+                const int tag = Pstream::msgType(),
+                const label comm = UPstream::worldComm
             );
 
 
@@ -249,7 +269,8 @@ public:
             (
                 const List<commsStruct>& comms,
                 List<T>& Values,
-                const int tag
+                const int tag,
+                const label comm
             );
 
             //- Like above but switches between linear/tree communication
@@ -257,7 +278,8 @@ public:
             static void gatherList
             (
                 List<T>& Values,
-                const int tag = Pstream::msgType()
+                const int tag = Pstream::msgType(),
+                const label comm = UPstream::worldComm
             );
 
             //- Scatter data. Reverse of gatherList
@@ -266,7 +288,8 @@ public:
             (
                 const List<commsStruct>& comms,
                 List<T>& Values,
-                const int tag
+                const int tag,
+                const label comm
             );
 
             //- Like above but switches between linear/tree communication
@@ -274,7 +297,8 @@ public:
             static void scatterList
             (
                 List<T>& Values,
-                const int tag = Pstream::msgType()
+                const int tag = Pstream::msgType(),
+                const label comm = UPstream::worldComm
             );
 
 
@@ -291,6 +315,7 @@ public:
                 List<Container >&,
                 labelListList& sizes,
                 const int tag = UPstream::msgType(),
+                const label comm = UPstream::worldComm,
                 const bool block = true
             );
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.C
index 24eec63c35..cbb62ae55c 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.C
@@ -40,17 +40,19 @@ Foam::PstreamBuffers::PstreamBuffers
 (
     const UPstream::commsTypes commsType,
     const int tag,
+    const label comm,
     IOstream::streamFormat format,
     IOstream::versionNumber version
 )
 :
     commsType_(commsType),
     tag_(tag),
+    comm_(comm),
     format_(format),
     version_(version),
-    sendBuf_(UPstream::nProcs()),
-    recvBuf_(UPstream::nProcs()),
-    recvBufPos_(UPstream::nProcs(),  0),
+    sendBuf_(UPstream::nProcs(comm)),
+    recvBuf_(UPstream::nProcs(comm)),
+    recvBufPos_(UPstream::nProcs(comm),  0),
     finishedSendsCalled_(false)
 {}
 
@@ -90,6 +92,7 @@ void Foam::PstreamBuffers::finishedSends(const bool block)
             recvBuf_,
             sizes,
             tag_,
+            comm_,
             block
         );
     }
@@ -108,6 +111,7 @@ void Foam::PstreamBuffers::finishedSends(labelListList& sizes, const bool block)
             recvBuf_,
             sizes,
             tag_,
+            comm_,
             block
         );
     }
@@ -123,9 +127,9 @@ void Foam::PstreamBuffers::finishedSends(labelListList& sizes, const bool block)
 
         // Note: possible only if using different tag from write started
         // by ~UOPstream. Needs some work.
-        //sizes.setSize(UPstream::nProcs());
-        //labelList& nsTransPs = sizes[UPstream::myProcNo()];
-        //nsTransPs.setSize(UPstream::nProcs());
+        //sizes.setSize(UPstream::nProcs(comm));
+        //labelList& nsTransPs = sizes[UPstream::myProcNo(comm)];
+        //nsTransPs.setSize(UPstream::nProcs(comm));
         //
         //forAll(sendBuf_, procI)
         //{
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H
index 0b88a644f0..da6cb21acc 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H
@@ -95,6 +95,8 @@ class PstreamBuffers
 
         const int tag_;
 
+        const label comm_;
+
         const IOstream::streamFormat format_;
 
         const IOstream::versionNumber version_;
@@ -127,6 +129,7 @@ public:
         (
             const UPstream::commsTypes commsType,
             const int tag = UPstream::msgType(),
+            const label comm = UPstream::worldComm,
             IOstream::streamFormat format=IOstream::BINARY,
             IOstream::versionNumber version=IOstream::currentVersion
         );
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineReduceOps.H b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineReduceOps.H
index c657e0a140..4a2985ef3a 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineReduceOps.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineReduceOps.H
@@ -53,11 +53,12 @@ void combineReduce
     const List<UPstream::commsStruct>& comms,
     T& Value,
     const CombineOp& cop,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
-    Pstream::combineGather(comms, Value, cop, tag);
-    Pstream::combineScatter(comms, Value, tag);
+    Pstream::combineGather(comms, Value, cop, tag, comm);
+    Pstream::combineScatter(comms, Value, tag, comm);
 }
 
 
@@ -66,24 +67,45 @@ void combineReduce
 (
     T& Value,
     const CombineOp& cop,
-    const int tag = Pstream::msgType()
+    const int tag = Pstream::msgType(),
+    const label comm = Pstream::worldComm
 )
 {
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
         Pstream::combineGather
         (
-            UPstream::linearCommunication(),
+            UPstream::linearCommunication(comm),
             Value,
             cop,
-            tag
+            tag,
+            comm
+        );
+        Pstream::combineScatter
+        (
+            UPstream::linearCommunication(comm),
+            Value,
+            tag,
+            comm
         );
-        Pstream::combineScatter(UPstream::linearCommunication(), Value, tag);
     }
     else
     {
-        Pstream::combineGather(UPstream::treeCommunication(), Value, cop, tag);
-        Pstream::combineScatter(UPstream::treeCommunication(), Value, tag);
+        Pstream::combineGather
+        (
+            UPstream::treeCommunication(comm),
+            Value,
+            cop,
+            tag,
+            comm
+        );
+        Pstream::combineScatter
+        (
+            UPstream::treeCommunication(comm),
+            Value,
+            tag,
+            comm
+        );
     }
 }
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamReduceOps.H b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamReduceOps.H
index 47c7c51acd..bd8f5ab1dc 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamReduceOps.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamReduceOps.H
@@ -44,11 +44,12 @@ void reduce
     const List<UPstream::commsStruct>& comms,
     T& Value,
     const BinaryOp& bop,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
-    Pstream::gather(comms, Value, bop, tag);
-    Pstream::scatter(comms, Value, tag);
+    Pstream::gather(comms, Value, bop, tag, comm);
+    Pstream::scatter(comms, Value, tag, comm);
 }
 
 
@@ -58,16 +59,17 @@ void reduce
 (
     T& Value,
     const BinaryOp& bop,
-    const int tag = Pstream::msgType()
+    const int tag = Pstream::msgType(),
+    const label comm = UPstream::worldComm
 )
 {
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
-        reduce(UPstream::linearCommunication(), Value, bop, tag);
+        reduce(UPstream::linearCommunication(comm), Value, bop, tag, comm);
     }
     else
     {
-        reduce(UPstream::treeCommunication(), Value, bop, tag);
+        reduce(UPstream::treeCommunication(comm), Value, bop, tag, comm);
     }
 }
 
@@ -78,18 +80,33 @@ T returnReduce
 (
     const T& Value,
     const BinaryOp& bop,
-    const int tag = Pstream::msgType()
+    const int tag = Pstream::msgType(),
+    const label comm = UPstream::worldComm
 )
 {
     T WorkValue(Value);
 
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
-        reduce(UPstream::linearCommunication(), WorkValue, bop, tag);
+        reduce
+        (
+            UPstream::linearCommunication(comm),
+            WorkValue,
+            bop,
+            tag,
+            comm
+        );
     }
     else
     {
-        reduce(UPstream::treeCommunication(), WorkValue, bop, tag);
+        reduce
+        (
+            UPstream::treeCommunication(comm),
+            WorkValue,
+            bop,
+            tag,
+            comm
+        );
     }
 
     return WorkValue;
@@ -102,11 +119,12 @@ void sumReduce
 (
     T& Value,
     label& Count,
-    const int tag = Pstream::msgType()
+    const int tag = Pstream::msgType(),
+    const label comm = UPstream::worldComm
 )
 {
-    reduce(Value, sumOp<T>(), tag);
-    reduce(Count, sumOp<label>(), tag);
+    reduce(Value, sumOp<T>(), tag, comm);
+    reduce(Count, sumOp<label>(), tag, comm);
 }
 
 
@@ -117,10 +135,14 @@ void reduce
     T& Value,
     const BinaryOp& bop,
     const int tag,
+    const label comm,
     label& request
 )
 {
-    notImplemented("reduce(T&, const BinaryOp&, const int, label&");
+    notImplemented
+    (
+        "reduce(T&, const BinaryOp&, const int, const label, label&"
+    );
 }
 
 
@@ -129,28 +151,32 @@ void reduce
 (
     scalar& Value,
     const sumOp<scalar>& bop,
-    const int tag = Pstream::msgType()
+    const int tag = Pstream::msgType(),
+    const label comm = UPstream::worldComm
 );
 
 void reduce
 (
     scalar& Value,
     const minOp<scalar>& bop,
-    const int tag = Pstream::msgType()
+    const int tag = Pstream::msgType(),
+    const label comm = UPstream::worldComm
 );
 
 void reduce
 (
     vector2D& Value,
     const sumOp<vector2D>& bop,
-    const int tag = Pstream::msgType()
+    const int tag = Pstream::msgType(),
+    const label comm = UPstream::worldComm
 );
 
 void sumReduce
 (
     scalar& Value,
     label& Count,
-    const int tag = Pstream::msgType()
+    const int tag = Pstream::msgType(),
+    const label comm = UPstream::worldComm
 );
 
 void reduce
@@ -158,6 +184,7 @@ void reduce
     scalar& Value,
     const sumOp<scalar>& bop,
     const int tag,
+    const label comm,
     label& request
 );
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.C
index 2167d15b6d..4e853274d8 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.C
@@ -330,7 +330,9 @@ Foam::Istream& Foam::UIPstream::rewind()
 void Foam::UIPstream::print(Ostream& os) const
 {
     os  << "Reading from processor " << fromProcNo_
-        << " to processor " << myProcNo() << Foam::endl;
+        << " to processor " << myProcNo << " using communicator " << comm_
+        <<  " and tag " << tag_
+        << Foam::endl;
 }
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H
index 7b4a7bfada..3e76c09552 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H
@@ -66,6 +66,8 @@ class UIPstream
 
         const int tag_;
 
+        const label comm_;
+
         const bool clearAtEnd_;
 
         int messageSize_;
@@ -97,6 +99,7 @@ public:
             DynamicList<char>& externalBuf,
             label& externalBufPosition,
             const int tag = UPstream::msgType(),
+            const label comm = UPstream::worldComm,
             const bool clearAtEnd = false,   // destroy externalBuf if at end
             streamFormat format=BINARY,
             versionNumber version=currentVersion
@@ -131,7 +134,8 @@ public:
                 const int fromProcNo,
                 char* buf,
                 const std::streamsize bufSize,
-                const int tag = UPstream::msgType()
+                const int tag = UPstream::msgType(),
+                const label communicator = 0
             );
 
             //- Return next token from stream
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.C
index e01e8de01b..2e6cdb0ec1 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.C
@@ -91,6 +91,7 @@ Foam::UOPstream::UOPstream
     const int toProcNo,
     DynamicList<char>& sendBuf,
     const int tag,
+    const label comm,
     const bool sendAtDestruct,
     streamFormat format,
     versionNumber version
@@ -101,6 +102,7 @@ Foam::UOPstream::UOPstream
     toProcNo_(toProcNo),
     sendBuf_(sendBuf),
     tag_(tag),
+    comm_(comm),
     sendAtDestruct_(sendAtDestruct)
 {
     setOpened();
@@ -115,6 +117,7 @@ Foam::UOPstream::UOPstream(const int toProcNo, PstreamBuffers& buffers)
     toProcNo_(toProcNo),
     sendBuf_(buffers.sendBuf_[toProcNo]),
     tag_(buffers.tag_),
+    comm_(buffers.comm_),
     sendAtDestruct_(buffers.commsType_ != UPstream::nonBlocking)
 {
     setOpened();
@@ -136,7 +139,8 @@ Foam::UOPstream::~UOPstream()
                 toProcNo_,
                 sendBuf_.begin(),
                 sendBuf_.size(),
-                tag_
+                tag_,
+                comm_
             )
         )
         {
@@ -287,7 +291,8 @@ Foam::Ostream& Foam::UOPstream::write(const char* data, std::streamsize count)
 void Foam::UOPstream::print(Ostream& os) const
 {
     os  << "Writing from processor " << toProcNo_
-        << " to processor " << myProcNo() << Foam::endl;
+        << " to processor " << myProcNo() << " in communicator " << comm_
+        << " and tag " << tag_ << Foam::endl;
 }
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
index 5cbde9aee5..4f2dcc9c6b 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
@@ -65,6 +65,8 @@ class UOPstream
 
         const int tag_;
 
+        const label comm_;
+
         const bool sendAtDestruct_;
 
 
@@ -93,6 +95,7 @@ public:
             const int toProcNo,
             DynamicList<char>& sendBuf,
             const int tag = UPstream::msgType(),
+            const label comm = UPstream::worldComm,
             const bool sendAtDestruct = true,
             streamFormat format=BINARY,
             versionNumber version=currentVersion
@@ -126,7 +129,8 @@ public:
                 const int toProcNo,
                 const char* buf,
                 const std::streamsize bufSize,
-                const int tag = UPstream::msgType()
+                const int tag = UPstream::msgType(),
+                const label communicator = 0
             );
 
             //- Write next token to stream
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
index 9cc6d9d6f5..20d4609265 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
@@ -54,18 +54,33 @@ const Foam::NamedEnum<Foam::UPstream::commsTypes, 3>
 
 // * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
 
-void Foam::UPstream::setParRun()
+void Foam::UPstream::setParRun(const label nProcs)
 {
     parRun_ = true;
 
-    Pout.prefix() = '[' +  name(myProcNo()) + "] ";
-    Perr.prefix() = '[' +  name(myProcNo()) + "] ";
+    // Redo worldComm communicator (this has been created at static
+    // initialisation time)
+    freeCommunicator(UPstream::worldComm);
+    label comm = allocateCommunicator(-1, identity(nProcs), true);
+    if (comm != UPstream::worldComm)
+    {
+        FatalErrorIn("UPstream::setParRun(const label)")
+            << "problem : comm:" << comm
+            << "  UPstream::worldComm:" << UPstream::worldComm
+            << Foam::exit(FatalError);
+    }
+
+    Pout.prefix() = '[' +  name(myProcNo(Pstream::worldComm)) + "] ";
+    Perr.prefix() = '[' +  name(myProcNo(Pstream::worldComm)) + "] ";
 }
 
 
-void Foam::UPstream::calcLinearComm(const label nProcs)
+Foam::List<Foam::UPstream::commsStruct> Foam::UPstream::calcLinearComm
+(
+    const label nProcs
+)
 {
-    linearCommunication_.setSize(nProcs);
+    List<commsStruct> linearCommunication(nProcs);
 
     // Master
     labelList belowIDs(nProcs - 1);
@@ -74,7 +89,7 @@ void Foam::UPstream::calcLinearComm(const label nProcs)
         belowIDs[i] = i + 1;
     }
 
-    linearCommunication_[0] = commsStruct
+    linearCommunication[0] = commsStruct
     (
         nProcs,
         0,
@@ -86,7 +101,7 @@ void Foam::UPstream::calcLinearComm(const label nProcs)
     // Slaves. Have no below processors, only communicate up to master
     for (label procID = 1; procID < nProcs; procID++)
     {
-        linearCommunication_[procID] = commsStruct
+        linearCommunication[procID] = commsStruct
         (
             nProcs,
             procID,
@@ -95,6 +110,7 @@ void Foam::UPstream::calcLinearComm(const label nProcs)
             labelList(0)
         );
     }
+    return linearCommunication;
 }
 
 
@@ -142,7 +158,10 @@ void Foam::UPstream::collectReceives
 //  5       -               4
 //  6       7               4
 //  7       -               6
-void Foam::UPstream::calcTreeComm(label nProcs)
+Foam::List<Foam::UPstream::commsStruct> Foam::UPstream::calcTreeComm
+(
+    label nProcs
+)
 {
     label nLevels = 1;
     while ((1 << nLevels) < nProcs)
@@ -188,11 +207,11 @@ void Foam::UPstream::calcTreeComm(label nProcs)
     }
 
 
-    treeCommunication_.setSize(nProcs);
+    List<commsStruct> treeCommunication(nProcs);
 
     for (label procID = 0; procID < nProcs; procID++)
     {
-        treeCommunication_[procID] = commsStruct
+        treeCommunication[procID] = commsStruct
         (
             nProcs,
             procID,
@@ -201,37 +220,159 @@ void Foam::UPstream::calcTreeComm(label nProcs)
             allReceives[procID].shrink()
         );
     }
+    return treeCommunication;
 }
 
 
-// Callback from UPstream::init() : initialize linear and tree communication
-// schedules now that nProcs is known.
-void Foam::UPstream::initCommunicationSchedule()
+//// Callback from UPstream::init() : initialize linear and tree communication
+//// schedules now that nProcs is known.
+//void Foam::UPstream::initCommunicationSchedule()
+//{
+//    calcLinearComm(nProcs());
+//    calcTreeComm(nProcs());
+//}
+
+
+Foam::label Foam::UPstream::allocateCommunicator
+(
+    const label parentIndex,
+    const labelList& subRanks,
+    const bool doPstream
+)
 {
-    calcLinearComm(nProcs());
-    calcTreeComm(nProcs());
+    label index;
+    if (!freeComms_.empty())
+    {
+        index = freeComms_.pop();
+    }
+    else
+    {
+        // Extend storage
+        index = parentCommunicator_.size();
+
+        myProcNo_.append(-1);
+        procIDs_.append(List<int>(0));
+        parentCommunicator_.append(-1);
+        linearCommunication_.append(List<commsStruct>(0));
+        treeCommunication_.append(List<commsStruct>(0));
+    }
+
+    Pout<< "Communicators : Allocating communicator " << index << endl
+        << "    parent : " << parentIndex << endl
+        << "    procs  : " << subRanks << endl
+        << endl;
+
+    // Initialise; overwritten by allocatePstreamCommunicator
+    myProcNo_[index] = 0;
+
+    // Convert from label to int
+    procIDs_[index].setSize(subRanks.size());
+    forAll(procIDs_[index], i)
+    {
+        procIDs_[index][i] = subRanks[i];
+    }
+    parentCommunicator_[index] = parentIndex;
+
+    linearCommunication_[index] = calcLinearComm(procIDs_[index].size());
+    treeCommunication_[index] = calcTreeComm(procIDs_[index].size());
+
+
+    if (doPstream)
+    {
+        allocatePstreamCommunicator(parentIndex, index);
+    }
+
+    return index;
+}
+
+
+void Foam::UPstream::freeCommunicator
+(
+    const label communicator,
+    const bool doPstream
+)
+{
+    Pout<< "Communicators : Freeing communicator " << communicator << endl
+        << "    parent   : " << parentCommunicator_[communicator] << endl
+        << "    myProcNo : " << myProcNo_[communicator] << endl
+        << endl;
+
+    if (doPstream)
+    {
+        freePstreamCommunicator(communicator);
+    }
+    myProcNo_[communicator] = -1;
+    //procIDs_[communicator].clear();
+    parentCommunicator_[communicator] = -1;
+    linearCommunication_[communicator].clear();
+    treeCommunication_[communicator].clear();
+
+    freeComms_.push(communicator);
+}
+
+
+void Foam::UPstream::freeCommunicators(const bool doPstream)
+{
+    Pout<< "Communicators : Freeing all communicators" << endl
+        << endl;
+
+    forAll(myProcNo_, communicator)
+    {
+        if (myProcNo_[communicator] != -1)
+        {
+            freeCommunicator(communicator, doPstream);
+        }
+    }
 }
 
 
 // * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //
 
-// Initialise my process number to 0 (the master)
-int Foam::UPstream::myProcNo_(0);
-
 // By default this is not a parallel run
 bool Foam::UPstream::parRun_(false);
 
+//// Initialise my process number to 0 (the master)
+//int Foam::UPstream::myProcNo_(0);
+//
+//// List of process IDs
+//Foam::List<int> Foam::UPstream::procIDs_(label(1), 0);
+
+// Free communicators
+Foam::LIFOStack<Foam::label> Foam::UPstream::freeComms_;
+
+// My processor number
+Foam::DynamicList<int> Foam::UPstream::myProcNo_(10);
+
 // List of process IDs
-Foam::List<int> Foam::UPstream::procIDs_(label(1), 0);
+Foam::DynamicList<Foam::List<int> > Foam::UPstream::procIDs_(10);
+
+// Parent communicator
+Foam::DynamicList<Foam::label> Foam::UPstream::parentCommunicator_(10);
 
 // Standard transfer message type
 int Foam::UPstream::msgType_(1);
 
+//// Linear communication schedule
+//Foam::List<Foam::UPstream::commsStruct>
+//      Foam::UPstream::linearCommunication_(0);
+//// Multi level communication schedule
+//Foam::List<Foam::UPstream::commsStruct>
+//      Foam::UPstream::treeCommunication_(0);
+
 // Linear communication schedule
-Foam::List<Foam::UPstream::commsStruct> Foam::UPstream::linearCommunication_(0);
+Foam::DynamicList<Foam::List<Foam::UPstream::commsStruct> >
+Foam::UPstream::linearCommunication_(10);
 
 // Multi level communication schedule
-Foam::List<Foam::UPstream::commsStruct> Foam::UPstream::treeCommunication_(0);
+Foam::DynamicList<Foam::List<Foam::UPstream::commsStruct> >
+Foam::UPstream::treeCommunication_(10);
+
+
+// Allocate a serial communicator. This gets overwritten in parallel mode
+// (by UPstream::setParRun())
+Foam::UPstream::communicator serialComm(-1, Foam::labelList(1, 0), false);
+
+
 
 // Should compact transfer be used in which floats replace doubles
 // reducing the bandwidth requirement at the expense of some loss
@@ -292,6 +433,10 @@ public:
 addcommsTypeToOpt addcommsTypeToOpt_("commsType");
 
 
+// Default communicator
+Foam::label Foam::UPstream::worldComm(0);
+
+
 // Number of polling cycles in processor updates
 int Foam::UPstream::nPollProcInterfaces
 (
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
index 1ad4aa99cc..2eaa66847e 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
@@ -29,7 +29,6 @@ Description
 
 SourceFiles
     UPstream.C
-    UPstreamsPrint.C
     UPstreamCommsStruct.C
     gatherScatter.C
     combineGatherScatter.C
@@ -45,6 +44,8 @@ SourceFiles
 #include "HashTable.H"
 #include "string.H"
 #include "NamedEnum.H"
+#include "ListOps.H"
+#include "LIFOStack.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
@@ -180,26 +181,33 @@ private:
 
     // Private data
 
-        static int myProcNo_;
         static bool parRun_;
-
-        static List<int> procIDs_;
         static int msgType_;
 
-        static List<commsStruct> linearCommunication_;
-        static List<commsStruct> treeCommunication_;
+//        static int myProcNo_;
+//        static List<int> procIDs_;
+//        static List<commsStruct> linearCommunication_;
+//        static List<commsStruct> treeCommunication_;
+        // Communicator specific data
+        static LIFOStack<label> freeComms_;
+        static DynamicList<int> myProcNo_;
+        static DynamicList<List<int> > procIDs_;
+        static DynamicList<label> parentCommunicator_;
+
+        static DynamicList<List<commsStruct> > linearCommunication_;
+        static DynamicList<List<commsStruct> > treeCommunication_;
 
 
     // Private Member Functions
 
         //- Set data for parallel running
-        static void setParRun();
+        static void setParRun(const label nProcs);
 
         //- Calculate linear communication schedule
-        static void calcLinearComm(const label nProcs);
+        static List<commsStruct> calcLinearComm(const label nProcs);
 
         //- Calculate tree communication schedule
-        static void calcTreeComm(const label nProcs);
+        static List<commsStruct> calcTreeComm(const label nProcs);
 
         //- Helper function for tree communication schedule determination
         //  Collects all processorIDs below a processor
@@ -210,9 +218,22 @@ private:
             DynamicList<label>& allReceives
         );
 
-        //- Initialize all communication schedules. Callback from
-        //  UPstream::init()
-        static void initCommunicationSchedule();
+        //- Allocate a communicator with index
+        static void allocatePstreamCommunicator
+        (
+            const label parentIndex,
+            const label index
+        );
+
+        //- Free a communicator
+        static void freePstreamCommunicator
+        (
+            const label index
+        );
+
+//        //- Initialize all communication schedules. Callback from
+//        //  UPstream::init()
+//        static void initCommunicationSchedule();
 
 
 protected:
@@ -245,6 +266,9 @@ public:
         //- Number of polling cycles in processor updates
         static int nPollProcInterfaces;
 
+        //- Default communicator (all processors)
+        static label worldComm;
+
     // Constructors
 
         //- Construct given optional buffer size
@@ -256,6 +280,69 @@ public:
 
     // Member functions
 
+        //- Allocate a new communicator
+        static label allocateCommunicator
+        (
+            const label parent,
+            const labelList& subRanks,
+            const bool doPstream = true
+        );
+
+        //- Free a previously allocated communicator
+        static void freeCommunicator
+        (
+            const label communicator,
+            const bool doPstream = true
+        );
+
+        //- Free all communicators
+        static void freeCommunicators(const bool doPstream);
+
+        //- Helper class for allocating/freeing communicators
+        class communicator
+        {
+            label comm_;
+
+            //- Disallow copy and assignment
+            communicator(const communicator&);
+            void operator=(const communicator&);
+
+        public:
+
+            communicator
+            (
+                const label parent,
+                const labelList& subRanks,
+                const bool doPstream
+            )
+            :
+                comm_(allocateCommunicator(parent, subRanks, doPstream))
+            {}
+
+            communicator(const label parent)
+            :
+                comm_
+                (
+                    allocateCommunicator
+                    (
+                        parent,
+                        identity(UPstream::nProcs(parent))
+                    )
+                )
+            {}
+
+            ~communicator()
+            {
+                freeCommunicator(comm_);
+            }
+
+            operator label() const
+            {
+                return comm_;
+            }
+        };
+
+
         //- Add the valid option this type of communications library
         //  adds/requires on the command line
         static void addValidParOptions(HashTable<string>& validParOptions);
@@ -281,6 +368,14 @@ public:
             //- Non-blocking comms: has request i finished?
             static bool finishedRequest(const label i);
 
+            static int allocateTag(const char*);
+
+            static int allocateTag(const word&);
+
+            static void freeTag(const char*, const int tag);
+
+            static void freeTag(const word&, const int tag);
+
 
         //- Is this a parallel run?
         static bool& parRun()
@@ -289,15 +384,9 @@ public:
         }
 
         //- Number of processes in parallel run
-        static label nProcs()
+        static label nProcs(const label communicator = 0)
         {
-            return procIDs_.size();
-        }
-
-        //- Am I the master process
-        static bool master()
-        {
-            return myProcNo_ == 0;
+            return procIDs_[communicator].size();
         }
 
         //- Process index of the master
@@ -306,23 +395,29 @@ public:
             return 0;
         }
 
+        //- Am I the master process
+        static bool master(const label communicator = 0)
+        {
+            return myProcNo_[communicator] == masterNo();
+        }
+
         //- Number of this process (starting from masterNo() = 0)
-        static int myProcNo()
+        static int myProcNo(const label communicator = 0)
         {
-            return myProcNo_;
+            return myProcNo_[communicator];
         }
 
-        //- Process IDs
-        static const List<int>& procIDs()
-        {
-            return procIDs_;
-        }
-
-        //- Process ID of given process index
-        static int procID(int procNo)
-        {
-            return procIDs_[procNo];
-        }
+//        //- Process IDs
+//        static const List<int>& procIDs()
+//        {
+//            return procIDs_;
+//        }
+//
+//        //- Process ID of given process index
+//        static int procID(int procNo)
+//        {
+//            return procIDs_[procNo];
+//        }
 
         //- Process index of first slave
         static int firstSlave()
@@ -331,21 +426,27 @@ public:
         }
 
         //- Process index of last slave
-        static int lastSlave()
+        static int lastSlave(const label communicator = 0)
         {
-            return nProcs() - 1;
+            return nProcs(communicator) - 1;
         }
 
         //- Communication schedule for linear all-to-master (proc 0)
-        static const List<commsStruct>& linearCommunication()
+        static const List<commsStruct>& linearCommunication
+        (
+            const label communicator = 0
+        )
         {
-            return linearCommunication_;
+            return linearCommunication_[communicator];
         }
 
         //- Communication schedule for tree all-to-master (proc 0)
-        static const List<commsStruct>& treeCommunication()
+        static const List<commsStruct>& treeCommunication
+        (
+            const label communicator = 0
+        )
         {
-            return treeCommunication_;
+            return treeCommunication_[communicator];
         }
 
         //- Message tag of standard messages
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/combineGatherScatter.C b/src/OpenFOAM/db/IOstreams/Pstreams/combineGatherScatter.C
index fbe246f1ea..952cfbb47d 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/combineGatherScatter.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/combineGatherScatter.C
@@ -51,13 +51,14 @@ void Pstream::combineGather
     const List<UPstream::commsStruct>& comms,
     T& Value,
     const CombineOp& cop,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
     if (UPstream::parRun())
     {
         // Get my communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo()];
+        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
 
         // Receive from my downstairs neighbours
         forAll(myComm.below(), belowI)
@@ -73,7 +74,8 @@ void Pstream::combineGather
                     belowID,
                     reinterpret_cast<char*>(&value),
                     sizeof(T),
-                    tag
+                    tag,
+                    comm
                 );
 
                 if (debug & 2)
@@ -86,7 +88,7 @@ void Pstream::combineGather
             }
             else
             {
-                IPstream fromBelow(UPstream::scheduled, belowID, 0, tag);
+                IPstream fromBelow(UPstream::scheduled, belowID, 0, tag, comm);
                 T value(fromBelow);
 
                 if (debug & 2)
@@ -116,12 +118,20 @@ void Pstream::combineGather
                     myComm.above(),
                     reinterpret_cast<const char*>(&Value),
                     sizeof(T),
-                    tag
+                    tag,
+                    comm
                 );
             }
             else
             {
-                OPstream toAbove(UPstream::scheduled, myComm.above(), 0, tag);
+                OPstream toAbove
+                (
+                    UPstream::scheduled,
+                    myComm.above(),
+                    0,
+                    tag,
+                    comm
+                );
                 toAbove << Value;
             }
         }
@@ -130,15 +140,35 @@ void Pstream::combineGather
 
 
 template <class T, class CombineOp>
-void Pstream::combineGather(T& Value, const CombineOp& cop, const int tag)
+void Pstream::combineGather
+(
+    T& Value,
+    const CombineOp& cop,
+    const int tag,
+    const label comm
+)
 {
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
-        combineGather(UPstream::linearCommunication(), Value, cop, tag);
+        combineGather
+        (
+            UPstream::linearCommunication(comm),
+            Value,
+            cop,
+            tag,
+            comm
+        );
     }
     else
     {
-        combineGather(UPstream::treeCommunication(), Value, cop, tag);
+        combineGather
+        (
+            UPstream::treeCommunication(comm),
+            Value,
+            cop,
+            tag,
+            comm
+        );
     }
 }
 
@@ -148,13 +178,14 @@ void Pstream::combineScatter
 (
     const List<UPstream::commsStruct>& comms,
     T& Value,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
     if (UPstream::parRun())
     {
         // Get my communication order
-        const UPstream::commsStruct& myComm = comms[UPstream::myProcNo()];
+        const UPstream::commsStruct& myComm = comms[UPstream::myProcNo(comm)];
 
         // Reveive from up
         if (myComm.above() != -1)
@@ -167,12 +198,20 @@ void Pstream::combineScatter
                     myComm.above(),
                     reinterpret_cast<char*>(&Value),
                     sizeof(T),
-                    tag
+                    tag,
+                    comm
                 );
             }
             else
             {
-                IPstream fromAbove(UPstream::scheduled, myComm.above(), 0, tag);
+                IPstream fromAbove
+                (
+                    UPstream::scheduled,
+                    myComm.above(),
+                    0,
+                    tag,
+                    comm
+                );
                 Value = T(fromAbove);
             }
 
@@ -201,12 +240,13 @@ void Pstream::combineScatter
                     belowID,
                     reinterpret_cast<const char*>(&Value),
                     sizeof(T),
-                    tag
+                    tag,
+                    comm
                 );
             }
             else
             {
-                OPstream toBelow(UPstream::scheduled, belowID, 0, tag);
+                OPstream toBelow(UPstream::scheduled, belowID, 0, tag, comm);
                 toBelow << Value;
             }
         }
@@ -215,15 +255,20 @@ void Pstream::combineScatter
 
 
 template <class T>
-void Pstream::combineScatter(T& Value, const int tag)
+void Pstream::combineScatter
+(
+    T& Value,
+    const int tag,
+    const label comm
+)
 {
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
-        combineScatter(UPstream::linearCommunication(), Value, tag);
+        combineScatter(UPstream::linearCommunication(comm), Value, tag, comm);
     }
     else
     {
-        combineScatter(UPstream::treeCommunication(), Value, tag);
+        combineScatter(UPstream::treeCommunication(comm), Value, tag, comm);
     }
 }
 
@@ -238,13 +283,14 @@ void Pstream::listCombineGather
     const List<UPstream::commsStruct>& comms,
     List<T>& Values,
     const CombineOp& cop,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
     if (UPstream::parRun())
     {
         // Get my communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo()];
+        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
 
         // Receive from my downstairs neighbours
         forAll(myComm.below(), belowI)
@@ -261,7 +307,8 @@ void Pstream::listCombineGather
                     belowID,
                     reinterpret_cast<char*>(receivedValues.begin()),
                     receivedValues.byteSize(),
-                    tag
+                    tag,
+                    comm
                 );
 
                 if (debug & 2)
@@ -277,7 +324,7 @@ void Pstream::listCombineGather
             }
             else
             {
-                IPstream fromBelow(UPstream::scheduled, belowID, 0, tag);
+                IPstream fromBelow(UPstream::scheduled, belowID, 0, tag, comm);
                 List<T> receivedValues(fromBelow);
 
                 if (debug & 2)
@@ -310,12 +357,20 @@ void Pstream::listCombineGather
                     myComm.above(),
                     reinterpret_cast<const char*>(Values.begin()),
                     Values.byteSize(),
-                    tag
+                    tag,
+                    comm
                 );
             }
             else
             {
-                OPstream toAbove(UPstream::scheduled, myComm.above(), 0, tag);
+                OPstream toAbove
+                (
+                    UPstream::scheduled,
+                    myComm.above(),
+                    0,
+                    tag,
+                    comm
+                );
                 toAbove << Values;
             }
         }
@@ -328,16 +383,31 @@ void Pstream::listCombineGather
 (
     List<T>& Values,
     const CombineOp& cop,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
-        listCombineGather(UPstream::linearCommunication(), Values, cop, tag);
+        listCombineGather
+        (
+            UPstream::linearCommunication(comm),
+            Values,
+            cop,
+            tag,
+            comm
+        );
     }
     else
     {
-        listCombineGather(UPstream::treeCommunication(), Values, cop, tag);
+        listCombineGather
+        (
+            UPstream::treeCommunication(comm),
+            Values,
+            cop,
+            tag,
+            comm
+        );
     }
 }
 
@@ -347,13 +417,14 @@ void Pstream::listCombineScatter
 (
     const List<UPstream::commsStruct>& comms,
     List<T>& Values,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
     if (UPstream::parRun())
     {
         // Get my communication order
-        const UPstream::commsStruct& myComm = comms[UPstream::myProcNo()];
+        const UPstream::commsStruct& myComm = comms[UPstream::myProcNo(comm)];
 
         // Reveive from up
         if (myComm.above() != -1)
@@ -366,12 +437,20 @@ void Pstream::listCombineScatter
                     myComm.above(),
                     reinterpret_cast<char*>(Values.begin()),
                     Values.byteSize(),
-                    tag
+                    tag,
+                    comm
                 );
             }
             else
             {
-                IPstream fromAbove(UPstream::scheduled, myComm.above(), 0, tag);
+                IPstream fromAbove
+                (
+                    UPstream::scheduled,
+                    myComm.above(),
+                    0,
+                    tag,
+                    comm
+                );
                 fromAbove >> Values;
             }
 
@@ -400,12 +479,13 @@ void Pstream::listCombineScatter
                     belowID,
                     reinterpret_cast<const char*>(Values.begin()),
                     Values.byteSize(),
-                    tag
+                    tag,
+                    comm
                 );
             }
             else
             {
-                OPstream toBelow(UPstream::scheduled, belowID, 0, tag);
+                OPstream toBelow(UPstream::scheduled, belowID, 0, tag, comm);
                 toBelow << Values;
             }
         }
@@ -414,15 +494,32 @@ void Pstream::listCombineScatter
 
 
 template <class T>
-void Pstream::listCombineScatter(List<T>& Values, const int tag)
+void Pstream::listCombineScatter
+(
+    List<T>& Values,
+    const int tag,
+    const label comm
+)
 {
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
-        listCombineScatter(UPstream::linearCommunication(), Values, tag);
+        listCombineScatter
+        (
+            UPstream::linearCommunication(comm),
+            Values,
+            tag,
+            comm
+        );
     }
     else
     {
-        listCombineScatter(UPstream::treeCommunication(), Values, tag);
+        listCombineScatter
+        (
+            UPstream::treeCommunication(comm),
+            Values,
+            tag,
+            comm
+        );
     }
 }
 
@@ -439,20 +536,21 @@ void Pstream::mapCombineGather
     const List<UPstream::commsStruct>& comms,
     Container& Values,
     const CombineOp& cop,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
     if (UPstream::parRun())
     {
         // Get my communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo()];
+        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
 
         // Receive from my downstairs neighbours
         forAll(myComm.below(), belowI)
         {
             label belowID = myComm.below()[belowI];
 
-            IPstream fromBelow(UPstream::scheduled, belowID, 0, tag);
+            IPstream fromBelow(UPstream::scheduled, belowID, 0, tag, comm);
             Container receivedValues(fromBelow);
 
             if (debug & 2)
@@ -492,7 +590,7 @@ void Pstream::mapCombineGather
                     << " data:" << Values << endl;
             }
 
-            OPstream toAbove(UPstream::scheduled, myComm.above(), 0, tag);
+            OPstream toAbove(UPstream::scheduled, myComm.above(), 0, tag, comm);
             toAbove << Values;
         }
     }
@@ -504,16 +602,31 @@ void Pstream::mapCombineGather
 (
     Container& Values,
     const CombineOp& cop,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
-        mapCombineGather(UPstream::linearCommunication(), Values, cop, tag);
+        mapCombineGather
+        (
+            UPstream::linearCommunication(comm),
+            Values,
+            cop,
+            tag,
+            comm
+        );
     }
     else
     {
-        mapCombineGather(UPstream::treeCommunication(), Values, cop, tag);
+        mapCombineGather
+        (
+            UPstream::treeCommunication(comm),
+            Values,
+            cop,
+            tag,
+            comm
+        );
     }
 }
 
@@ -523,18 +636,26 @@ void Pstream::mapCombineScatter
 (
     const List<UPstream::commsStruct>& comms,
     Container& Values,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
     if (UPstream::parRun())
     {
         // Get my communication order
-        const UPstream::commsStruct& myComm = comms[UPstream::myProcNo()];
+        const UPstream::commsStruct& myComm = comms[UPstream::myProcNo(comm)];
 
         // Reveive from up
         if (myComm.above() != -1)
         {
-            IPstream fromAbove(UPstream::scheduled, myComm.above(), 0, tag);
+            IPstream fromAbove
+            (
+                UPstream::scheduled,
+                myComm.above(),
+                0,
+                tag,
+                comm
+            );
             fromAbove >> Values;
 
             if (debug & 2)
@@ -554,7 +675,7 @@ void Pstream::mapCombineScatter
                 Pout<< " sending to " << belowID << " data:" << Values << endl;
             }
 
-            OPstream toBelow(UPstream::scheduled, belowID, 0, tag);
+            OPstream toBelow(UPstream::scheduled, belowID, 0, tag, comm);
             toBelow << Values;
         }
     }
@@ -562,15 +683,32 @@ void Pstream::mapCombineScatter
 
 
 template <class Container>
-void Pstream::mapCombineScatter(Container& Values, const int tag)
+void Pstream::mapCombineScatter
+(
+    Container& Values,
+    const int tag,
+    const label comm
+)
 {
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
-        mapCombineScatter(UPstream::linearCommunication(), Values, tag);
+        mapCombineScatter
+        (
+            UPstream::linearCommunication(comm),
+            Values,
+            tag,
+            comm
+        );
     }
     else
     {
-        mapCombineScatter(UPstream::treeCommunication(), Values, tag);
+        mapCombineScatter
+        (
+            UPstream::treeCommunication(comm),
+            Values,
+            tag,
+            comm
+        );
     }
 }
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/exchange.C b/src/OpenFOAM/db/IOstreams/Pstreams/exchange.C
index 48c166935a..b64de740f9 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/exchange.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/exchange.C
@@ -46,6 +46,7 @@ void Pstream::exchange
     List<Container>& recvBufs,
     labelListList& sizes,
     const int tag,
+    const label comm,
     const bool block
 )
 {
@@ -57,20 +58,20 @@ void Pstream::exchange
         )   << "Continuous data only." << Foam::abort(FatalError);
     }
 
-    if (sendBufs.size() != UPstream::nProcs())
+    if (sendBufs.size() != UPstream::nProcs(comm))
     {
         FatalErrorIn
         (
             "Pstream::exchange(..)"
         )   << "Size of list:" << sendBufs.size()
             << " does not equal the number of processors:"
-            << UPstream::nProcs()
+            << UPstream::nProcs(comm)
             << Foam::abort(FatalError);
     }
 
-    sizes.setSize(UPstream::nProcs());
-    labelList& nsTransPs = sizes[UPstream::myProcNo()];
-    nsTransPs.setSize(UPstream::nProcs());
+    sizes.setSize(UPstream::nProcs(comm));
+    labelList& nsTransPs = sizes[UPstream::myProcNo(comm)];
+    nsTransPs.setSize(UPstream::nProcs(comm));
 
     forAll(sendBufs, procI)
     {
@@ -78,7 +79,7 @@ void Pstream::exchange
     }
 
     // Send sizes across. Note: blocks.
-    combineReduce(sizes, UPstream::listEq(), tag);
+    combineReduce(sizes, UPstream::listEq(), tag, comm);
 
     if (Pstream::parRun())
     {
@@ -90,9 +91,9 @@ void Pstream::exchange
         recvBufs.setSize(sendBufs.size());
         forAll(sizes, procI)
         {
-            label nRecv = sizes[procI][UPstream::myProcNo()];
+            label nRecv = sizes[procI][UPstream::myProcNo(comm)];
 
-            if (procI != Pstream::myProcNo() && nRecv > 0)
+            if (procI != Pstream::myProcNo(comm) && nRecv > 0)
             {
                 recvBufs[procI].setSize(nRecv);
                 UIPstream::read
@@ -101,7 +102,8 @@ void Pstream::exchange
                     procI,
                     reinterpret_cast<char*>(recvBufs[procI].begin()),
                     nRecv*sizeof(T),
-                    tag
+                    tag,
+                    comm
                 );
             }
         }
@@ -112,7 +114,7 @@ void Pstream::exchange
 
         forAll(sendBufs, procI)
         {
-            if (procI != Pstream::myProcNo() && sendBufs[procI].size() > 0)
+            if (procI != Pstream::myProcNo(comm) && sendBufs[procI].size() > 0)
             {
                 if
                 (
@@ -122,7 +124,8 @@ void Pstream::exchange
                         procI,
                         reinterpret_cast<const char*>(sendBufs[procI].begin()),
                         sendBufs[procI].size()*sizeof(T),
-                        tag
+                        tag,
+                        comm
                     )
                 )
                 {
@@ -146,7 +149,7 @@ void Pstream::exchange
     }
 
     // Do myself
-    recvBufs[Pstream::myProcNo()] = sendBufs[Pstream::myProcNo()];
+    recvBufs[Pstream::myProcNo(comm)] = sendBufs[Pstream::myProcNo(comm)];
 }
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/gatherScatter.C b/src/OpenFOAM/db/IOstreams/Pstreams/gatherScatter.C
index 7065383d38..3e412f416f 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/gatherScatter.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/gatherScatter.C
@@ -48,13 +48,14 @@ void Pstream::gather
     const List<UPstream::commsStruct>& comms,
     T& Value,
     const BinaryOp& bop,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
     if (UPstream::parRun())
     {
         // Get my communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo()];
+        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
 
         // Receive from my downstairs neighbours
         forAll(myComm.below(), belowI)
@@ -69,7 +70,8 @@ void Pstream::gather
                     myComm.below()[belowI],
                     reinterpret_cast<char*>(&value),
                     sizeof(T),
-                    tag
+                    tag,
+                    comm
                 );
             }
             else
@@ -79,7 +81,8 @@ void Pstream::gather
                     UPstream::scheduled,
                     myComm.below()[belowI],
                     0,
-                    tag
+                    tag,
+                    comm
                 );
                 fromBelow >> value;
             }
@@ -98,12 +101,20 @@ void Pstream::gather
                     myComm.above(),
                     reinterpret_cast<const char*>(&Value),
                     sizeof(T),
-                    tag
+                    tag,
+                    comm
                 );
             }
             else
             {
-                OPstream toAbove(UPstream::scheduled, myComm.above(), 0, tag);
+                OPstream toAbove
+                (
+                    UPstream::scheduled,
+                    myComm.above(),
+                    0,
+                    tag,
+                    comm
+                );
                 toAbove << Value;
             }
         }
@@ -112,15 +123,21 @@ void Pstream::gather
 
 
 template <class T, class BinaryOp>
-void Pstream::gather(T& Value, const BinaryOp& bop, const int tag)
+void Pstream::gather
+(
+    T& Value,
+    const BinaryOp& bop,
+    const int tag,
+    const label comm
+)
 {
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
-        gather(UPstream::linearCommunication(), Value, bop, tag);
+        gather(UPstream::linearCommunication(comm), Value, bop, tag, comm);
     }
     else
     {
-        gather(UPstream::treeCommunication(), Value, bop, tag);
+        gather(UPstream::treeCommunication(comm), Value, bop, tag, comm);
     }
 }
 
@@ -130,13 +147,14 @@ void Pstream::scatter
 (
     const List<UPstream::commsStruct>& comms,
     T& Value,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
     if (UPstream::parRun())
     {
         // Get my communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo()];
+        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
 
         // Reveive from up
         if (myComm.above() != -1)
@@ -149,12 +167,20 @@ void Pstream::scatter
                     myComm.above(),
                     reinterpret_cast<char*>(&Value),
                     sizeof(T),
-                    tag
+                    tag,
+                    comm
                 );
             }
             else
             {
-                IPstream fromAbove(UPstream::scheduled, myComm.above(), 0, tag);
+                IPstream fromAbove
+                (
+                    UPstream::scheduled,
+                    myComm.above(),
+                    0,
+                    tag,
+                    comm
+                );
                 fromAbove >> Value;
             }
         }
@@ -170,7 +196,8 @@ void Pstream::scatter
                     myComm.below()[belowI],
                     reinterpret_cast<const char*>(&Value),
                     sizeof(T),
-                    tag
+                    tag,
+                    comm
                 );
             }
             else
@@ -180,7 +207,8 @@ void Pstream::scatter
                     UPstream::scheduled,
                     myComm.below()[belowI],
                     0,
-                    tag
+                    tag,
+                    comm
                 );
                 toBelow << Value;
             }
@@ -190,15 +218,15 @@ void Pstream::scatter
 
 
 template <class T>
-void Pstream::scatter(T& Value, const int tag)
+void Pstream::scatter(T& Value, const int tag, const label comm)
 {
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
-        scatter(UPstream::linearCommunication(), Value, tag);
+        scatter(UPstream::linearCommunication(comm), Value, tag, comm);
     }
     else
     {
-        scatter(UPstream::treeCommunication(), Value, tag);
+        scatter(UPstream::treeCommunication(comm), Value, tag, comm);
     }
 }
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/gatherScatterList.C b/src/OpenFOAM/db/IOstreams/Pstreams/gatherScatterList.C
index b309ea7932..c7e2d026b6 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/gatherScatterList.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/gatherScatterList.C
@@ -26,7 +26,7 @@ Description
     communication schedule (usually linear-to-master or tree-to-master).
     The gathered data will be a list with element procID the data from processor
     procID. Before calling every processor should insert its value into
-    Values[UPstream::myProcNo()].
+    Values[UPstream::myProcNo(comm)].
     Note: after gather every processor only knows its own data and that of the
     processors below it. Only the 'master' of the communication schedule holds
     a fully filled List. Use scatter to distribute the data.
@@ -49,12 +49,13 @@ void Pstream::gatherList
 (
     const List<UPstream::commsStruct>& comms,
     List<T>& Values,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
     if (UPstream::parRun())
     {
-        if (Values.size() != UPstream::nProcs())
+        if (Values.size() != UPstream::nProcs(comm))
         {
             FatalErrorIn
             (
@@ -62,12 +63,12 @@ void Pstream::gatherList
                 ", List<T>)"
             )   << "Size of list:" << Values.size()
                 << " does not equal the number of processors:"
-                << UPstream::nProcs()
+                << UPstream::nProcs(comm)
                 << Foam::abort(FatalError);
         }
 
         // Get my communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo()];
+        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
 
         // Receive from my downstairs neighbours
         forAll(myComm.below(), belowI)
@@ -85,7 +86,8 @@ void Pstream::gatherList
                     belowID,
                     reinterpret_cast<char*>(receivedValues.begin()),
                     receivedValues.byteSize(),
-                    tag
+                    tag,
+                    comm
                 );
 
                 Values[belowID] = receivedValues[0];
@@ -97,7 +99,7 @@ void Pstream::gatherList
             }
             else
             {
-                IPstream fromBelow(UPstream::scheduled, belowID, 0, tag);
+                IPstream fromBelow(UPstream::scheduled, belowID, 0, tag, comm);
                 fromBelow >> Values[belowID];
 
                 if (debug & 2)
@@ -133,14 +135,14 @@ void Pstream::gatherList
             if (debug & 2)
             {
                 Pout<< " sending to " << myComm.above()
-                    << " data from me:" << UPstream::myProcNo()
-                    << " data:" << Values[UPstream::myProcNo()] << endl;
+                    << " data from me:" << UPstream::myProcNo(comm)
+                    << " data:" << Values[UPstream::myProcNo(comm)] << endl;
             }
 
             if (contiguous<T>())
             {
                 List<T> sendingValues(belowLeaves.size() + 1);
-                sendingValues[0] = Values[UPstream::myProcNo()];
+                sendingValues[0] = Values[UPstream::myProcNo(comm)];
 
                 forAll(belowLeaves, leafI)
                 {
@@ -153,13 +155,21 @@ void Pstream::gatherList
                     myComm.above(),
                     reinterpret_cast<const char*>(sendingValues.begin()),
                     sendingValues.byteSize(),
-                    tag
+                    tag,
+                    comm
                 );
             }
             else
             {
-                OPstream toAbove(UPstream::scheduled, myComm.above(), 0, tag);
-                toAbove << Values[UPstream::myProcNo()];
+                OPstream toAbove
+                (
+                    UPstream::scheduled,
+                    myComm.above(),
+                    0,
+                    tag,
+                    comm
+                );
+                toAbove << Values[UPstream::myProcNo(comm)];
 
                 forAll(belowLeaves, leafI)
                 {
@@ -180,15 +190,15 @@ void Pstream::gatherList
 
 
 template <class T>
-void Pstream::gatherList(List<T>& Values, const int tag)
+void Pstream::gatherList(List<T>& Values, const int tag, const label comm)
 {
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
-        gatherList(UPstream::linearCommunication(), Values, tag);
+        gatherList(UPstream::linearCommunication(comm), Values, tag, comm);
     }
     else
     {
-        gatherList(UPstream::treeCommunication(), Values, tag);
+        gatherList(UPstream::treeCommunication(comm), Values, tag, comm);
     }
 }
 
@@ -198,12 +208,13 @@ void Pstream::scatterList
 (
     const List<UPstream::commsStruct>& comms,
     List<T>& Values,
-    const int tag
+    const int tag,
+    const label comm
 )
 {
     if (UPstream::parRun())
     {
-        if (Values.size() != UPstream::nProcs())
+        if (Values.size() != UPstream::nProcs(comm))
         {
             FatalErrorIn
             (
@@ -211,12 +222,12 @@ void Pstream::scatterList
                 ", List<T>)"
             )   << "Size of list:" << Values.size()
                 << " does not equal the number of processors:"
-                << UPstream::nProcs()
+                << UPstream::nProcs(comm)
                 << Foam::abort(FatalError);
         }
 
         // Get my communication order
-        const commsStruct& myComm = comms[UPstream::myProcNo()];
+        const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
 
         // Reveive from up
         if (myComm.above() != -1)
@@ -233,7 +244,8 @@ void Pstream::scatterList
                     myComm.above(),
                     reinterpret_cast<char*>(receivedValues.begin()),
                     receivedValues.byteSize(),
-                    tag
+                    tag,
+                    comm
                 );
 
                 forAll(notBelowLeaves, leafI)
@@ -243,7 +255,14 @@ void Pstream::scatterList
             }
             else
             {
-                IPstream fromAbove(UPstream::scheduled, myComm.above(), 0, tag);
+                IPstream fromAbove
+                (
+                    UPstream::scheduled,
+                    myComm.above(),
+                    0,
+                    tag,
+                    comm
+                );
 
                 forAll(notBelowLeaves, leafI)
                 {
@@ -281,12 +300,13 @@ void Pstream::scatterList
                     belowID,
                     reinterpret_cast<const char*>(sendingValues.begin()),
                     sendingValues.byteSize(),
-                    tag
+                    tag,
+                    comm
                 );
             }
             else
             {
-                OPstream toBelow(UPstream::scheduled, belowID, 0, tag);
+                OPstream toBelow(UPstream::scheduled, belowID, 0, tag, comm);
 
                 // Send data destined for all other processors below belowID
                 forAll(notBelowLeaves, leafI)
@@ -308,15 +328,15 @@ void Pstream::scatterList
 
 
 template <class T>
-void Pstream::scatterList(List<T>& Values, const int tag)
+void Pstream::scatterList(List<T>& Values, const int tag, const label comm)
 {
-    if (UPstream::nProcs() < UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(comm) < UPstream::nProcsSimpleSum)
     {
-        scatterList(UPstream::linearCommunication(), Values, tag);
+        scatterList(UPstream::linearCommunication(comm), Values, tag, comm);
     }
     else
     {
-        scatterList(UPstream::treeCommunication(), Values, tag);
+        scatterList(UPstream::treeCommunication(comm), Values, tag, comm);
     }
 }
 
diff --git a/src/OpenFOAM/db/regIOobject/regIOobjectRead.C b/src/OpenFOAM/db/regIOobject/regIOobjectRead.C
index 39fba32a29..5b273d7a80 100644
--- a/src/OpenFOAM/db/regIOobject/regIOobjectRead.C
+++ b/src/OpenFOAM/db/regIOobject/regIOobjectRead.C
@@ -215,9 +215,10 @@ bool Foam::regIOobject::read()
         (
             comms,
             const_cast<word&>(headerClassName()),
-            Pstream::msgType()
+            Pstream::msgType(),
+            Pstream::worldComm
         );
-        Pstream::scatter(comms, note(), Pstream::msgType());
+        Pstream::scatter(comms, note(), Pstream::msgType(), Pstream::worldComm);
 
 
         // Get my communication order
diff --git a/src/OpenFOAM/matrices/LUscalarMatrix/procLduInterface.C b/src/OpenFOAM/matrices/LUscalarMatrix/procLduInterface.C
index 1d02976a15..6eab2e4cbd 100644
--- a/src/OpenFOAM/matrices/LUscalarMatrix/procLduInterface.C
+++ b/src/OpenFOAM/matrices/LUscalarMatrix/procLduInterface.C
@@ -40,7 +40,8 @@ Foam::procLduInterface::procLduInterface
     coeffs_(coeffs),
     myProcNo_(-1),
     neighbProcNo_(-1),
-    tag_(-1)
+    tag_(-1),
+    comm_(-1)
 {
     if (isA<processorLduInterface>(interface.interface()))
     {
@@ -50,6 +51,7 @@ Foam::procLduInterface::procLduInterface
         myProcNo_ = pldui.myProcNo();
         neighbProcNo_ = pldui.neighbProcNo();
         tag_ = pldui.tag();
+        comm_ = pldui.comm();
     }
     else if (isA<cyclicLduInterface>(interface.interface()))
     {
@@ -73,7 +75,8 @@ Foam::procLduInterface::procLduInterface(Istream& is)
     coeffs_(is),
     myProcNo_(readLabel(is)),
     neighbProcNo_(readLabel(is)),
-    tag_(readLabel(is))
+    tag_(readLabel(is)),
+    comm_(readLabel(is))
 {}
 
 
@@ -85,7 +88,8 @@ Foam::Ostream& Foam::operator<<(Ostream& os, const procLduInterface& cldui)
         << cldui.coeffs_
         << cldui.myProcNo_
         << cldui.neighbProcNo_
-        << cldui.tag_;
+        << cldui.tag_
+        << cldui.comm_;
 
     return os;
 }
diff --git a/src/OpenFOAM/matrices/LUscalarMatrix/procLduInterface.H b/src/OpenFOAM/matrices/LUscalarMatrix/procLduInterface.H
index 7541b53b0f..8965f989db 100644
--- a/src/OpenFOAM/matrices/LUscalarMatrix/procLduInterface.H
+++ b/src/OpenFOAM/matrices/LUscalarMatrix/procLduInterface.H
@@ -25,7 +25,7 @@ Class
     Foam::procLduInterface
 
 Description
-    Foam::procLduInterface
+    IO interface for processorLduInterface
 
 SourceFiles
     procLduInterface.C
@@ -58,6 +58,7 @@ class procLduInterface
         label myProcNo_;
         label neighbProcNo_;
         label tag_;
+        label comm_;
 
 
     // Private Member Functions
diff --git a/src/OpenFOAM/matrices/lduMatrix/lduAddressing/lduInterface/processorLduInterface.H b/src/OpenFOAM/matrices/lduMatrix/lduAddressing/lduInterface/processorLduInterface.H
index 26dc4c1eb8..ac0f895900 100644
--- a/src/OpenFOAM/matrices/lduMatrix/lduAddressing/lduInterface/processorLduInterface.H
+++ b/src/OpenFOAM/matrices/lduMatrix/lduAddressing/lduInterface/processorLduInterface.H
@@ -84,10 +84,13 @@ public:
 
         // Access
 
-            //- Return processor number
+            //- Return communicator used for sending
+            virtual int comm() const = 0;
+
+            //- Return processor number (rank in communicator)
             virtual int myProcNo() const = 0;
 
-            //- Return neigbour processor number
+            //- Return neigbour processor number (rank in communicator)
             virtual int neighbProcNo() const = 0;
 
             //- Return face transformation tensor
diff --git a/src/OpenFOAM/matrices/lduMatrix/lduAddressing/lduInterface/processorLduInterfaceTemplates.C b/src/OpenFOAM/matrices/lduMatrix/lduAddressing/lduInterface/processorLduInterfaceTemplates.C
index 396845ffe8..fe552501e1 100644
--- a/src/OpenFOAM/matrices/lduMatrix/lduAddressing/lduInterface/processorLduInterfaceTemplates.C
+++ b/src/OpenFOAM/matrices/lduMatrix/lduAddressing/lduInterface/processorLduInterfaceTemplates.C
@@ -46,7 +46,8 @@ void Foam::processorLduInterface::send
             neighbProcNo(),
             reinterpret_cast<const char*>(f.begin()),
             nBytes,
-            tag()
+            tag(),
+            comm()
         );
     }
     else if (commsType == Pstream::nonBlocking)
@@ -59,7 +60,8 @@ void Foam::processorLduInterface::send
             neighbProcNo(),
             receiveBuf_.begin(),
             nBytes,
-            tag()
+            tag(),
+            comm()
         );
 
         resizeBuf(sendBuf_, nBytes);
@@ -71,7 +73,8 @@ void Foam::processorLduInterface::send
             neighbProcNo(),
             sendBuf_.begin(),
             nBytes,
-            tag()
+            tag(),
+            comm()
         );
     }
     else
@@ -98,7 +101,8 @@ void Foam::processorLduInterface::receive
             neighbProcNo(),
             reinterpret_cast<char*>(f.begin()),
             f.byteSize(),
-            tag()
+            tag(),
+            comm()
         );
     }
     else if (commsType == Pstream::nonBlocking)
@@ -162,7 +166,8 @@ void Foam::processorLduInterface::compressedSend
                 neighbProcNo(),
                 sendBuf_.begin(),
                 nBytes,
-                tag()
+                tag(),
+                comm()
             );
         }
         else if (commsType == Pstream::nonBlocking)
@@ -175,7 +180,8 @@ void Foam::processorLduInterface::compressedSend
                 neighbProcNo(),
                 receiveBuf_.begin(),
                 nBytes,
-                tag()
+                tag(),
+                comm()
             );
 
             OPstream::write
@@ -184,7 +190,8 @@ void Foam::processorLduInterface::compressedSend
                 neighbProcNo(),
                 sendBuf_.begin(),
                 nBytes,
-                tag()
+                tag(),
+                comm()
             );
         }
         else
@@ -225,7 +232,8 @@ void Foam::processorLduInterface::compressedReceive
                 neighbProcNo(),
                 receiveBuf_.begin(),
                 nBytes,
-                tag()
+                tag(),
+                comm()
             );
         }
         else if (commsType != Pstream::nonBlocking)
diff --git a/src/OpenFOAM/matrices/lduMatrix/lduAddressing/lduInterfaceFields/processorLduInterfaceField/processorLduInterfaceField.H b/src/OpenFOAM/matrices/lduMatrix/lduAddressing/lduInterfaceFields/processorLduInterfaceField/processorLduInterfaceField.H
index a5b9066b82..c1cc1c2188 100644
--- a/src/OpenFOAM/matrices/lduMatrix/lduAddressing/lduInterfaceFields/processorLduInterfaceField/processorLduInterfaceField.H
+++ b/src/OpenFOAM/matrices/lduMatrix/lduAddressing/lduInterfaceFields/processorLduInterfaceField/processorLduInterfaceField.H
@@ -71,6 +71,9 @@ public:
 
         // Access
 
+            //- Return communicator used for comms
+            virtual int comm() const = 0;
+
             //- Return processor number
             virtual int myProcNo() const = 0;
 
diff --git a/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/interfaceFields/processorGAMGInterfaceField/processorGAMGInterfaceField.H b/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/interfaceFields/processorGAMGInterfaceField/processorGAMGInterfaceField.H
index c64f0738ee..68007b2c81 100644
--- a/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/interfaceFields/processorGAMGInterfaceField/processorGAMGInterfaceField.H
+++ b/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/interfaceFields/processorGAMGInterfaceField/processorGAMGInterfaceField.H
@@ -146,6 +146,12 @@ public:
 
         //- Processor interface functions
 
+            //- Return communicator used for comms
+            virtual int comm() const
+            {
+                return procInterface_.comm();
+            }
+
             //- Return processor number
             virtual int myProcNo() const
             {
diff --git a/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/interfaces/processorGAMGInterface/processorGAMGInterface.H b/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/interfaces/processorGAMGInterface/processorGAMGInterface.H
index 8ef90c74fe..6c332909cf 100644
--- a/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/interfaces/processorGAMGInterface/processorGAMGInterface.H
+++ b/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/interfaces/processorGAMGInterface/processorGAMGInterface.H
@@ -55,7 +55,7 @@ class processorGAMGInterface
 {
     // Private data
 
-        //- Reference tor the processorLduInterface from which this is
+        //- Reference for the processorLduInterface from which this is
         //  agglomerated
         const processorLduInterface& fineProcInterface_;
 
@@ -137,6 +137,12 @@ public:
             {
                 return fineProcInterface_.tag();
             }
+
+            //- Return communicator used for sending
+            virtual int comm() const
+            {
+                return fineProcInterface_.comm();
+            }
 };
 
 
diff --git a/src/OpenFOAM/meshes/ProcessorTopology/ProcessorTopology.C b/src/OpenFOAM/meshes/ProcessorTopology/ProcessorTopology.C
index f5a6c72fa8..7f099685bf 100644
--- a/src/OpenFOAM/meshes/ProcessorTopology/ProcessorTopology.C
+++ b/src/OpenFOAM/meshes/ProcessorTopology/ProcessorTopology.C
@@ -34,6 +34,7 @@ License
 template<class Patch, class ProcPatch>
 Foam::labelList Foam::ProcessorTopology<Patch, ProcPatch>::procNeighbours
 (
+    const label nProcs,
     const PtrList<Patch>& patches
 )
 {
@@ -43,7 +44,7 @@ Foam::labelList Foam::ProcessorTopology<Patch, ProcPatch>::procNeighbours
 
     label maxNb = 0;
 
-    boolList isNeighbourProc(Pstream::nProcs(), false);
+    boolList isNeighbourProc(nProcs, false);
 
     forAll(patches, patchi)
     {
@@ -106,20 +107,21 @@ Foam::labelList Foam::ProcessorTopology<Patch, ProcPatch>::procNeighbours
 template<class Patch, class ProcPatch>
 Foam::ProcessorTopology<Patch, ProcPatch>::ProcessorTopology
 (
-    const PtrList<Patch>& patches
+    const PtrList<Patch>& patches,
+    const label comm
 )
 :
-    labelListList(Pstream::nProcs()),
+    labelListList(Pstream::nProcs(comm)),
     patchSchedule_(2*patches.size())
 {
     if (Pstream::parRun())
     {
         // Fill my 'slot' with my neighbours
-        operator[](Pstream::myProcNo()) = procNeighbours(patches);
+        operator[](Pstream::myProcNo()) = procNeighbours(this->size(), patches);
 
         // Distribute to all processors
-        Pstream::gatherList(*this);
-        Pstream::scatterList(*this);
+        Pstream::gatherList(*this, Pstream::msgType(), comm);
+        Pstream::scatterList(*this, Pstream::msgType(), comm);
     }
 
     if (Pstream::parRun() && Pstream::defaultCommsType == Pstream::scheduled)
@@ -172,7 +174,7 @@ Foam::ProcessorTopology<Patch, ProcPatch>::ProcessorTopology
         (
             commSchedule
             (
-                Pstream::nProcs(),
+                Pstream::nProcs(comm),
                 comms
             ).procSchedule()[Pstream::myProcNo()]
         );
diff --git a/src/OpenFOAM/meshes/ProcessorTopology/ProcessorTopology.H b/src/OpenFOAM/meshes/ProcessorTopology/ProcessorTopology.H
index 4a46bfcbd0..e39e9e1914 100644
--- a/src/OpenFOAM/meshes/ProcessorTopology/ProcessorTopology.H
+++ b/src/OpenFOAM/meshes/ProcessorTopology/ProcessorTopology.H
@@ -76,14 +76,14 @@ private:
 
         //- Return all neighbouring processors of this processor. Set
         //  procPatchMap_.
-        labelList procNeighbours(const PtrList<Patch>&);
+        labelList procNeighbours(const label nProcs, const PtrList<Patch>&);
 
 public:
 
     // Constructors
 
         //- Construct from boundaryMesh
-        ProcessorTopology(const PtrList<Patch>& patches);
+        ProcessorTopology(const PtrList<Patch>& patches, const label comm);
 
 
     // Member Functions
diff --git a/src/OpenFOAM/meshes/polyMesh/globalMeshData/globalMeshData.C b/src/OpenFOAM/meshes/polyMesh/globalMeshData/globalMeshData.C
index 2c0ae4c102..35a88a646d 100644
--- a/src/OpenFOAM/meshes/polyMesh/globalMeshData/globalMeshData.C
+++ b/src/OpenFOAM/meshes/polyMesh/globalMeshData/globalMeshData.C
@@ -1740,7 +1740,7 @@ void Foam::globalMeshData::calcGlobalCoPointSlaves() const
 // Construct from polyMesh
 Foam::globalMeshData::globalMeshData(const polyMesh& mesh)
 :
-    processorTopology(mesh.boundaryMesh()),
+    processorTopology(mesh.boundaryMesh(), UPstream::worldComm),
     mesh_(mesh),
     nTotalPoints_(-1),
     nTotalFaces_(-1),
diff --git a/src/OpenFOAM/meshes/polyMesh/mapPolyMesh/mapDistribute/mapDistribute.C b/src/OpenFOAM/meshes/polyMesh/mapPolyMesh/mapDistribute/mapDistribute.C
index a4ababac70..94c8d176df 100644
--- a/src/OpenFOAM/meshes/polyMesh/mapPolyMesh/mapDistribute/mapDistribute.C
+++ b/src/OpenFOAM/meshes/polyMesh/mapPolyMesh/mapDistribute/mapDistribute.C
@@ -548,7 +548,8 @@ void Foam::mapDistribute::exchangeAddressing
         wantedRemoteElements,
         subMap_,
         sendSizes,
-        tag
+        tag,
+        Pstream::worldComm  //TBD
     );
 
     // Renumber elements
@@ -627,7 +628,8 @@ void Foam::mapDistribute::exchangeAddressing
         wantedRemoteElements,
         subMap_,
         sendSizes,
-        tag
+        tag,
+        Pstream::worldComm      //TBD
     );
 
     // Renumber elements
diff --git a/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processor/processorPolyPatch.C b/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processor/processorPolyPatch.C
index 5da47a6ce4..ab44c8d45b 100644
--- a/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processor/processorPolyPatch.C
+++ b/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processor/processorPolyPatch.C
@@ -54,12 +54,14 @@ Foam::processorPolyPatch::processorPolyPatch
     const label start,
     const label index,
     const polyBoundaryMesh& bm,
+    const label comm,
     const int myProcNo,
     const int neighbProcNo,
     const transformType transform
 )
 :
     coupledPolyPatch(name, size, start, index, bm, typeName, transform),
+    comm_(comm),
     myProcNo_(myProcNo),
     neighbProcNo_(neighbProcNo),
     neighbFaceCentres_(),
@@ -78,6 +80,10 @@ Foam::processorPolyPatch::processorPolyPatch
 )
 :
     coupledPolyPatch(name, dict, index, bm, patchType),
+    comm_
+    (
+        dict.lookupOrDefault("communicator", UPstream::worldComm)
+    ),
     myProcNo_(readLabel(dict.lookup("myProcNo"))),
     neighbProcNo_(readLabel(dict.lookup("neighbProcNo"))),
     neighbFaceCentres_(),
@@ -93,6 +99,7 @@ Foam::processorPolyPatch::processorPolyPatch
 )
 :
     coupledPolyPatch(pp, bm),
+    comm_(pp.comm_),
     myProcNo_(pp.myProcNo_),
     neighbProcNo_(pp.neighbProcNo_),
     neighbFaceCentres_(),
@@ -111,6 +118,7 @@ Foam::processorPolyPatch::processorPolyPatch
 )
 :
     coupledPolyPatch(pp, bm, index, newSize, newStart),
+    comm_(pp.comm_),
     myProcNo_(pp.myProcNo_),
     neighbProcNo_(pp.neighbProcNo_),
     neighbFaceCentres_(),
@@ -129,6 +137,7 @@ Foam::processorPolyPatch::processorPolyPatch
 )
 :
     coupledPolyPatch(pp, bm, index, mapAddressing, newStart),
+    comm_(pp.comm_),
     myProcNo_(pp.myProcNo_),
     neighbProcNo_(pp.neighbProcNo_),
     neighbFaceCentres_(),
@@ -1082,6 +1091,11 @@ bool Foam::processorPolyPatch::order
 void Foam::processorPolyPatch::write(Ostream& os) const
 {
     coupledPolyPatch::write(os);
+    if (comm_ != UPstream::worldComm)
+    {
+        os.writeKeyword("communicator") << comm_
+            << token::END_STATEMENT << nl;
+    }
     os.writeKeyword("myProcNo") << myProcNo_
         << token::END_STATEMENT << nl;
     os.writeKeyword("neighbProcNo") << neighbProcNo_
diff --git a/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processor/processorPolyPatch.H b/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processor/processorPolyPatch.H
index f40f97c94f..7c74f9f28e 100644
--- a/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processor/processorPolyPatch.H
+++ b/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processor/processorPolyPatch.H
@@ -58,6 +58,9 @@ class processorPolyPatch
 {
     // Private data
 
+        //- Communicator to use
+        label comm_;
+
         int myProcNo_;
         int neighbProcNo_;
 
@@ -133,6 +136,7 @@ public:
             const label start,
             const label index,
             const polyBoundaryMesh& bm,
+            const label comm,
             const int myProcNo,
             const int neighbProcNo,
             const transformType transform = UNKNOWN  // transformation type
@@ -244,6 +248,13 @@ public:
             }
         }
 
+
+        //- Return communicator used for communication
+        label comm() const
+        {
+            return comm_;
+        }
+
         //- Return processor number
         int myProcNo() const
         {
diff --git a/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processorCyclic/processorCyclicPolyPatch.C b/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processorCyclic/processorCyclicPolyPatch.C
index 8988a06eb2..305f01826d 100644
--- a/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processorCyclic/processorCyclicPolyPatch.C
+++ b/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processorCyclic/processorCyclicPolyPatch.C
@@ -49,6 +49,7 @@ Foam::processorCyclicPolyPatch::processorCyclicPolyPatch
     const label start,
     const label index,
     const polyBoundaryMesh& bm,
+    const label comm,
     const int myProcNo,
     const int neighbProcNo,
     const word& referPatchName,
@@ -62,6 +63,7 @@ Foam::processorCyclicPolyPatch::processorCyclicPolyPatch
         start,
         index,
         bm,
+        comm,
         myProcNo,
         neighbProcNo,
         transform
diff --git a/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processorCyclic/processorCyclicPolyPatch.H b/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processorCyclic/processorCyclicPolyPatch.H
index fa59414586..139080b35c 100644
--- a/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processorCyclic/processorCyclicPolyPatch.H
+++ b/src/OpenFOAM/meshes/polyMesh/polyPatches/constraint/processorCyclic/processorCyclicPolyPatch.H
@@ -121,6 +121,7 @@ public:
             const label start,
             const label index,
             const polyBoundaryMesh& bm,
+            const label comm,
             const int myProcNo,
             const int neighbProcNo,
             const word& referPatchName,
diff --git a/src/Pstream/dummy/UIPread.C b/src/Pstream/dummy/UIPread.C
index fc3b4466e2..6fa253f38c 100644
--- a/src/Pstream/dummy/UIPread.C
+++ b/src/Pstream/dummy/UIPread.C
@@ -37,6 +37,7 @@ Foam::UIPstream::UIPstream
     DynamicList<char>& externalBuf,
     label& externalBufPosition,
     const int tag,
+    const label comm,
     const bool clearAtEnd,
     streamFormat format,
     versionNumber version
@@ -48,6 +49,7 @@ Foam::UIPstream::UIPstream
     externalBuf_(externalBuf),
     externalBufPosition_(externalBufPosition),
     tag_(tag),
+    comm_(comm),
     clearAtEnd_(clearAtEnd),
     messageSize_(0)
 {
@@ -60,6 +62,7 @@ Foam::UIPstream::UIPstream
             "DynamicList<char>&,\n"
             "label&,\n"
             "const int,\n"
+            "const label,\n"
             "const bool,\n"
             "streamFormat,\n"
             "versionNumber\n"
@@ -68,11 +71,7 @@ Foam::UIPstream::UIPstream
 }
 
 
-Foam::UIPstream::UIPstream
-(
-    const int fromProcNo,
-    PstreamBuffers& buffers
-)
+Foam::UIPstream::UIPstream(const int fromProcNo, PstreamBuffers& buffers)
 :
     UPstream(buffers.commsType_),
     Istream(buffers.format_, buffers.version_),
@@ -80,6 +79,7 @@ Foam::UIPstream::UIPstream
     externalBuf_(buffers.recvBuf_[fromProcNo]),
     externalBufPosition_(buffers.recvBufPos_[fromProcNo]),
     tag_(buffers.tag_),
+    comm_(buffers.comm_),
     clearAtEnd_(true),
     messageSize_(0)
 {
@@ -102,7 +102,8 @@ Foam::label Foam::UIPstream::read
     const int fromProcNo,
     char* buf,
     const std::streamsize bufSize,
-    const int tag
+    const int tag,
+    const label communicator
 )
 {
     notImplemented
@@ -113,7 +114,8 @@ Foam::label Foam::UIPstream::read
             "const int fromProcNo,"
             "char* buf,"
             "const label bufSize,"
-            "const int tag"
+            "const int tag,"
+            "const label communicator"
         ")"
      );
 
diff --git a/src/Pstream/dummy/UOPwrite.C b/src/Pstream/dummy/UOPwrite.C
index 3766f82e7c..79dfb9719b 100644
--- a/src/Pstream/dummy/UOPwrite.C
+++ b/src/Pstream/dummy/UOPwrite.C
@@ -36,7 +36,8 @@ bool Foam::UOPstream::write
     const int toProcNo,
     const char* buf,
     const std::streamsize bufSize,
-    const int tag
+    const int tag,
+    const label communicator
 )
 {
     notImplemented
@@ -47,7 +48,8 @@ bool Foam::UOPstream::write
             "const int fromProcNo,"
             "char* buf,"
             "const label bufSize,"
-            "const int tag"
+            "const int tag,"
+            "const label communicator"
         ")"
     );
 
diff --git a/src/Pstream/dummy/UPstream.C b/src/Pstream/dummy/UPstream.C
index 183a86cb9e..1e4cd4899a 100644
--- a/src/Pstream/dummy/UPstream.C
+++ b/src/Pstream/dummy/UPstream.C
@@ -55,28 +55,41 @@ void Foam::UPstream::abort()
 }
 
 
-void Foam::reduce(scalar&, const sumOp<scalar>&, const int)
+void Foam::reduce(scalar&, const sumOp<scalar>&, const int, const label)
 {}
 
 
-void Foam::reduce(scalar&, const minOp<scalar>&, const int)
+void Foam::reduce(scalar&, const minOp<scalar>&, const int, const label)
 {}
 
 
-void Foam::reduce(vector2D&, const sumOp<vector2D>&, const int)
+void Foam::reduce(vector2D&, const sumOp<vector2D>&, const int, const label)
 {}
 
 
 void Foam::sumReduce
 (
-    scalar& Value,
-    label& Count,
-    const int tag
+    scalar&,
+    label&,
+    const int,
+    const label
 )
 {}
 
 
-void Foam::reduce(scalar&, const sumOp<scalar>&, const int, label&)
+void Foam::reduce(scalar&, const sumOp<scalar>&, const int, const label, label&)
+{}
+
+
+void Foam::UPstream::allocatePstreamCommunicator
+(
+    const label,
+    const label
+)
+{}
+
+
+void Foam::UPstream::freePstreamCommunicator(const label)
 {}
 
 
diff --git a/src/Pstream/mpi/PstreamGlobals.C b/src/Pstream/mpi/PstreamGlobals.C
index 40eb6adacf..c57e7aef24 100644
--- a/src/Pstream/mpi/PstreamGlobals.C
+++ b/src/Pstream/mpi/PstreamGlobals.C
@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2011 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2013 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -37,6 +37,35 @@ namespace Foam
 DynamicList<MPI_Request> PstreamGlobals::outstandingRequests_;
 //! \endcond
 
+// Allocated communicators.
+//! \cond fileScope
+DynamicList<MPI_Comm> PstreamGlobals::MPICommunicators_;
+DynamicList<MPI_Group> PstreamGlobals::MPIGroups_;
+//! \endcond
+
+void PstreamGlobals::checkCommunicator
+(
+    const label comm,
+    const label otherProcNo
+)
+{
+    if
+    (
+        comm < 0
+     || comm >= PstreamGlobals::MPICommunicators_.size()
+    )
+    {
+        FatalErrorIn
+        (
+            "PstreamGlobals::checkCommunicator(const label, const label)"
+        )   << "otherProcNo:" << otherProcNo << " : illegal communicator "
+            << comm << endl
+            << "Communicator should be within range 0.."
+            << PstreamGlobals::MPICommunicators_.size()-1 << abort(FatalError);
+    }
+}
+
+
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 } // End namespace Foam
diff --git a/src/Pstream/mpi/PstreamGlobals.H b/src/Pstream/mpi/PstreamGlobals.H
index 9d12ca9da0..9448a948a5 100644
--- a/src/Pstream/mpi/PstreamGlobals.H
+++ b/src/Pstream/mpi/PstreamGlobals.H
@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2011 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2013 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -26,7 +26,7 @@ Namespace
 
 Description
     Global functions and variables for working with parallel streams,
-    but principally for gamma/mpi
+    but principally for mpi
 
 SourceFiles
     PstreamGlobals.C
@@ -54,6 +54,13 @@ namespace PstreamGlobals
 
 extern DynamicList<MPI_Request> outstandingRequests_;
 
+
+// Current communicators. First element will be MPI_COMM_WORLD
+extern DynamicList<MPI_Comm> MPICommunicators_;
+extern DynamicList<MPI_Group> MPIGroups_;
+
+void checkCommunicator(const label, const label procNo);
+
 };
 
 
diff --git a/src/Pstream/mpi/UIPread.C b/src/Pstream/mpi/UIPread.C
index 19d6b16112..c1cae71e16 100644
--- a/src/Pstream/mpi/UIPread.C
+++ b/src/Pstream/mpi/UIPread.C
@@ -41,6 +41,7 @@ Foam::UIPstream::UIPstream
     DynamicList<char>& externalBuf,
     label& externalBufPosition,
     const int tag,
+    const label comm,
     const bool clearAtEnd,
     streamFormat format,
     versionNumber version
@@ -52,6 +53,7 @@ Foam::UIPstream::UIPstream
     externalBuf_(externalBuf),
     externalBufPosition_(externalBufPosition),
     tag_(tag),
+    comm_(comm),
     clearAtEnd_(clearAtEnd),
     messageSize_(0)
 {
@@ -80,7 +82,7 @@ Foam::UIPstream::UIPstream
         // and set it
         if (!wantedSize)
         {
-            MPI_Probe(procID(fromProcNo_), tag_, MPI_COMM_WORLD, &status);
+            MPI_Probe(fromProcNo_, tag_, MPI_COMM_WORLD, &status);
             MPI_Get_count(&status, MPI_BYTE, &messageSize_);
 
             externalBuf_.setCapacity(messageSize_);
@@ -99,7 +101,8 @@ Foam::UIPstream::UIPstream
             fromProcNo_,
             externalBuf_.begin(),
             wantedSize,
-            tag_
+            tag_,
+            comm_
         );
 
         // Set addressed size. Leave actual allocated memory intact.
@@ -121,6 +124,7 @@ Foam::UIPstream::UIPstream(const int fromProcNo, PstreamBuffers& buffers)
     externalBuf_(buffers.recvBuf_[fromProcNo]),
     externalBufPosition_(buffers.recvBufPos_[fromProcNo]),
     tag_(buffers.tag_),
+    comm_(buffers.comm_),
     clearAtEnd_(true),
     messageSize_(0)
 {
@@ -167,7 +171,7 @@ Foam::UIPstream::UIPstream(const int fromProcNo, PstreamBuffers& buffers)
         // and set it
         if (!wantedSize)
         {
-            MPI_Probe(procID(fromProcNo_), tag_, MPI_COMM_WORLD, &status);
+            MPI_Probe(fromProcNo_, tag_, MPI_COMM_WORLD, &status);
             MPI_Get_count(&status, MPI_BYTE, &messageSize_);
 
             externalBuf_.setCapacity(messageSize_);
@@ -186,7 +190,8 @@ Foam::UIPstream::UIPstream(const int fromProcNo, PstreamBuffers& buffers)
             fromProcNo_,
             externalBuf_.begin(),
             wantedSize,
-            tag_
+            tag_,
+            comm_
         );
 
         // Set addressed size. Leave actual allocated memory intact.
@@ -208,13 +213,15 @@ Foam::label Foam::UIPstream::read
     const int fromProcNo,
     char* buf,
     const std::streamsize bufSize,
-    const int tag
+    const int tag,
+    const label communicator
 )
 {
     if (debug)
     {
         Pout<< "UIPstream::read : starting read from:" << fromProcNo
-            << " tag:" << tag << " wanted size:" << label(bufSize)
+            << " tag:" << tag << " comm:" << communicator
+            << " wanted size:" << label(bufSize)
             << " commsType:" << UPstream::commsTypeNames[commsType]
             << Foam::endl;
     }
@@ -230,9 +237,9 @@ Foam::label Foam::UIPstream::read
                 buf,
                 bufSize,
                 MPI_PACKED,
-                procID(fromProcNo),
+                fromProcNo,
                 tag,
-                MPI_COMM_WORLD,
+                PstreamGlobals::MPICommunicators_[communicator],
                 &status
             )
         )
@@ -286,9 +293,9 @@ Foam::label Foam::UIPstream::read
                 buf,
                 bufSize,
                 MPI_PACKED,
-                procID(fromProcNo),
+                fromProcNo,
                 tag,
-                MPI_COMM_WORLD,
+                PstreamGlobals::MPICommunicators_[communicator],
                 &request
             )
         )
diff --git a/src/Pstream/mpi/UOPwrite.C b/src/Pstream/mpi/UOPwrite.C
index 2a1b3f7195..24f425e38b 100644
--- a/src/Pstream/mpi/UOPwrite.C
+++ b/src/Pstream/mpi/UOPwrite.C
@@ -39,17 +39,22 @@ bool Foam::UOPstream::write
     const int toProcNo,
     const char* buf,
     const std::streamsize bufSize,
-    const int tag
+    const int tag,
+    const label communicator
 )
 {
     if (debug)
     {
         Pout<< "UOPstream::write : starting write to:" << toProcNo
-            << " tag:" << tag << " size:" << label(bufSize)
+            << " tag:" << tag
+            << " comm:" << communicator << " size:" << label(bufSize)
             << " commsType:" << UPstream::commsTypeNames[commsType]
             << Foam::endl;
     }
 
+    PstreamGlobals::checkCommunicator(communicator, toProcNo);
+
+
     bool transferFailed = true;
 
     if (commsType == blocking)
@@ -59,9 +64,9 @@ bool Foam::UOPstream::write
             const_cast<char*>(buf),
             bufSize,
             MPI_PACKED,
-            procID(toProcNo),
+            toProcNo,   //procID(toProcNo),
             tag,
-            MPI_COMM_WORLD
+            PstreamGlobals::MPICommunicators_[communicator] //MPI_COMM_WORLD
         );
 
         if (debug)
@@ -79,9 +84,9 @@ bool Foam::UOPstream::write
             const_cast<char*>(buf),
             bufSize,
             MPI_PACKED,
-            procID(toProcNo),
+            toProcNo,   //procID(toProcNo),
             tag,
-            MPI_COMM_WORLD
+            PstreamGlobals::MPICommunicators_[communicator] //MPI_COMM_WORLD
         );
 
         if (debug)
@@ -101,9 +106,9 @@ bool Foam::UOPstream::write
             const_cast<char*>(buf),
             bufSize,
             MPI_PACKED,
-            procID(toProcNo),
+            toProcNo,   //procID(toProcNo),
             tag,
-            MPI_COMM_WORLD,
+            PstreamGlobals::MPICommunicators_[communicator],//MPI_COMM_WORLD,
             &request
         );
 
diff --git a/src/Pstream/mpi/UPstream.C b/src/Pstream/mpi/UPstream.C
index bf7af5dd12..efe8488483 100644
--- a/src/Pstream/mpi/UPstream.C
+++ b/src/Pstream/mpi/UPstream.C
@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2011-2012 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2011-2013 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -55,7 +55,6 @@ void Foam::UPstream::addValidParOptions(HashTable<string>& validParOptions)
     validParOptions.insert("p4wd", "directory");
     validParOptions.insert("p4amslave", "");
     validParOptions.insert("p4yourname", "hostname");
-    validParOptions.insert("GAMMANP", "number of instances");
     validParOptions.insert("machinefile", "machine file");
 }
 
@@ -66,12 +65,13 @@ bool Foam::UPstream::init(int& argc, char**& argv)
 
     int numprocs;
     MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
-    MPI_Comm_rank(MPI_COMM_WORLD, &myProcNo_);
+    int myRank;
+    MPI_Comm_rank(MPI_COMM_WORLD, &myRank);
 
     if (debug)
     {
         Pout<< "UPstream::init : initialised with numProcs:" << numprocs
-            << " myProcNo:" << myProcNo_ << endl;
+            << " myRank:" << myRank << endl;
     }
 
     if (numprocs <= 1)
@@ -82,14 +82,9 @@ bool Foam::UPstream::init(int& argc, char**& argv)
             << Foam::abort(FatalError);
     }
 
-    procIDs_.setSize(numprocs);
 
-    forAll(procIDs_, procNo)
-    {
-        procIDs_[procNo] = procNo;
-    }
-
-    setParRun();
+    // Initialise parallel structure
+    setParRun(numprocs);
 
 #   ifndef SGIMPI
     string bufferSizeName = getEnv("MPI_BUFFER_SIZE");
@@ -116,11 +111,9 @@ bool Foam::UPstream::init(int& argc, char**& argv)
     char processorName[MPI_MAX_PROCESSOR_NAME];
 
     MPI_Get_processor_name(processorName, &processorNameLen);
+    processorName[processorNameLen] = '\0';
 
-    //signal(SIGABRT, stop);
-
-    // Now that nprocs is known construct communication tables.
-    initCommunicationSchedule();
+    Pout<< "Processor name:" << processorName << endl;
 
     return true;
 }
@@ -153,6 +146,15 @@ void Foam::UPstream::exit(int errnum)
             << endl;
     }
 
+    // Clean mpi communicators
+    forAll(myProcNo_, communicator)
+    {
+        if (myProcNo_[communicator] != -1)
+        {
+            freePstreamCommunicator(communicator);
+        }
+    }
+
     if (errnum == 0)
     {
         MPI_Finalize();
@@ -171,21 +173,39 @@ void Foam::UPstream::abort()
 }
 
 
-void Foam::reduce(scalar& Value, const sumOp<scalar>& bop, const int tag)
+void Foam::reduce
+(
+    scalar& Value,
+    const sumOp<scalar>& bop,
+    const int tag,
+    const label communicator
+)
 {
-    allReduce(Value, 1, MPI_SCALAR, MPI_SUM, bop, tag);
+    allReduce(Value, 1, MPI_SCALAR, MPI_SUM, bop, tag, communicator);
 }
 
 
-void Foam::reduce(scalar& Value, const minOp<scalar>& bop, const int tag)
+void Foam::reduce
+(
+    scalar& Value,
+    const minOp<scalar>& bop,
+    const int tag,
+    const label communicator
+)
 {
-    allReduce(Value, 1, MPI_SCALAR, MPI_MIN, bop, tag);
+    allReduce(Value, 1, MPI_SCALAR, MPI_MIN, bop, tag, communicator);
 }
 
 
-void Foam::reduce(vector2D& Value, const sumOp<vector2D>& bop, const int tag)
+void Foam::reduce
+(
+    vector2D& Value,
+    const sumOp<vector2D>& bop,
+    const int tag,
+    const label communicator
+)
 {
-    allReduce(Value, 2, MPI_SCALAR, MPI_SUM, bop, tag);
+    allReduce(Value, 2, MPI_SCALAR, MPI_SUM, bop, tag, communicator);
 }
 
 
@@ -193,11 +213,12 @@ void Foam::sumReduce
 (
     scalar& Value,
     label& Count,
-    const int tag
+    const int tag,
+    const label communicator
 )
 {
     vector2D twoScalars(Value, scalar(Count));
-    reduce(twoScalars, sumOp<vector2D>());
+    reduce(twoScalars, sumOp<vector2D>(), tag, communicator);
 
     Value = twoScalars.x();
     Count = twoScalars.y();
@@ -209,6 +230,7 @@ void Foam::reduce
     scalar& Value,
     const sumOp<scalar>& bop,
     const int tag,
+    const label communicator,
     label& requestID
 )
 {
@@ -225,7 +247,7 @@ void Foam::reduce
         MPI_SCALAR,
         MPI_SUM,
         0,              //root
-        MPI_COMM_WORLD,
+        PstreamGlobals::MPICommunicators_[communicator],
         &request
     );
 
@@ -233,12 +255,141 @@ void Foam::reduce
     PstreamGlobals::outstandingRequests_.append(request);
 #else
     // Non-blocking not yet implemented in mpi
-    reduce(Value, bop, tag);
+    reduce(Value, bop, tag, communicator);
     requestID = -1;
 #endif
 }
 
 
+void Foam::UPstream::allocatePstreamCommunicator
+(
+    const label parentIndex,
+    const label index
+)
+{
+    if (index == PstreamGlobals::MPIGroups_.size())
+    {
+        // Extend storage with dummy values
+        MPI_Group newGroup;
+        PstreamGlobals::MPIGroups_.append(newGroup);
+        MPI_Comm newComm;
+        PstreamGlobals::MPICommunicators_.append(newComm);
+    }
+    else if (index > PstreamGlobals::MPIGroups_.size())
+    {
+        FatalErrorIn
+        (
+            "UPstream::allocatePstreamCommunicator\n"
+            "(\n"
+            "    const label parentIndex,\n"
+            "    const labelList& subRanks\n"
+            ")\n"
+        )   << "PstreamGlobals out of sync with UPstream data. Problem."
+            << Foam::exit(FatalError);
+    }
+
+
+    if (parentIndex == -1)
+    {
+        // Allocate world communicator
+
+        //std::cout
+        //    << "MPI : Allocating world communicator at index " << index
+        //    << std::endl;
+
+        if (index != UPstream::worldComm)
+        {
+            FatalErrorIn
+            (
+                "UPstream::allocateCommunicator\n"
+                "(\n"
+                "    const label parentIndex,\n"
+                "    const labelList& subRanks\n"
+                ")\n"
+            )   << "world communicator should always be index "
+                << UPstream::worldComm << Foam::exit(FatalError);
+        }
+
+        PstreamGlobals::MPICommunicators_[index] = MPI_COMM_WORLD;
+        MPI_Comm_group(MPI_COMM_WORLD, &PstreamGlobals::MPIGroups_[index]);
+        MPI_Comm_rank
+        (
+            PstreamGlobals::MPICommunicators_[index],
+           &myProcNo_[index]
+        );
+
+        // Set the number of processes to the actual number
+        int numProcs;
+        MPI_Comm_size(PstreamGlobals::MPICommunicators_[index], &numProcs);
+        procIDs_[index] = identity(numProcs);
+    }
+    else
+    {
+        //std::cout
+        //    << "MPI : Allocating new communicator at index " << index
+        //    << " from parent " << parentIndex
+        //    << std::endl;
+
+        // Create new group
+        MPI_Group_incl
+        (
+            PstreamGlobals::MPIGroups_[parentIndex],
+            procIDs_[index].size(),
+            procIDs_[index].begin(),
+           &PstreamGlobals::MPIGroups_[index]
+        );
+ 
+       //std::cout
+       //     << "MPI : New group " << long(PstreamGlobals::MPIGroups_[index])
+       //     << std::endl;
+
+
+        // Create new communicator
+        MPI_Comm_create
+        (
+            PstreamGlobals::MPICommunicators_[parentIndex],
+            PstreamGlobals::MPIGroups_[index],
+           &PstreamGlobals::MPICommunicators_[index]
+        );
+
+        if (PstreamGlobals::MPICommunicators_[index] == MPI_COMM_NULL)
+        {
+            //std::cout
+            //     << "MPI : NULL : not in group"
+            //    << std::endl;
+            myProcNo_[index] = -1;
+        }
+        else
+        {
+            //std::cout
+            //    << "MPI : New comm "
+            //    << long(PstreamGlobals::MPICommunicators_[index])
+            //    << std::endl;
+            MPI_Comm_rank
+            (
+                PstreamGlobals::MPICommunicators_[index],
+               &myProcNo_[index]
+            );
+        }
+    }
+
+    //std::cout<< "MPI : I am rank " << myProcNo_[index] << std::endl;
+}
+
+
+void Foam::UPstream::freePstreamCommunicator(const label communicator)
+{
+    if (communicator != UPstream::worldComm)
+    {
+        if (PstreamGlobals::MPICommunicators_[communicator] != MPI_COMM_NULL)
+        {
+            MPI_Comm_free(&PstreamGlobals::MPICommunicators_[communicator]);
+        }
+        MPI_Group_free(&PstreamGlobals::MPIGroups_[communicator]);
+    }
+}
+
+
 Foam::label Foam::UPstream::nRequests()
 {
     return PstreamGlobals::outstandingRequests_.size();
diff --git a/src/Pstream/mpi/allReduce.H b/src/Pstream/mpi/allReduce.H
index bb4c693113..53bd102ae1 100644
--- a/src/Pstream/mpi/allReduce.H
+++ b/src/Pstream/mpi/allReduce.H
@@ -52,7 +52,8 @@ void allReduce
     MPI_Datatype MPIType,
     MPI_Op op,
     const BinaryOp& bop,
-    const int tag
+    const int tag,
+    const label communicator
 );
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
diff --git a/src/Pstream/mpi/allReduceTemplates.C b/src/Pstream/mpi/allReduceTemplates.C
index 98b8d97b0a..9271e8b90b 100644
--- a/src/Pstream/mpi/allReduceTemplates.C
+++ b/src/Pstream/mpi/allReduceTemplates.C
@@ -35,7 +35,8 @@ void Foam::allReduce
     MPI_Datatype MPIType,
     MPI_Op MPIOp,
     const BinaryOp& bop,
-    const int tag
+    const int tag,
+    const label communicator
 )
 {
     if (!UPstream::parRun())
@@ -43,14 +44,14 @@ void Foam::allReduce
         return;
     }
 
-    if (UPstream::nProcs() <= UPstream::nProcsSimpleSum)
+    if (UPstream::nProcs(communicator) <= UPstream::nProcsSimpleSum)
     {
-        if (UPstream::master())
+        if (UPstream::master(communicator))
         {
             for
             (
                 int slave=UPstream::firstSlave();
-                slave<=UPstream::lastSlave();
+                slave<=UPstream::lastSlave(communicator);
                 slave++
             )
             {
@@ -63,9 +64,9 @@ void Foam::allReduce
                         &value,
                         MPICount,
                         MPIType,
-                        UPstream::procID(slave),
+                        slave,  //UPstream::procID(slave),
                         tag,
-                        MPI_COMM_WORLD,
+                        PstreamGlobals::MPICommunicators_[communicator],
                         MPI_STATUS_IGNORE
                     )
                 )
@@ -97,9 +98,9 @@ void Foam::allReduce
                     &Value,
                     MPICount,
                     MPIType,
-                    UPstream::procID(UPstream::masterNo()),
+                    UPstream::masterNo(),//UPstream::procID(masterNo()),
                     tag,
-                    MPI_COMM_WORLD
+                    PstreamGlobals::MPICommunicators_[communicator]
                 )
             )
             {
@@ -120,12 +121,12 @@ void Foam::allReduce
         }
 
 
-        if (UPstream::master())
+        if (UPstream::master(communicator))
         {
             for
             (
                 int slave=UPstream::firstSlave();
-                slave<=UPstream::lastSlave();
+                slave<=UPstream::lastSlave(communicator);
                 slave++
             )
             {
@@ -136,9 +137,9 @@ void Foam::allReduce
                         &Value,
                         MPICount,
                         MPIType,
-                        UPstream::procID(slave),
+                        slave,      //UPstream::procID(slave),
                         tag,
-                        MPI_COMM_WORLD
+                        PstreamGlobals::MPICommunicators_[communicator]
                     )
                 )
                 {
@@ -167,9 +168,9 @@ void Foam::allReduce
                     &Value,
                     MPICount,
                     MPIType,
-                    UPstream::procID(UPstream::masterNo()),
+                    UPstream::masterNo(),//UPstream::procID(masterNo()),
                     tag,
-                    MPI_COMM_WORLD,
+                    PstreamGlobals::MPICommunicators_[communicator],
                     MPI_STATUS_IGNORE
                 )
             )
@@ -193,7 +194,15 @@ void Foam::allReduce
     else
     {
         Type sum;
-        MPI_Allreduce(&Value, &sum, MPICount, MPIType, MPIOp, MPI_COMM_WORLD);
+        MPI_Allreduce
+        (
+            &Value,
+            &sum,
+            MPICount,
+            MPIType,
+            MPIOp,
+            PstreamGlobals::MPICommunicators_[communicator]
+        );
         Value = sum;
     }
 }
diff --git a/src/dynamicMesh/fvMeshDistribute/fvMeshDistribute.C b/src/dynamicMesh/fvMeshDistribute/fvMeshDistribute.C
index a21cfb6300..f3e97ba285 100644
--- a/src/dynamicMesh/fvMeshDistribute/fvMeshDistribute.C
+++ b/src/dynamicMesh/fvMeshDistribute/fvMeshDistribute.C
@@ -945,6 +945,7 @@ void Foam::fvMeshDistribute::addProcPatches
                         mesh_.nFaces(),
                         mesh_.boundaryMesh().size(),
                         mesh_.boundaryMesh(),
+                        Pstream::worldComm,
                         Pstream::myProcNo(),
                         nbrProc[bFaceI]
                     );
@@ -988,6 +989,7 @@ void Foam::fvMeshDistribute::addProcPatches
                         mesh_.nFaces(),
                         mesh_.boundaryMesh().size(),
                         mesh_.boundaryMesh(),
+                        Pstream::worldComm,
                         Pstream::myProcNo(),
                         nbrProc[bFaceI],
                         cycName,
diff --git a/src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchField.C b/src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchField.C
index c9fb2563c1..b0c2cd26d3 100644
--- a/src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchField.C
+++ b/src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchField.C
@@ -244,23 +244,25 @@ void processorFvPatchField<Type>::initEvaluate
             // Fast path. Receive into *this
             this->setSize(sendBuf_.size());
             outstandingRecvRequest_ = UPstream::nRequests();
-            IPstream::read
+            UIPstream::read
             (
                 Pstream::nonBlocking,
                 procPatch_.neighbProcNo(),
                 reinterpret_cast<char*>(this->begin()),
                 this->byteSize(),
-                procPatch_.tag()
+                procPatch_.tag(),
+                procPatch_.comm()
             );
 
             outstandingSendRequest_ = UPstream::nRequests();
-            OPstream::write
+            UOPstream::write
             (
                 Pstream::nonBlocking,
                 procPatch_.neighbProcNo(),
                 reinterpret_cast<const char*>(sendBuf_.begin()),
                 this->byteSize(),
-                procPatch_.tag()
+                procPatch_.tag(),
+                procPatch_.comm()
             );
         }
         else
@@ -342,23 +344,25 @@ void processorFvPatchField<Type>::initInterfaceMatrixUpdate
 
         scalarReceiveBuf_.setSize(scalarSendBuf_.size());
         outstandingRecvRequest_ = UPstream::nRequests();
-        IPstream::read
+        UIPstream::read
         (
             Pstream::nonBlocking,
             procPatch_.neighbProcNo(),
             reinterpret_cast<char*>(scalarReceiveBuf_.begin()),
             scalarReceiveBuf_.byteSize(),
-            procPatch_.tag()
+            procPatch_.tag(),
+            procPatch_.comm()
         );
 
         outstandingSendRequest_ = UPstream::nRequests();
-        OPstream::write
+        UOPstream::write
         (
             Pstream::nonBlocking,
             procPatch_.neighbProcNo(),
             reinterpret_cast<const char*>(scalarSendBuf_.begin()),
             scalarSendBuf_.byteSize(),
-            procPatch_.tag()
+            procPatch_.tag(),
+            procPatch_.comm()
         );
     }
     else
@@ -467,7 +471,8 @@ void processorFvPatchField<Type>::initInterfaceMatrixUpdate
             procPatch_.neighbProcNo(),
             reinterpret_cast<char*>(receiveBuf_.begin()),
             receiveBuf_.byteSize(),
-            procPatch_.tag()
+            procPatch_.tag(),
+            procPatch_.comm()
         );
 
         outstandingSendRequest_ = UPstream::nRequests();
@@ -477,7 +482,8 @@ void processorFvPatchField<Type>::initInterfaceMatrixUpdate
             procPatch_.neighbProcNo(),
             reinterpret_cast<const char*>(sendBuf_.begin()),
             sendBuf_.byteSize(),
-            procPatch_.tag()
+            procPatch_.tag(),
+            procPatch_.comm()
         );
     }
     else
diff --git a/src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchField.H b/src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchField.H
index a913aaf606..848b8e5165 100644
--- a/src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchField.H
+++ b/src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchField.H
@@ -244,6 +244,12 @@ public:
 
         //- Processor coupled interface functions
 
+            //- Return communicator used for comms
+            virtual int comm() const
+            {
+                return procPatch_.comm();
+            }
+
             //- Return processor number
             virtual int myProcNo() const
             {
diff --git a/src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchScalarField.C b/src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchScalarField.C
index 82dcf7ca36..2d51603100 100644
--- a/src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchScalarField.C
+++ b/src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchScalarField.C
@@ -60,23 +60,25 @@ void processorFvPatchField<scalar>::initInterfaceMatrixUpdate
 
         scalarReceiveBuf_.setSize(scalarSendBuf_.size());
         outstandingRecvRequest_ = UPstream::nRequests();
-        IPstream::read
+        UIPstream::read
         (
             Pstream::nonBlocking,
             procPatch_.neighbProcNo(),
             reinterpret_cast<char*>(scalarReceiveBuf_.begin()),
             scalarReceiveBuf_.byteSize(),
-            procPatch_.tag()
+            procPatch_.tag(),
+            procPatch_.comm()
         );
 
         outstandingSendRequest_ = UPstream::nRequests();
-        OPstream::write
+        UOPstream::write
         (
             Pstream::nonBlocking,
             procPatch_.neighbProcNo(),
             reinterpret_cast<const char*>(scalarSendBuf_.begin()),
             scalarSendBuf_.byteSize(),
-            procPatch_.tag()
+            procPatch_.tag(),
+            procPatch_.comm()
         );
     }
     else
diff --git a/src/finiteVolume/fvMesh/fvPatches/constraint/processor/processorFvPatch.H b/src/finiteVolume/fvMesh/fvPatches/constraint/processor/processorFvPatch.H
index d0f3b04c7c..5df5a25f77 100644
--- a/src/finiteVolume/fvMesh/fvPatches/constraint/processor/processorFvPatch.H
+++ b/src/finiteVolume/fvMesh/fvPatches/constraint/processor/processorFvPatch.H
@@ -84,14 +84,20 @@ public:
 
     // Member functions
 
+        //- Return communicator used for comms
+        virtual int comm() const
+        {
+            return procPolyPatch_.comm();
+        }
+
         //- Return processor number
-        int myProcNo() const
+        virtual int myProcNo() const
         {
             return procPolyPatch_.myProcNo();
         }
 
         //- Return neigbour processor number
-        int neighbProcNo() const
+        virtual int neighbProcNo() const
         {
             return procPolyPatch_.neighbProcNo();
         }