diff --git a/applications/test/parallel-comm0/Make/files b/applications/test/parallel-comm0/Make/files
new file mode 100644
index 0000000000..cdf4e68ab1
--- /dev/null
+++ b/applications/test/parallel-comm0/Make/files
@@ -0,0 +1,3 @@
+Test-parallel-comm0.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-parallel-comm0
diff --git a/applications/test/parallel-communicators/Make/options b/applications/test/parallel-comm0/Make/options
similarity index 100%
rename from applications/test/parallel-communicators/Make/options
rename to applications/test/parallel-comm0/Make/options
diff --git a/applications/test/parallel-comm0/Test-parallel-comm0.C b/applications/test/parallel-comm0/Test-parallel-comm0.C
new file mode 100644
index 0000000000..dc6fdb064f
--- /dev/null
+++ b/applications/test/parallel-comm0/Test-parallel-comm0.C
@@ -0,0 +1,161 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2022 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Application
+    Test-parallel-comm0
+
+Description
+    Very basic checks on standard communicators
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "Time.H"
+#include "IPstream.H"
+#include "OPstream.H"
+#include "Pair.H"
+#include "Tuple2.H"
+#include "IOstreams.H"
+#include "PstreamReduceOps.H"
+
+using namespace Foam;
+
+
+void printInfo(const label comm)
+{
+    Info<< "comm:" << comm
+        << " nprocs:" << UPstream::nProcs(comm)
+        << " all:" << UPstream::allProcs(comm)
+        << " sub:" << UPstream::subProcs(comm) << nl;
+
+
+    if (UPstream::selfComm == comm)
+    {
+        Pout<< "self all:" << UPstream::allProcs(comm)
+            << " sub:" << UPstream::subProcs(comm) << nl;
+    }
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+int main(int argc, char *argv[])
+{
+    argList::noBanner();
+    argList::noCheckProcessorDirectories();
+    argList::addBoolOption("verbose", "Set debug level");
+
+    // Capture manually. We need values before proper startup
+    int nVerbose = 0;
+    for (int argi = 1; argi < argc; ++argi)
+    {
+        if (strcmp(argv[argi], "-verbose") == 0)
+        {
+            ++nVerbose;
+        }
+    }
+
+    UPstream::debug = nVerbose;
+
+
+    #include "setRootCase.H"
+
+    Info<< nl
+        << "nProcs = " << UPstream::nProcs()
+        << " with " << UPstream::nComms() << " predefined comm(s)" << nl;
+
+    Info<< "worldComm : ";
+    printInfo(UPstream::worldComm);
+
+    Info<< "selfComm  : ";
+    printInfo(UPstream::selfComm);
+
+    Info<< nl;
+
+    // Reductions (using MPI intrinsics)
+    {
+        label val = Pstream::myProcNo(UPstream::worldComm);
+
+        label worldVal = returnReduce
+        (
+            val,
+            sumOp<label>(),
+            Pstream::msgType(),
+            UPstream::worldComm
+        );
+
+        label selfVal = returnReduce
+        (
+            val,
+            sumOp<label>(),
+            Pstream::msgType(),
+            UPstream::selfComm
+        );
+
+        Pout<< "value " << val
+            << " (world) reduced " << worldVal
+            << " (self) reduced " << selfVal << nl;
+    }
+
+    // Reductions (not using MPI intrinsics)
+    {
+        Pair<label> val
+        (
+            Pstream::myProcNo(UPstream::worldComm),
+            Pstream::myProcNo(UPstream::worldComm)
+        );
+
+        Pair<label> worldVal = val;
+
+        Pstream::combineReduce
+        (
+            worldVal,
+            minFirstEqOp<label>(),
+            Pstream::msgType(),
+            UPstream::worldComm
+        );
+
+        Pair<label> selfVal = val;
+
+        Pstream::combineReduce
+        (
+            worldVal,
+            minFirstEqOp<label>(),
+            Pstream::msgType(),
+            UPstream::selfComm
+        );
+
+        Pout<< "value " << val
+            << " (world) reduced " << worldVal
+            << " (self) reduced " << selfVal << nl;
+    }
+
+    Pout<< "\nEnd\n" << endl;
+
+    return 0;
+}
+
+
+// ************************************************************************* //
diff --git a/applications/test/parallel-comm1/Make/files b/applications/test/parallel-comm1/Make/files
new file mode 100644
index 0000000000..38e9320286
--- /dev/null
+++ b/applications/test/parallel-comm1/Make/files
@@ -0,0 +1,3 @@
+Test-parallel-comm1.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-parallel-comm1
diff --git a/applications/test/parallel-comm1/Make/options b/applications/test/parallel-comm1/Make/options
new file mode 100644
index 0000000000..18e6fe47af
--- /dev/null
+++ b/applications/test/parallel-comm1/Make/options
@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */
diff --git a/applications/test/parallel-communicators/Test-parallel-communicators.C b/applications/test/parallel-comm1/Test-parallel-comm1.C
similarity index 100%
rename from applications/test/parallel-communicators/Test-parallel-communicators.C
rename to applications/test/parallel-comm1/Test-parallel-comm1.C
diff --git a/applications/test/parallel-communicators/Make/files b/applications/test/parallel-communicators/Make/files
deleted file mode 100644
index ab261d4da9..0000000000
--- a/applications/test/parallel-communicators/Make/files
+++ /dev/null
@@ -1,3 +0,0 @@
-Test-parallel-communicators.C
-
-EXE = $(FOAM_USER_APPBIN)/Test-parallel-communicators
diff --git a/applications/test/parallel-external-init/Test-parallel-external-init.C b/applications/test/parallel-external-init/Test-parallel-external-init.C
index 731aaaf64c..dbcead0d8b 100644
--- a/applications/test/parallel-external-init/Test-parallel-external-init.C
+++ b/applications/test/parallel-external-init/Test-parallel-external-init.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2019 OpenCFD Ltd.
+    Copyright (C) 2019-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -47,16 +47,88 @@ using namespace Foam;
 
 bool startMPI()
 {
-    int nprocs = 0, rank = 0;
+    enum whichComm : int { worldComm = 0, selfComm, nullComm };
+
+    int nprocs[3];
+    int rank[3];
+
+    int group_nprocs[3];
+    int group_rank[3];
+
+    MPI_Group mpiGroup;
 
     MPI_Init(nullptr, nullptr);
 
-    MPI_Comm_size(MPI_COMM_WORLD, &nprocs);
-    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+    MPI_Comm_size(MPI_COMM_WORLD, &nprocs[worldComm]);
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank[worldComm]);
 
-    if (nprocs && rank == 0)
+    const bool isMaster = (rank[worldComm] == 0);
+    const string prefix = '[' + Foam::name(rank[worldComm]) + "] ";
+
+    MPI_Comm_group(MPI_COMM_WORLD, &mpiGroup);
+    MPI_Group_size(mpiGroup, &group_nprocs[worldComm]);
+    MPI_Group_rank(mpiGroup, &group_rank[worldComm]);
+
+
+    if (isMaster && nprocs[worldComm])
     {
-        std::cout<< nl << "Using MPI with " << nprocs << " procs" << nl << nl;
+        std::cout
+            << nl << "Using MPI with " << nprocs[worldComm]
+            << " procs, group:"
+            << group_nprocs[worldComm] << nl
+            << "World group: " << Foam::name(mpiGroup) << nl
+            << nl;
+    }
+
+    MPI_Comm worldMpiComm;
+
+    MPI_Comm_dup(MPI_COMM_WORLD, &worldMpiComm);
+
+    MPI_Comm_group(MPI_COMM_WORLD, &mpiGroup);
+
+    if (isMaster && nprocs[worldComm])
+    {
+        std::cout
+            << "dup comm group: " << Foam::name(mpiGroup) << nl;
+    }
+
+    MPI_Comm_free(&worldMpiComm);
+
+    // May be a bad idea
+    MPI_Group_free(&mpiGroup);
+
+    MPI_Comm_size(MPI_COMM_SELF, &nprocs[selfComm]);
+    MPI_Comm_rank(MPI_COMM_SELF, &rank[selfComm]);
+
+    MPI_Comm_group(MPI_COMM_SELF, &mpiGroup);
+    MPI_Group_size(mpiGroup, &group_nprocs[selfComm]);
+    MPI_Group_rank(mpiGroup, &group_rank[selfComm]);
+
+    if (isMaster && nprocs[worldComm])
+    {
+        std::cout
+            << nl
+            << "Self group: " << Foam::name(mpiGroup) << nl;
+    }
+
+    // Should be a bad idea
+    MPI_Group_free(&mpiGroup);
+
+    // if (nprocs && isMaster)
+    {
+        std::cout
+            << prefix
+            << "Self: " << rank[selfComm] << " from " << nprocs[selfComm]
+            << " procs, group:"
+            << group_nprocs[selfComm] << nl;
+    }
+
+    if (isMaster)
+    {
+        std::cout
+            << "MPI_COMM_NULL:  " << MPI_COMM_NULL << nl
+            << "MPI_COMM_SELF:  " << MPI_COMM_SELF << nl
+            << "MPI_COMM_WORLD: " << MPI_COMM_WORLD << nl;
     }
 
     return true;
@@ -77,8 +149,8 @@ string message()
 {
     return
     (
-        "rank " + name(Pstream::myProcNo())
-      + " / " + name(Pstream::nProcs()) + "\n"
+        "rank " + Foam::name(Pstream::myProcNo())
+      + " / " + Foam::name(Pstream::nProcs()) + "\n"
     );
 }
 
@@ -87,9 +159,21 @@ string message()
 
 int main(int argc, char *argv[])
 {
+    argList::noBanner();
     argList::noCheckProcessorDirectories();
+    argList::addBoolOption("verbose", "Set debug level");
 
-    UPstream::debug = 1;
+    // Need to capture manually, since we need values before proper startup
+    int nVerbose = 0;
+    for (int argi = 1; argi < argc; ++argi)
+    {
+        if (strcmp(argv[argi], "-verbose") == 0)
+        {
+            ++nVerbose;
+        }
+    }
+
+    UPstream::debug = nVerbose;
 
     startMPI();
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
index e4db3ee022..d35520f1e2 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
@@ -58,43 +58,78 @@ Foam::UPstream::commsTypeNames
 
 void Foam::UPstream::setParRun(const label nProcs, const bool haveThreads)
 {
-    if (nProcs == 0)
-    {
-        parRun_ = false;
-        haveThreads_ = haveThreads;
+    parRun_ = (nProcs > 0);
+    haveThreads_ = haveThreads;
 
-        freeCommunicator(UPstream::worldComm);
-        label comm = allocateCommunicator(-1, labelList(Foam::one{}, 0), false);
-        if (comm != UPstream::worldComm)
+    label comm = -1;
+
+    if (!parRun_)
+    {
+        // These are already correct from the static initialisation,
+        // but just in case of future changes
+
+        // Using (world, self) ordering
+        freeCommunicator(UPstream::selfComm);
+        freeCommunicator(UPstream::globalComm);
+
+        // 0: worldComm
+        comm = allocateCommunicator(-1, Foam::labelList(Foam::one{}, 0), false);
+        if (comm != UPstream::globalComm)
         {
+            // Failed sanity check
             FatalErrorInFunction
                 << "problem : comm:" << comm
-                << "  UPstream::worldComm:" << UPstream::worldComm
+                << "  UPstream::globalComm:" << UPstream::globalComm
                 << Foam::exit(FatalError);
         }
 
-        Pout.prefix() = "";
-        Perr.prefix() = "";
+        // 1: selfComm
+        comm = allocateCommunicator(-2, Foam::labelList(Foam::one{}, 0), false);
+        if (comm != UPstream::selfComm)
+        {
+            // Failed sanity check
+            FatalErrorInFunction
+                << "problem : comm:" << comm
+                << "  UPstream::selfComm:" << UPstream::selfComm
+                << Foam::exit(FatalError);
+        }
+
+        Pout.prefix().clear();
+        Perr.prefix().clear();
     }
     else
     {
-        parRun_ = true;
-        haveThreads_ = haveThreads;
+        // Redo communicators that were created during static initialisation
+        // but this time with Pstream components
 
-        // 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)
+        // Using (world, self) ordering
+        freeCommunicator(UPstream::selfComm);
+        freeCommunicator(UPstream::globalComm);
+
+        // 0: worldComm
+        comm = allocateCommunicator(-1, identity(nProcs), true);
+        if (comm != UPstream::globalComm)
         {
+            // Failed sanity check
             FatalErrorInFunction
                 << "problem : comm:" << comm
-                << "  UPstream::worldComm:" << UPstream::worldComm
+                << "  UPstream::globalComm:" << UPstream::globalComm
                 << Foam::exit(FatalError);
         }
 
-        Pout.prefix() = '[' +  name(myProcNo(comm)) + "] ";
-        Perr.prefix() = '[' +  name(myProcNo(comm)) + "] ";
+        Pout.prefix() = '[' +  Foam::name(myProcNo(comm)) + "] ";
+        Perr.prefix() = Pout.prefix();
+
+        // 1: selfComm
+        comm = allocateCommunicator(-2, Foam::labelList(Foam::one{}, 0), true);
+        if (comm != UPstream::selfComm)
+        {
+            // Failed sanity check
+            FatalErrorInFunction
+                << "problem : comm:" << comm
+                << "  UPstream::selfComm:" << UPstream::selfComm
+                << Foam::exit(FatalError);
+        }
     }
 
     if (debug)
@@ -110,23 +145,25 @@ void Foam::UPstream::setParRun(const label nProcs, const bool haveThreads)
 Foam::label Foam::UPstream::allocateCommunicator
 (
     const label parentIndex,
-    const labelList& subRanks,
+    const labelUList& subRanks,
     const bool doPstream
 )
 {
     label index;
     if (!freeComms_.empty())
     {
-        index = freeComms_.remove();  // LIFO pop
+        // LIFO pop
+        index = freeComms_.back();
+        freeComms_.pop_back();
     }
     else
     {
         // Extend storage
-        index = parentCommunicator_.size();
+        index = parentComm_.size();
 
         myProcNo_.append(-1);
         procIDs_.append(List<int>());
-        parentCommunicator_.append(-1);
+        parentComm_.append(-1);
         linearCommunication_.append(List<commsStruct>());
         treeCommunication_.append(List<commsStruct>());
     }
@@ -142,8 +179,10 @@ Foam::label Foam::UPstream::allocateCommunicator
     // Initialise; overwritten by allocatePstreamCommunicator
     myProcNo_[index] = 0;
 
+    const label numSubRanks = subRanks.size();
+
     // Convert from label to int
-    procIDs_[index].setSize(subRanks.size());
+    procIDs_[index].resize_nocopy(numSubRanks);
     forAll(procIDs_[index], i)
     {
         procIDs_[index][i] = subRanks[i];
@@ -158,11 +197,11 @@ Foam::label Foam::UPstream::allocateCommunicator
                 << Foam::abort(FatalError);
         }
     }
-    parentCommunicator_[index] = parentIndex;
+    parentComm_[index] = parentIndex;
 
     // Size but do not fill structure - this is done on-the-fly
-    linearCommunication_[index] = List<commsStruct>(procIDs_[index].size());
-    treeCommunication_[index] = List<commsStruct>(procIDs_[index].size());
+    linearCommunication_[index] = List<commsStruct>(numSubRanks);
+    treeCommunication_[index] = List<commsStruct>(numSubRanks);
 
     if (doPstream && parRun())
     {
@@ -179,10 +218,16 @@ void Foam::UPstream::freeCommunicator
     const bool doPstream
 )
 {
+    // Filter out any placeholders
+    if (communicator < 0)
+    {
+        return;
+    }
+
     if (debug)
     {
         Pout<< "Communicators : Freeing communicator " << communicator << endl
-            << "    parent   : " << parentCommunicator_[communicator] << endl
+            << "    parent   : " << parentComm_[communicator] << endl
             << "    myProcNo : " << myProcNo_[communicator] << endl
             << endl;
     }
@@ -191,13 +236,15 @@ void Foam::UPstream::freeCommunicator
     {
         freePstreamCommunicator(communicator);
     }
+
     myProcNo_[communicator] = -1;
     //procIDs_[communicator].clear();
-    parentCommunicator_[communicator] = -1;
+    parentComm_[communicator] = -1;
     linearCommunication_[communicator].clear();
     treeCommunication_[communicator].clear();
 
-    freeComms_.append(communicator);  // LIFO push
+    // LIFO push
+    freeComms_.push_back(communicator);
 }
 
 
@@ -213,48 +260,44 @@ void Foam::UPstream::freeCommunicators(const bool doPstream)
 }
 
 
-int Foam::UPstream::baseProcNo(const label myComm, const int myProcID)
+int Foam::UPstream::baseProcNo(label comm, int procID)
 {
-    int procID = myProcID;
-    label comm = myComm;
-
-    while (parent(comm) != -1)
+    while (parent(comm) >= 0)
     {
-        const List<int>& parentRanks = UPstream::procID(comm);
+        const auto& parentRanks = UPstream::procID(comm);
         procID = parentRanks[procID];
-        comm = UPstream::parent(comm);
+        comm = parent(comm);
     }
 
     return procID;
 }
 
 
-Foam::label Foam::UPstream::procNo(const label myComm, const int baseProcID)
+Foam::label Foam::UPstream::procNo(const label comm, const int baseProcID)
 {
-    const List<int>& parentRanks = procID(myComm);
-    label parentComm = parent(myComm);
+    const auto& parentRanks = procID(comm);
+    label parentComm = parent(comm);
 
-    if (parentComm == -1)
+    int procID = baseProcID;
+
+    if (parentComm >= 0)
     {
-        return parentRanks.find(baseProcID);
-    }
-    else
-    {
-        const label parentRank = procNo(parentComm, baseProcID);
-        return parentRanks.find(parentRank);
+        procID = procNo(parentComm, baseProcID);
     }
+
+    return parentRanks.find(procID);
 }
 
 
 Foam::label Foam::UPstream::procNo
 (
-    const label myComm,
+    const label comm,
     const label currentComm,
     const int currentProcID
 )
 {
     label physProcID = UPstream::baseProcNo(currentComm, currentProcID);
-    return procNo(myComm, physProcID);
+    return procNo(comm, physProcID);
 }
 
 
@@ -380,7 +423,7 @@ Foam::DynamicList<int> Foam::UPstream::myProcNo_(10);
 
 Foam::DynamicList<Foam::List<int>> Foam::UPstream::procIDs_(10);
 
-Foam::DynamicList<Foam::label> Foam::UPstream::parentCommunicator_(10);
+Foam::DynamicList<Foam::label> Foam::UPstream::parentComm_(10);
 
 Foam::DynamicList<Foam::label> Foam::UPstream::freeComms_;
 
@@ -394,14 +437,25 @@ 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(Foam::one{}, 0),
-    false
-);
+Foam::label Foam::UPstream::worldComm(0);
+
+Foam::label Foam::UPstream::warnComm(-1);
+
+
+// Predefine worldComm, selfComm slots.
+// These are overwritten in parallel mode (by UPstream::setParRun())
+const Foam::label nPredefinedComm = []()
+{
+    const Foam::labelList singleProc(Foam::one{}, 0);
+
+    // 0: worldComm
+    (void) Foam::UPstream::allocateCommunicator(-1, singleProc, false);
+
+    // 1: selfComm
+    (void) Foam::UPstream::allocateCommunicator(-2, singleProc, false);
+
+    return Foam::UPstream::nComms();
+}();
 
 
 bool Foam::UPstream::floatTransfer
@@ -466,10 +520,6 @@ namespace Foam
     addcommsTypeToOpt addcommsTypeToOpt_("commsType");
 }
 
-Foam::label Foam::UPstream::worldComm(0);
-
-Foam::label Foam::UPstream::warnComm(-1);
-
 int Foam::UPstream::nPollProcInterfaces
 (
     Foam::debug::optimisationSwitch("nPollProcInterfaces", 0)
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
index a36a77d97b..04ac5f6065 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
@@ -107,9 +107,9 @@ public:
                 commsStruct
                 (
                     const label above,
-                    const labelList& below,
-                    const labelList& allBelow,
-                    const labelList& allNotBelow
+                    const labelUList& below,
+                    const labelUList& allBelow,
+                    const labelUList& allNotBelow
                 );
 
                 //- Construct from components; construct allNotBelow_
@@ -118,13 +118,16 @@ public:
                     const label nProcs,
                     const label myProcID,
                     const label above,
-                    const labelList& below,
-                    const labelList& allBelow
+                    const labelUList& below,
+                    const labelUList& allBelow
                 );
 
 
             // Member Functions
 
+                //- Reset to default constructed state
+                void clear();
+
                 //- The procID of the processor directly above
                 label above() const noexcept
                 {
@@ -203,7 +206,7 @@ private:
         //- Names of all worlds
         static wordList allWorlds_;
 
-        //- Per processor the name of the world
+        //- Per processor the world index (into allWorlds_)
         static labelList worldIDs_;
 
 
@@ -216,7 +219,7 @@ private:
         static DynamicList<List<int>> procIDs_;
 
         //- Parent communicator
-        static DynamicList<label> parentCommunicator_;
+        static DynamicList<label> parentComm_;
 
         //- Free communicators
         static DynamicList<label> freeComms_;
@@ -233,21 +236,6 @@ private:
         //- Set data for parallel running
         static void setParRun(const label nProcs, const bool haveThreads);
 
-        //- Calculate linear communication schedule
-        static List<commsStruct> calcLinearComm(const label nProcs);
-
-        //- Calculate tree communication schedule
-        static List<commsStruct> calcTreeComm(const label nProcs);
-
-        //- Helper function for tree communication schedule determination
-        //  Collects all processorIDs below a processor
-        static void collectReceives
-        (
-            const label procID,
-            const List<DynamicList<label>>& receives,
-            DynamicList<label>& allReceives
-        );
-
         //- Allocate a communicator with index
         static void allocatePstreamCommunicator
         (
@@ -255,11 +243,9 @@ private:
             const label index
         );
 
-        //- Free a communicator
-        static void freePstreamCommunicator
-        (
-            const label index
-        );
+        //- Free MPI components of communicator.
+        //  Does not touch the first two communicators (SELF, WORLD)
+        static void freePstreamCommunicator(const label index);
 
 
 public:
@@ -290,12 +276,31 @@ public:
         //- MPI buffer-size (bytes)
         static const int mpiBufferSize;
 
-        //- Default communicator (all processors)
+
+    // Standard Communicators
+
+        //- Default world communicator (all processors).
+        //- May differ from globalComm if local worlds are in use
         static label worldComm;
 
         //- Debugging: warn for use of any communicator differing from warnComm
         static label warnComm;
 
+        //- Communicator for all processors, irrespective of any local worlds
+        static constexpr label globalComm = 0;
+
+        //- A communicator within the current rank only
+        static constexpr label selfComm = 1;
+
+        //- Number of currently defined communicators
+        static label nComms() noexcept { return parentComm_.size(); }
+
+        //- True if communicator appears to be user-allocated
+        static bool isUserComm(label communicator) noexcept
+        {
+            return (communicator > worldComm && communicator > selfComm);
+        }
+
 
     // Constructors
 
@@ -312,11 +317,12 @@ public:
         static label allocateCommunicator
         (
             const label parent,
-            const labelList& subRanks,
+            const labelUList& subRanks,
             const bool doPstream = true
         );
 
-        //- Free a previously allocated communicator
+        //- Free a previously allocated communicator.
+        //  Ignores placeholder (negative) communicators.
         static void freeCommunicator
         (
             const label communicator,
@@ -326,43 +332,52 @@ public:
         //- Free all communicators
         static void freeCommunicators(const bool doPstream);
 
-        //- Helper class for allocating/freeing communicators
+        //- Wrapper class for allocating/freeing communicators
         class communicator
         {
             label comm_;
 
+        public:
+
+            //- Default construct (a placeholder communicator)
+            communicator() : comm_(-1) {}
+
+            //- Move construct
+            communicator(communicator&&) = default;
+
+            //- Move assignment
+            communicator& operator=(communicator&&) = default;
+
             //- No copy construct
             communicator(const communicator&) = delete;
 
             //- No copy assignment
             void operator=(const communicator&) = delete;
 
-        public:
-
+            //- Allocate a communicator from given parent
             communicator
             (
                 const label parent,
-                const labelList& subRanks,
+                const labelUList& subRanks,
                 const bool doPstream
             )
             :
                 comm_(allocateCommunicator(parent, subRanks, doPstream))
             {}
 
+            //- Free allocated communicator and group
             ~communicator()
             {
                 freeCommunicator(comm_);
             }
 
-            operator label() const noexcept
-            {
-                return comm_;
-            }
+            operator label() const noexcept { return comm_; }
         };
 
+
         //- Return physical processor number (i.e. processor number in
-        //- worldComm) given communicator and procssor
-        static int baseProcNo(const label myComm, const int procID);
+        //- worldComm) given communicator and processor
+        static int baseProcNo(label comm, int procID);
 
         //- Return processor number in communicator (given physical processor
         //- number) (= reverse of baseProcNo)
@@ -372,7 +387,7 @@ public:
         //- and communicator)
         static label procNo
         (
-            const label myComm,
+            const label comm,
             const label currentComm,
             const int currentProcID
         );
@@ -414,13 +429,8 @@ public:
             //  A no-op and returns true if parRun() == false
             static bool finishedRequest(const label i);
 
-            static int allocateTag(const char*);
-
-            static int allocateTag(const std::string&);
-
-            static void freeTag(const char*, const int tag);
-
-            static void freeTag(const std::string&, const int tag);
+            static int allocateTag(const char* const msg = nullptr);
+            static void freeTag(const int tag, const char* const msg = nullptr);
 
 
         //- Set as parallel run on/off.
@@ -445,7 +455,8 @@ public:
             return haveThreads_;
         }
 
-        //- Number of processes in parallel run, and 1 for serial run
+        //- Number of ranks in parallel run (for given communicator)
+        //- is 1 for serial run
         static label nProcs(const label communicator = worldComm)
         {
             return procIDs_[communicator].size();
@@ -469,13 +480,14 @@ public:
             return myProcNo_[communicator];
         }
 
+        //- The parent communicator
         static label parent(const label communicator)
         {
-            return parentCommunicator_(communicator);
+            return parentComm_(communicator);
         }
 
-        //- Process ID of given process index
-        static List<int>& procID(label communicator)
+        //- Process IDs within a given communicator
+        static List<int>& procID(const label communicator)
         {
             return procIDs_[communicator];
         }
@@ -489,7 +501,7 @@ public:
                 return allWorlds_;
             }
 
-            //- worldID (index in allWorlds) of all processes
+            //- The indices into allWorlds for all processes
             static const labelList& worldIDs() noexcept
             {
                 return worldIDs_;
@@ -498,13 +510,13 @@ public:
             //- My worldID
             static label myWorldID()
             {
-                return worldIDs_[myProcNo(0)];
+                return worldIDs_[myProcNo(globalComm)];
             }
 
             //- My world
             static const word& myWorld()
             {
-                return allWorlds()[myWorldID()];
+                return allWorlds_[worldIDs_[myProcNo(globalComm)]];
             }
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamCommsStruct.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamCommsStruct.C
index c9c5a8cfe0..e854bc7182 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamCommsStruct.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamCommsStruct.C
@@ -42,9 +42,9 @@ Foam::UPstream::commsStruct::commsStruct() noexcept
 Foam::UPstream::commsStruct::commsStruct
 (
     const label above,
-    const labelList& below,
-    const labelList& allBelow,
-    const labelList& allNotBelow
+    const labelUList& below,
+    const labelUList& allBelow,
+    const labelUList& allNotBelow
 )
 :
     above_(above),
@@ -59,8 +59,8 @@ Foam::UPstream::commsStruct::commsStruct
     const label nProcs,
     const label myProcID,
     const label above,
-    const labelList& below,
-    const labelList& allBelow
+    const labelUList& below,
+    const labelUList& allBelow
 )
 :
     above_(above),
@@ -90,6 +90,17 @@ Foam::UPstream::commsStruct::commsStruct
 }
 
 
+// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
+
+void Foam::UPstream::commsStruct::clear()
+{
+    above_ = -1;
+    below_.clear();
+    allBelow_.clear();
+    allNotBelow_.clear();
+}
+
+
 // * * * * * * * * * * * * * * * Member Operators  * * * * * * * * * * * * * //
 
 bool Foam::UPstream::commsStruct::operator==(const commsStruct& comm) const
diff --git a/src/Pstream/mpi/PstreamGlobals.H b/src/Pstream/mpi/PstreamGlobals.H
index 3f80c63a9d..c472379e34 100644
--- a/src/Pstream/mpi/PstreamGlobals.H
+++ b/src/Pstream/mpi/PstreamGlobals.H
@@ -58,10 +58,14 @@ extern int nTags_;
 //- Free'd message tags
 extern DynamicList<int> freedTags_;
 
-// Current communicators. First element will be MPI_COMM_WORLD
+// Current communicators, which may be allocated or predefined
+// (eg, MPI_COMM_SELF, MPI_COMM_WORLD)
 extern DynamicList<MPI_Comm> MPICommunicators_;
+
+// Groups associated with the currrent communicators.
 extern DynamicList<MPI_Group> MPIGroups_;
 
+//- Fatal if comm is outside the allocated range
 void checkCommunicator(const label comm, const label toProcNo);
 
 
diff --git a/src/Pstream/mpi/UPstream.C b/src/Pstream/mpi/UPstream.C
index 81fdff28ec..5ecd11b155 100644
--- a/src/Pstream/mpi/UPstream.C
+++ b/src/Pstream/mpi/UPstream.C
@@ -30,9 +30,9 @@ License
 #include "PstreamReduceOps.H"
 #include "PstreamGlobals.H"
 #include "profilingPstream.H"
+#include "int.H"
 #include "SubList.H"
 #include "UPstreamWrapping.H"
-#include "int.H"
 #include "collatedFileOperation.H"
 
 #include <mpi.h>
@@ -293,41 +293,56 @@ bool Foam::UPstream::init(int& argc, char**& argv, const bool needsThread)
 
     if (worldIndex != -1)
     {
+        // During startup, so worldComm == globalComm
+
         wordList worlds(numprocs);
-        worlds[Pstream::myProcNo()] = world;
-        Pstream::gatherList(worlds);
-        Pstream::broadcast(worlds);
+        worlds[Pstream::myProcNo(UPstream::globalComm)] = world;
+        Pstream::gatherList(worlds, UPstream::msgType(), UPstream::globalComm);
 
         // Compact
-        if (Pstream::master())
+        if (Pstream::master(UPstream::globalComm))
         {
-            DynamicList<word> allWorlds(numprocs);
-            for (const word& world : worlds)
-            {
-                allWorlds.appendUniq(world);
-            }
-            allWorlds_ = std::move(allWorlds);
+            DynamicList<word> worldNames(numprocs);
+            worldIDs_.resize_nocopy(numprocs);
 
-            worldIDs_.setSize(numprocs);
             forAll(worlds, proci)
             {
                 const word& world = worlds[proci];
-                worldIDs_[proci] = allWorlds_.find(world);
+
+                worldIDs_[proci] = worldNames.find(world);
+
+                if (worldIDs_[proci] == -1)
+                {
+                    worldIDs_[proci] = worldNames.size();
+                    worldNames.push_back(world);
+                }
             }
+
+            allWorlds_.transfer(worldNames);
         }
-        Pstream::broadcasts(UPstream::worldComm, allWorlds_, worldIDs_);
+        Pstream::broadcasts(UPstream::globalComm, allWorlds_, worldIDs_);
+
+        const label myWorldId =
+            worldIDs_[Pstream::myProcNo(UPstream::globalComm)];
 
         DynamicList<label> subRanks;
-        forAll(worlds, proci)
+        forAll(worldIDs_, proci)
         {
-            if (worlds[proci] == worlds[Pstream::myProcNo()])
+            if (worldIDs_[proci] == myWorldId)
             {
-                subRanks.append(proci);
+                subRanks.push_back(proci);
             }
         }
 
-        // Allocate new communicator 1 with parent 0 (= mpi_world)
-        const label subComm = allocateCommunicator(0, subRanks, true);
+        // Allocate new communicator with globalComm as its parent
+        const label subComm =
+            UPstream::allocateCommunicator
+            (
+                UPstream::globalComm,  // parent
+                subRanks,
+                true
+            );
+
 
         // Override worldComm
         UPstream::worldComm = subComm;
@@ -337,11 +352,11 @@ bool Foam::UPstream::init(int& argc, char**& argv, const bool needsThread)
         if (debug)
         {
             // Check
-            int subNProcs, subRank;
+            int subNumProcs, subRank;
             MPI_Comm_size
             (
                 PstreamGlobals::MPICommunicators_[subComm],
-                &subNProcs
+                &subNumProcs
             );
             MPI_Comm_rank
             (
@@ -351,19 +366,20 @@ bool Foam::UPstream::init(int& argc, char**& argv, const bool needsThread)
 
             Pout<< "UPstream::init : in world:" << world
                 << " using local communicator:" << subComm
-                << " with procs:" << subNProcs
-                << " and rank:" << subRank
+                << " rank " << subRank
+                << " of " << subNumProcs
                 << endl;
         }
 
         // Override Pout prefix (move to setParRun?)
         Pout.prefix() = '[' + world + '/' +  name(myProcNo(subComm)) + "] ";
-        Perr.prefix() = '[' + world + '/' +  name(myProcNo(subComm)) + "] ";
+        Perr.prefix() = Pout.prefix();
     }
     else
     {
         // All processors use world 0
-        worldIDs_.setSize(numprocs, 0);
+        worldIDs_.resize_nocopy(numprocs);
+        worldIDs_ = 0;
     }
 
     attachOurBuffers();
@@ -490,10 +506,10 @@ void Foam::UPstream::allocatePstreamCommunicator
     if (index == PstreamGlobals::MPIGroups_.size())
     {
         // Extend storage with dummy values
-        MPI_Group newGroup = MPI_GROUP_NULL;
-        PstreamGlobals::MPIGroups_.append(newGroup);
         MPI_Comm newComm = MPI_COMM_NULL;
-        PstreamGlobals::MPICommunicators_.append(newComm);
+        MPI_Group newGroup = MPI_GROUP_NULL;
+        PstreamGlobals::MPIGroups_.push_back(newGroup);
+        PstreamGlobals::MPICommunicators_.push_back(newComm);
     }
     else if (index > PstreamGlobals::MPIGroups_.size())
     {
@@ -505,34 +521,57 @@ void Foam::UPstream::allocatePstreamCommunicator
 
     if (parentIndex == -1)
     {
-        // Allocate world communicator
+        // Global communicator. Same as world communicator for single-world
 
-        if (index != UPstream::worldComm)
+        if (index != UPstream::globalComm)
         {
             FatalErrorInFunction
                 << "world communicator should always be index "
-                << UPstream::worldComm << Foam::exit(FatalError);
+                << UPstream::globalComm
+                << 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]
-        );
+        MPI_Comm_rank(MPI_COMM_WORLD, &myProcNo_[index]);
 
-        // Set the number of processes to the actual number
+        // Set the number of ranks to the actual number
         int numProcs;
-        MPI_Comm_size(PstreamGlobals::MPICommunicators_[index], &numProcs);
+        MPI_Comm_size(MPI_COMM_WORLD, &numProcs);
 
         //procIDs_[index] = identity(numProcs);
-        procIDs_[index].setSize(numProcs);
+        procIDs_[index].resize_nocopy(numProcs);
         forAll(procIDs_[index], i)
         {
             procIDs_[index][i] = i;
         }
     }
+    else if (parentIndex == -2)
+    {
+        // Self communicator
+
+        PstreamGlobals::MPICommunicators_[index] = MPI_COMM_SELF;
+        MPI_Comm_group(MPI_COMM_SELF, &PstreamGlobals::MPIGroups_[index]);
+        MPI_Comm_rank(MPI_COMM_SELF, &myProcNo_[index]);
+
+        // Number of ranks is always 1 (self communicator)
+
+        #ifdef FULLDEBUG
+        int numProcs;
+        MPI_Comm_size(MPI_COMM_SELF, &numProcs);
+
+        if (numProcs != 1)
+        {
+            // Already finalized - this is an error
+            FatalErrorInFunction
+                << "MPI_COMM_SELF had " << numProcs << " != 1 ranks!\n"
+                << Foam::abort(FatalError);
+        }
+        #endif
+
+        procIDs_[index].resize_nocopy(1);
+        procIDs_[index] = 0;
+    }
     else
     {
         // Create new group
@@ -558,7 +597,7 @@ void Foam::UPstream::allocatePstreamCommunicator
         (
             PstreamGlobals::MPICommunicators_[parentIndex],
             PstreamGlobals::MPIGroups_[index],
-            Pstream::msgType(),
+            UPstream::msgType(),
            &PstreamGlobals::MPICommunicators_[index]
         );
         #endif
@@ -593,16 +632,26 @@ void Foam::UPstream::allocatePstreamCommunicator
 
 void Foam::UPstream::freePstreamCommunicator(const label communicator)
 {
-    if (communicator != 0)
+    // Skip placeholders and pre-defined (not allocated) communicators
+
+    if (UPstream::debug)
     {
-        if (PstreamGlobals::MPICommunicators_[communicator] != MPI_COMM_NULL)
+        Pout<< "freePstreamCommunicator: " << communicator
+            << " from " << PstreamGlobals::MPICommunicators_.size() << endl;
+    }
+
+    // Not touching the first two communicators (SELF, WORLD)
+    if (communicator > 1)
+    {
+        if (MPI_COMM_NULL != PstreamGlobals::MPICommunicators_[communicator])
         {
             // Free communicator. Sets communicator to MPI_COMM_NULL
             MPI_Comm_free(&PstreamGlobals::MPICommunicators_[communicator]);
         }
-        if (PstreamGlobals::MPIGroups_[communicator] != MPI_GROUP_NULL)
+
+        if (MPI_GROUP_NULL != PstreamGlobals::MPIGroups_[communicator])
         {
-            // Free greoup. Sets group to MPI_GROUP_NULL
+            // Free group. Sets group to MPI_GROUP_NULL
             MPI_Group_free(&PstreamGlobals::MPIGroups_[communicator]);
         }
     }
@@ -715,7 +764,7 @@ void Foam::UPstream::waitRequest(const label i)
 
     profilingPstream::addWaitTime();
     // Push index onto free cache
-    PstreamGlobals::freedRequests_.append(i);
+    PstreamGlobals::freedRequests_.push_back(i);
 
     if (debug)
     {
@@ -766,69 +815,39 @@ bool Foam::UPstream::finishedRequest(const label i)
 }
 
 
-int Foam::UPstream::allocateTag(const char* s)
+int Foam::UPstream::allocateTag(const char* const msg)
 {
     int tag;
     if (PstreamGlobals::freedTags_.size())
     {
-        tag = PstreamGlobals::freedTags_.remove();
+        tag = PstreamGlobals::freedTags_.back();
+        (void)PstreamGlobals::freedTags_.pop_back();
     }
     else
     {
-        tag = PstreamGlobals::nTags_++;
+        tag = ++PstreamGlobals::nTags_;
     }
 
     if (debug)
     {
-        Pout<< "UPstream::allocateTag "
-            << s << " : tag:" << tag << endl;
+        Pout<< "UPstream::allocateTag";
+        if (msg) Pout<< ' ' << msg;
+        Pout<< " : tag:" << tag << endl;
     }
 
     return tag;
 }
 
 
-int Foam::UPstream::allocateTag(const std::string& s)
-{
-    int tag;
-    if (PstreamGlobals::freedTags_.size())
-    {
-        tag = PstreamGlobals::freedTags_.remove();
-    }
-    else
-    {
-        tag = PstreamGlobals::nTags_++;
-    }
-
-    if (debug)
-    {
-        Pout<< "UPstream::allocateTag "
-            << s.c_str() << " : tag:" << tag << endl;
-    }
-
-    return tag;
-}
-
-
-void Foam::UPstream::freeTag(const char* s, const int tag)
+void Foam::UPstream::freeTag(const int tag, const char* const msg)
 {
     if (debug)
     {
-        Pout<< "UPstream::freeTag "
-            << s << " tag:" << tag << endl;
+        Pout<< "UPstream::freeTag ";
+        if (msg) Pout<< ' ' << msg;
+        Pout<< " : tag:" << tag << endl;
     }
-    PstreamGlobals::freedTags_.append(tag);
-}
-
-
-void Foam::UPstream::freeTag(const std::string& s, const int tag)
-{
-    if (debug)
-    {
-        Pout<< "UPstream::freeTag "
-            << s.c_str() << " tag:" << tag << endl;
-    }
-    PstreamGlobals::freedTags_.append(tag);
+    PstreamGlobals::freedTags_.push_back(tag);
 }
 
 
diff --git a/src/meshTools/multiWorld/multiWorldConnectionsObject.C b/src/meshTools/multiWorld/multiWorldConnectionsObject.C
index 21efa3169e..3f0bcf293c 100644
--- a/src/meshTools/multiWorld/multiWorldConnectionsObject.C
+++ b/src/meshTools/multiWorld/multiWorldConnectionsObject.C
@@ -161,8 +161,8 @@ Foam::label Foam::multiWorldConnections::createCommunicator(const edge& worlds)
         }
     }
 
-    // Allocate new communicator with parent 0 (= world)
-    comm = UPstream::allocateCommunicator(0, subRanks, true);
+    // Allocate new communicator with global world
+    comm = UPstream::allocateCommunicator(UPstream::globalComm, subRanks, true);
 
     if (debug & 2)
     {
@@ -234,10 +234,10 @@ void Foam::multiWorldConnections::createComms()
 
 
     // Use MPI_COMM_WORLD
-    const label oldWorldComm(Pstream::worldComm);
-    const label oldWarnComm(Pstream::warnComm);
-    Pstream::worldComm = 0;
-    Pstream::warnComm = Pstream::worldComm;
+    const label oldWorldComm(UPstream::worldComm);
+    const label oldWarnComm(UPstream::warnComm);
+    UPstream::worldComm = UPstream::globalComm;
+    UPstream::warnComm = UPstream::worldComm;
 
     if (Pstream::parRun())
     {