From 5eebc758452b733699272f75b9a7e8b0dec2a12e Mon Sep 17 00:00:00 2001
From: Mark Olesen <Mark.Olesen@esi-group.com>
Date: Wed, 17 May 2023 14:26:05 +0200
Subject: [PATCH] ENH: improved UPstream gather/scatter functionality

- added UPstream::allGatherValues() with a direct call to MPI_Allgather.
  This enables possible benefit from a variety of internal algorithms
  and simplifies the caller

    Old:
        labelList nPerProc
        (
            UPstream::listGatherValues<label>(patch_.size(), myComm)
        );
        Pstream::broadcast(nPerProc, myComm);

    New:

        const labelList nPerProc
        (
            UPstream::allGatherValues<label>(patch_.size(), myComm)
        );

- Pstream::allGatherList uses MPI_Allgather for contiguous values
  instead of the hand-rolled tree walking involved with
  gatherList/scatterList.

-
- simplified the calling parameters for mpiGather/mpiScatter.

  Since send/recv data types are identical, the send/recv count
  is also always identical. Eliminates the possibility of any
  discrepancies.

  Since this is a low-level call, it does not affect much code.
  Currently just Foam::profilingPstream and a UPstream internal.

BUG: call to MPI_Allgather had hard-coded MPI_BYTE (not the data type)

- a latent bug since it is currently only passed char data anyhow
---
 .../test/parallel-comm2/Test-parallel-comm2.C |  3 +-
 .../surfaceRedistributePar.C                  |  8 +-
 .../db/IOstreams/Pstreams/PstreamGatherList.C | 27 +++---
 src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C |  3 +-
 src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H | 19 +++-
 .../db/IOstreams/Pstreams/UPstreamTemplates.C | 61 +++++++++----
 .../global/profiling/profilingPstream.C       |  6 +-
 .../parallel/globalIndex/globalIndex.C        | 10 +--
 .../parallel/globalIndex/globalIndex.H        |  1 -
 .../parallel/globalIndex/globalIndexI.H       |  5 +-
 src/Pstream/dummy/UPstreamGatherScatter.C     | 12 +--
 src/Pstream/mpi/UPstreamGatherScatter.C       | 12 +--
 src/Pstream/mpi/UPstreamWrapping.H            | 18 ++--
 src/Pstream/mpi/UPstreamWrappingTemplates.C   | 90 ++++++++-----------
 .../mappedPolyPatch/mappedPatchBase.C         | 10 +--
 15 files changed, 140 insertions(+), 145 deletions(-)

diff --git a/applications/test/parallel-comm2/Test-parallel-comm2.C b/applications/test/parallel-comm2/Test-parallel-comm2.C
index 7ccd11cf29..d38f35c1b3 100644
--- a/applications/test/parallel-comm2/Test-parallel-comm2.C
+++ b/applications/test/parallel-comm2/Test-parallel-comm2.C
@@ -266,9 +266,8 @@ int main(int argc, char *argv[])
             UPstream::mpiGather
             (
                 myDigest.cdata_bytes(),     // Send
-                SHA1Digest::max_size(),     // Num send per proc
                 digests.data_bytes(),       // Recv
-                SHA1Digest::max_size(),     // Num recv per proc
+                SHA1Digest::max_size(),     // Num send/recv per rank
                 UPstream::commGlobal()
             );
         }
diff --git a/applications/utilities/surface/surfaceRedistributePar/surfaceRedistributePar.C b/applications/utilities/surface/surfaceRedistributePar/surfaceRedistributePar.C
index 80f3bf8b18..83f7fb6644 100644
--- a/applications/utilities/surface/surfaceRedistributePar/surfaceRedistributePar.C
+++ b/applications/utilities/surface/surfaceRedistributePar/surfaceRedistributePar.C
@@ -62,10 +62,10 @@ void writeProcStats
 )
 {
     // Determine surface bounding boxes, faces, points
-    List<treeBoundBox> surfBb(Pstream::nProcs());
-    surfBb[Pstream::myProcNo()] = treeBoundBox(s.points());
-    Pstream::gatherList(surfBb);
-
+    List<treeBoundBox> surfBb
+    (
+        UPstream::listGatherValues<treeBoundBox>(treeBoundBox(s.points()))
+    );
     labelList nPoints(UPstream::listGatherValues<label>(s.points().size()));
     labelList nFaces(UPstream::listGatherValues<label>(s.size()));
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C
index 44d94933d6..e4dd9f8440 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C
@@ -350,20 +350,19 @@ void Foam::Pstream::allGatherList
 {
     if (UPstream::is_parallel(comm))
     {
-        // TBD
-        // if (std::is_arithmetic<T>::value)   // or is_contiguous ?
-        // {
-        //     if (values.size() != UPstream::nProcs(comm))
-        //     {
-        //         FatalErrorInFunction
-        //             << "Size of list:" << values.size()
-        //             << " != number of processors:"
-        //             << UPstream::nProcs(comm)
-        //             << Foam::abort(FatalError);
-        //     }
-        //     UPstream::mpiAllGather(values.data_bytes(), sizeof(T), comm);
-        //     return;
-        // }
+        if (is_contiguous<T>::value)
+        {
+            if (values.size() < UPstream::nProcs(comm))
+            {
+                FatalErrorInFunction
+                    << "List of values is too small:" << values.size()
+                    << " vs numProcs:" << UPstream::nProcs(comm) << nl
+                    << Foam::abort(FatalError);
+            }
+
+            UPstream::mpiAllGather(values.data_bytes(), sizeof(T), comm);
+            return;
+        }
 
         const auto& comms = UPstream::whichCommunication(comm);
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
index 331cf4da77..4700064f39 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
@@ -91,9 +91,8 @@ static List<int> getHostGroupIds(const label parentCommunicator)
     UPstream::mpiGather
     (
         myDigest.cdata_bytes(),     // Send
-        SHA1Digest::max_size(),     // Num send per proc
         digests.data_bytes(),       // Recv
-        SHA1Digest::max_size(),     // Num recv per proc
+        SHA1Digest::max_size(),     // Num send/recv data per rank
         parentCommunicator
     );
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
index 9e22089d9d..d164c7ad1a 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
@@ -1011,10 +1011,10 @@ public:
         (                                                                     \
             /*! On rank: individual value to send */                          \
             const Native* sendData,                                           \
-            int sendCount,                                                    \
             /*! On master: receive buffer with all values */                  \
             Native* recvData,                                                 \
-            int recvCount,                                                    \
+            /*! Number of send/recv data per rank. Globally consistent! */    \
+            int count,                                                        \
             const label communicator = worldComm                              \
         );                                                                    \
                                                                               \
@@ -1023,10 +1023,10 @@ public:
         (                                                                     \
             /*! On master: send buffer with all values */                     \
             const Native* sendData,                                           \
-            int sendCount,                                                    \
             /*! On rank: individual value to receive */                       \
             Native* recvData,                                                 \
-            int recvCount,                                                    \
+            /*! Number of send/recv data per rank. Globally consistent! */    \
+            int count,                                                        \
             const label communicator = worldComm                              \
         );                                                                    \
                                                                               \
@@ -1034,7 +1034,9 @@ public:
         /*! Send data from proc slot, receive into all slots */               \
         static void mpiAllGather                                              \
         (                                                                     \
+            /*! On all ranks: the base of the data locations */               \
             Native* allData,                                                  \
+            /*! Number of send/recv data per rank. Globally consistent! */    \
             int count,                                                        \
             const label communicator = worldComm                              \
         );
@@ -1080,6 +1082,15 @@ public:
 
     // Gather single, contiguous value(s)
 
+        //- Allgather individual values into list locations.
+        //  The returned list has size nProcs, identical on all ranks.
+        template<class T>
+        static List<T> allGatherValues
+        (
+            const T& localValue,
+            const label communicator = worldComm
+        );
+
         //- Gather individual values into list locations.
         //  On master list length == nProcs, otherwise zero length.
         //  \n
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamTemplates.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamTemplates.C
index a6b99f6ce9..10065bff3f 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamTemplates.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamTemplates.C
@@ -27,6 +27,42 @@ License
 
 // * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
 
+template<class T>
+Foam::List<T> Foam::UPstream::allGatherValues
+(
+    const T& localValue,
+    const label comm
+)
+{
+    if (!is_contiguous<T>::value)
+    {
+        FatalErrorInFunction
+            << "Cannot all-gather values for non-contiguous types" << endl
+            << Foam::abort(FatalError);
+    }
+
+
+    List<T> allValues;
+
+    if (UPstream::is_parallel(comm))
+    {
+        allValues.resize(UPstream::nProcs(comm));
+        allValues[UPstream::myProcNo(comm)] = localValue;
+
+        UPstream::mpiAllGather(allValues.data_bytes(), sizeof(T), comm);
+    }
+    else
+    {
+        // non-parallel: return own value
+        // TBD: only when UPstream::is_rank(comm) as well?
+        allValues.resize(1);
+        allValues[0] = localValue;
+    }
+
+    return allValues;
+}
+
+
 template<class T>
 Foam::List<T> Foam::UPstream::listGatherValues
 (
@@ -44,30 +80,25 @@ Foam::List<T> Foam::UPstream::listGatherValues
 
     List<T> allValues;
 
-    const label nproc =
-    (
-        UPstream::is_parallel(comm) ? UPstream::nProcs(comm) : 1
-    );
-
-    if (nproc > 1)
+    if (UPstream::is_parallel(comm))
     {
         if (UPstream::master(comm))
         {
-            allValues.resize(nproc);
+            allValues.resize(UPstream::nProcs(comm));
         }
 
         UPstream::mpiGather
         (
             reinterpret_cast<const char*>(&localValue),
-            sizeof(T),
             allValues.data_bytes(),
-            sizeof(T),
+            sizeof(T),  // The send/recv size per rank
             comm
         );
     }
     else
     {
         // non-parallel: return own value
+        // TBD: only when UPstream::is_rank(comm) as well?
         allValues.resize(1);
         allValues[0] = localValue;
     }
@@ -91,15 +122,12 @@ T Foam::UPstream::listScatterValues
     }
 
 
-    const label nproc =
-    (
-        UPstream::is_parallel(comm) ? UPstream::nProcs(comm) : 1
-    );
-
     T localValue;
 
-    if (nproc > 1)
+    if (UPstream::is_parallel(comm))
     {
+        const label nproc = UPstream::nProcs(comm);
+
         if (UPstream::master(comm) && allValues.size() < nproc)
         {
             FatalErrorInFunction
@@ -111,9 +139,8 @@ T Foam::UPstream::listScatterValues
         UPstream::mpiScatter
         (
             allValues.cdata_bytes(),
-            sizeof(T),
             reinterpret_cast<char*>(&localValue),
-            sizeof(T),
+            sizeof(T),  // The send/recv size per rank
             comm
         );
     }
diff --git a/src/OpenFOAM/global/profiling/profilingPstream.C b/src/OpenFOAM/global/profiling/profilingPstream.C
index cb6de74904..e7ad9050e0 100644
--- a/src/OpenFOAM/global/profiling/profilingPstream.C
+++ b/src/OpenFOAM/global/profiling/profilingPstream.C
@@ -229,9 +229,8 @@ void Foam::profilingPstream::report(const int reportLevel)
         UPstream::mpiGather
         (
             procValues.cdata_bytes(),   // Send
-            procValues.size_bytes(),    // Num send per proc
             allTimes.data_bytes(),      // Recv
-            procValues.size_bytes(),    // Num recv per proc
+            procValues.size_bytes(),    // Num send/recv data per rank
             UPstream::commWorld()
         );
     }
@@ -249,9 +248,8 @@ void Foam::profilingPstream::report(const int reportLevel)
         UPstream::mpiGather
         (
             procValues.cdata_bytes(),   // Send
-            procValues.size_bytes(),    // Num send per proc
             allCounts.data_bytes(),     // Recv
-            procValues.size_bytes(),    // Num recv per proc
+            procValues.size_bytes(),    // Num send/recv data per rank
             UPstream::commWorld()
         );
     }
diff --git a/src/OpenFOAM/parallel/globalIndex/globalIndex.C b/src/OpenFOAM/parallel/globalIndex/globalIndex.C
index c74a5b356a..924ca62b74 100644
--- a/src/OpenFOAM/parallel/globalIndex/globalIndex.C
+++ b/src/OpenFOAM/parallel/globalIndex/globalIndex.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2018-2022 OpenCFD Ltd.
+    Copyright (C) 2018-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -27,7 +27,6 @@ License
 \*---------------------------------------------------------------------------*/
 
 #include "globalIndex.H"
-#include "labelRange.H"
 
 // * * * * * * * * * * * * * Static Member Functions * * * * * * * * * * * * //
 
@@ -200,14 +199,13 @@ void Foam::globalIndex::reset
 {
     labelList localLens;
 
-    const label len = Pstream::nProcs(comm);
+    const label len = UPstream::nProcs(comm);
 
     if (len)
     {
-        if (parallel && UPstream::parRun())
+        if (parallel && UPstream::parRun())  // or UPstream::is_parallel()
         {
-            localLens = UPstream::listGatherValues(localSize, comm);
-            Pstream::broadcast(localLens, comm);
+            localLens = UPstream::allGatherValues(localSize, comm);
         }
         else
         {
diff --git a/src/OpenFOAM/parallel/globalIndex/globalIndex.H b/src/OpenFOAM/parallel/globalIndex/globalIndex.H
index 49934718d4..372b207ce4 100644
--- a/src/OpenFOAM/parallel/globalIndex/globalIndex.H
+++ b/src/OpenFOAM/parallel/globalIndex/globalIndex.H
@@ -55,7 +55,6 @@ namespace Foam
 
 // Forward Declarations
 class globalIndex;
-class labelRange;
 
 Istream& operator>>(Istream& is, globalIndex& gi);
 Ostream& operator<<(Ostream& os, const globalIndex& gi);
diff --git a/src/OpenFOAM/parallel/globalIndex/globalIndexI.H b/src/OpenFOAM/parallel/globalIndex/globalIndexI.H
index edf45f5cb8..72ed4a401c 100644
--- a/src/OpenFOAM/parallel/globalIndex/globalIndexI.H
+++ b/src/OpenFOAM/parallel/globalIndex/globalIndexI.H
@@ -27,7 +27,6 @@ License
 \*---------------------------------------------------------------------------*/
 
 #include "ListOps.H"
-#include "labelRange.H"
 
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
@@ -121,7 +120,7 @@ inline Foam::globalIndex::globalIndex
     const label comm
 )
 {
-    // one-sided: non-master only
+    // one-sided: non-master sizes only
     reset
     (
         UPstream::listGatherValues
@@ -401,7 +400,7 @@ inline void Foam::globalIndex::reset
     const label comm
 )
 {
-    // one-sided: non-master only
+    // one-sided: non-master sizes only
     reset
     (
         UPstream::listGatherValues
diff --git a/src/Pstream/dummy/UPstreamGatherScatter.C b/src/Pstream/dummy/UPstreamGatherScatter.C
index 7d2e2d63f9..2d1f125f30 100644
--- a/src/Pstream/dummy/UPstreamGatherScatter.C
+++ b/src/Pstream/dummy/UPstreamGatherScatter.C
@@ -36,28 +36,24 @@ License
 void Foam::UPstream::mpiGather                                                \
 (                                                                             \
     const Native* sendData,                                                   \
-    int sendCount,                                                            \
-                                                                              \
     Native* recvData,                                                         \
-    int recvCount,                                                            \
+    int count,                                                                \
     const label comm                                                          \
 )                                                                             \
 {                                                                             \
-    std::memmove(recvData, sendData, recvCount*sizeof(Native));               \
+    std::memmove(recvData, sendData, count*sizeof(Native));                   \
 }                                                                             \
                                                                               \
                                                                               \
 void Foam::UPstream::mpiScatter                                               \
 (                                                                             \
     const Native* sendData,                                                   \
-    int sendCount,                                                            \
-                                                                              \
     Native* recvData,                                                         \
-    int recvCount,                                                            \
+    int count,                                                                \
     const label comm                                                          \
 )                                                                             \
 {                                                                             \
-    std::memmove(recvData, sendData, recvCount*sizeof(Native));               \
+    std::memmove(recvData, sendData, count*sizeof(Native));                   \
 }                                                                             \
                                                                               \
                                                                               \
diff --git a/src/Pstream/mpi/UPstreamGatherScatter.C b/src/Pstream/mpi/UPstreamGatherScatter.C
index 180a98b9a3..4ed86d685a 100644
--- a/src/Pstream/mpi/UPstreamGatherScatter.C
+++ b/src/Pstream/mpi/UPstreamGatherScatter.C
@@ -38,16 +38,14 @@ License
 void Foam::UPstream::mpiGather                                                \
 (                                                                             \
     const Native* sendData,                                                   \
-    int sendCount,                                                            \
-                                                                              \
     Native* recvData,                                                         \
-    int recvCount,                                                            \
+    int count,                                                                \
     const label comm                                                          \
 )                                                                             \
 {                                                                             \
     PstreamDetail::gather                                                     \
     (                                                                         \
-        sendData, sendCount, recvData, recvCount,                             \
+        sendData, recvData, count,                                            \
         TaggedType, comm                                                      \
     );                                                                        \
 }                                                                             \
@@ -56,16 +54,14 @@ void Foam::UPstream::mpiGather                                                \
 void Foam::UPstream::mpiScatter                                               \
 (                                                                             \
     const Native* sendData,                                                   \
-    int sendCount,                                                            \
-                                                                              \
     Native* recvData,                                                         \
-    int recvCount,                                                            \
+    int count,                                                                \
     const label comm                                                          \
 )                                                                             \
 {                                                                             \
     PstreamDetail::scatter                                                    \
     (                                                                         \
-        sendData, sendCount, recvData, recvCount,                             \
+        sendData, recvData, count,                                            \
         TaggedType, comm                                                      \
     );                                                                        \
 }                                                                             \
diff --git a/src/Pstream/mpi/UPstreamWrapping.H b/src/Pstream/mpi/UPstreamWrapping.H
index 92791503e0..984a4f857e 100644
--- a/src/Pstream/mpi/UPstreamWrapping.H
+++ b/src/Pstream/mpi/UPstreamWrapping.H
@@ -164,12 +164,9 @@ void allToAllConsensus
 template<class Type>
 void gather
 (
-    const Type* sendData,
-    int sendCount,
-
-    Type* recvData,                     // Ignored on non-root rank
-    int recvCount,                      // Ignored on non-root rank
-
+    const Type* sendData,   // Local send value
+    Type* recvData,         // On master: recv buffer. Ignored elsewhere
+    int count,              // Per rank send/recv count. Globally consistent!
     MPI_Datatype datatype,              // The send/recv data type
     const label comm,                   // Communicator
     UPstream::Request* req = nullptr,   // Non-null for non-blocking
@@ -181,12 +178,9 @@ void gather
 template<class Type>
 void scatter
 (
-    const Type* sendData,               // Ignored on non-root rank
-    int sendCount,                      // Ignored on non-root rank
-
-    Type* recvData,
-    int recvCount,
-
+    const Type* sendData,   // On master: send buffer. Ignored elsewhere
+    Type* recvData,         // Local recv value
+    int count,              // Per rank send/recv count. Globally consistent!
     MPI_Datatype datatype,              // The send/recv data type
     const label comm,                   // Communicator
     UPstream::Request* req = nullptr,   // Non-null for non-blocking
diff --git a/src/Pstream/mpi/UPstreamWrappingTemplates.C b/src/Pstream/mpi/UPstreamWrappingTemplates.C
index dd22fa4c47..2e6eb8eb0a 100644
--- a/src/Pstream/mpi/UPstreamWrappingTemplates.C
+++ b/src/Pstream/mpi/UPstreamWrappingTemplates.C
@@ -924,14 +924,12 @@ template<class Type>
 void Foam::PstreamDetail::gather
 (
     const Type* sendData,
-    int sendCount,
-
     Type* recvData,
-    int recvCount,
 
+    int count,
     MPI_Datatype datatype,
-    const label comm,
 
+    const label comm,
     UPstream::Request* req,
     label* requestID
 )
@@ -940,13 +938,16 @@ void Foam::PstreamDetail::gather
 
     const bool immediate = (req || requestID);
 
-    if (!UPstream::is_rank(comm))
+    if (!UPstream::is_rank(comm) || !count)
     {
         return;
     }
     if (!UPstream::is_parallel(comm))
     {
-        std::memmove(recvData, sendData, recvCount*sizeof(Type));
+        if (recvData)
+        {
+            std::memmove(recvData, sendData, count*sizeof(Type));
+        }
         return;
     }
 
@@ -963,7 +964,7 @@ void Foam::PstreamDetail::gather
             Pout<< "** MPI_Gather (blocking):";
         }
         Pout<< " numProc:" << numProc
-            << " recvCount:" << recvCount
+            << " count:" << count
             << " with comm:" << comm
             << " warnComm:" << UPstream::warnComm
             << endl;
@@ -982,13 +983,9 @@ void Foam::PstreamDetail::gather
         (
             MPI_Igather
             (
-                const_cast<Type*>(sendData),
-                sendCount,
-                datatype,
-                recvData,
-                recvCount,
-                datatype,
-                0,  // (root rank) == UPstream::masterNo()
+                const_cast<Type*>(sendData), count, datatype,
+                recvData, count, datatype,
+                0,  // root: UPstream::masterNo()
                 PstreamGlobals::MPICommunicators_[comm],
                &request
             )
@@ -996,8 +993,7 @@ void Foam::PstreamDetail::gather
         {
             FatalErrorInFunction
                 << "MPI_Igather [comm: " << comm << "] failed."
-                << " sendCount " << sendCount
-                << " recvCount " << recvCount
+                << " count:" << count << nl
                 << Foam::abort(FatalError);
         }
 
@@ -1013,21 +1009,16 @@ void Foam::PstreamDetail::gather
         (
             MPI_Gather
             (
-                const_cast<Type*>(sendData),
-                sendCount,
-                datatype,
-                recvData,
-                recvCount,
-                datatype,
-                0,  // (root rank) == UPstream::masterNo()
+                const_cast<Type*>(sendData), count, datatype,
+                recvData, count, datatype,
+                0,  // root: UPstream::masterNo()
                 PstreamGlobals::MPICommunicators_[comm]
             )
         )
         {
             FatalErrorInFunction
                 << "MPI_Gather [comm: " << comm << "] failed."
-                << " sendCount " << sendCount
-                << " recvCount " << recvCount
+                << " count:" << count << nl
                 << Foam::abort(FatalError);
         }
 
@@ -1040,14 +1031,12 @@ template<class Type>
 void Foam::PstreamDetail::scatter
 (
     const Type* sendData,
-    int sendCount,
-
     Type* recvData,
-    int recvCount,
 
+    int count,
     MPI_Datatype datatype,
-    const label comm,
 
+    const label comm,
     UPstream::Request* req,
     label* requestID
 )
@@ -1056,13 +1045,16 @@ void Foam::PstreamDetail::scatter
 
     const bool immediate = (req || requestID);
 
-    if (!UPstream::is_rank(comm))
+    if (!UPstream::is_rank(comm) || !count)
     {
         return;
     }
     if (!UPstream::is_parallel(comm))
     {
-        std::memmove(recvData, sendData, recvCount*sizeof(Type));
+        if (recvData)
+        {
+            std::memmove(recvData, sendData, count*sizeof(Type));
+        }
         return;
     }
 
@@ -1079,7 +1071,7 @@ void Foam::PstreamDetail::scatter
             Pout<< "** MPI_Scatter (blocking):";
         }
         Pout<< " numProc:" << numProc
-            << " recvCount:" << recvCount
+            << " count:" << count
             << " with comm:" << comm
             << " warnComm:" << UPstream::warnComm
             << endl;
@@ -1098,13 +1090,9 @@ void Foam::PstreamDetail::scatter
         (
             MPI_Iscatter
             (
-                const_cast<Type*>(sendData),
-                sendCount,
-                datatype,
-                recvData,
-                recvCount,
-                datatype,
-                0,  // (root rank) == UPstream::masterNo()
+                const_cast<Type*>(sendData), count, datatype,
+                recvData, count, datatype,
+                0,  // root: UPstream::masterNo()
                 PstreamGlobals::MPICommunicators_[comm],
                &request
             )
@@ -1112,8 +1100,7 @@ void Foam::PstreamDetail::scatter
         {
             FatalErrorInFunction
                 << "MPI_Iscatter [comm: " << comm << "] failed."
-                << " sendCount " << sendCount
-                << " recvCount " << recvCount
+                << " count:" << count << nl
                 << Foam::abort(FatalError);
         }
 
@@ -1129,21 +1116,16 @@ void Foam::PstreamDetail::scatter
         (
             MPI_Scatter
             (
-                const_cast<Type*>(sendData),
-                sendCount,
-                datatype,
-                recvData,
-                recvCount,
-                datatype,
-                0,  // (root rank) == UPstream::masterNo()
+                const_cast<Type*>(sendData), count, datatype,
+                recvData, count, datatype,
+                0,  // root: UPstream::masterNo()
                 PstreamGlobals::MPICommunicators_[comm]
             )
         )
         {
             FatalErrorInFunction
                 << "MPI_Scatter [comm: " << comm << "] failed."
-                << " sendCount " << sendCount
-                << " recvCount " << recvCount
+                << " count:" << count << nl
                 << Foam::abort(FatalError);
         }
 
@@ -1483,8 +1465,8 @@ void Foam::PstreamDetail::allGather
         (
             MPI_Iallgather
             (
-                MPI_IN_PLACE, count, MPI_BYTE,
-                allData, count, MPI_BYTE,
+                MPI_IN_PLACE, count, datatype,
+                allData, count, datatype,
                 PstreamGlobals::MPICommunicators_[comm],
                &request
             )
@@ -1507,8 +1489,8 @@ void Foam::PstreamDetail::allGather
         (
             MPI_Allgather
             (
-                MPI_IN_PLACE, count, MPI_BYTE,
-                allData, count, MPI_BYTE,
+                MPI_IN_PLACE, count, datatype,
+                allData, count, datatype,
                 PstreamGlobals::MPICommunicators_[comm]
             )
         )
diff --git a/src/meshTools/mappedPatches/mappedPolyPatch/mappedPatchBase.C b/src/meshTools/mappedPatches/mappedPolyPatch/mappedPatchBase.C
index 92421c80f5..00acc4587a 100644
--- a/src/meshTools/mappedPatches/mappedPolyPatch/mappedPatchBase.C
+++ b/src/meshTools/mappedPatches/mappedPolyPatch/mappedPatchBase.C
@@ -240,17 +240,15 @@ void Foam::mappedPatchBase::collectSamples
     {
         labelList procToWorldIndex
         (
-            UPstream::listGatherValues<label>(mySampleWorld, myComm)
+            UPstream::allGatherValues<label>(mySampleWorld, myComm)
         );
         labelList nPerProc
         (
-            UPstream::listGatherValues<label>(patch_.size(), myComm)
+            UPstream::allGatherValues<label>(patch_.size(), myComm)
         );
 
-        Pstream::broadcasts(myComm, procToWorldIndex, nPerProc);
-
-        patchFaceWorlds.setSize(patchFaces.size());
-        patchFaceProcs.setSize(patchFaces.size());
+        patchFaceWorlds.resize(patchFaces.size());
+        patchFaceProcs.resize(patchFaces.size());
 
         label sampleI = 0;
         forAll(nPerProc, proci)