From 7006056eaea94394e6e75e7b2c7f4dfa2f63f0ba Mon Sep 17 00:00:00 2001
From: Mark Olesen <Mark.Olesen@esi-group.com>
Date: Thu, 29 Feb 2024 18:35:03 +0100
Subject: [PATCH] ENH: remove blocking communication for gather patterns

ENH: eliminate unnecessary duplicate communicator

- in globalMeshData previously had a comm_dup hack to avoid clashes
  with deltaCoeffs calculations. However, this was largely due to a
  manual implementation of reduce() that used point-to-point
  communication. This has since been updated to use an MPI_Allreduce
  and now an MPI_Allgather, neither of which need this hack.
---
 .../test/IndirectList/Test-IndirectList.C     |  19 +--
 .../test/parallel-nbx2/Test-parallel-nbx2.C   |   8 +-
 applications/test/parallel/Test-parallel.C    |  20 ++-
 .../test/router/{Gather => }/Gather.H         |  29 ++--
 .../test/router/{Gather => }/Gather.txx       |  79 ++++-------
 applications/test/router/Gather/GatherBase.H  | 102 --------------
 .../test/router/Gather/GatherBase.txx         | 115 ----------------
 applications/test/treeComms/Test-treeComms.C  |  12 +-
 .../manipulation/createPatch/createPatch.C    |  13 +-
 .../splitMeshRegions/splitMeshRegions.C       |   8 +-
 .../Lists/SortableList/ParSortableList.C      |   8 +-
 src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H |   2 +-
 src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H |  25 +---
 .../db/IOstreams/Pstreams/OPstreams.C         |  11 +-
 .../Pstreams/PstreamExchangeConsensus.C       |   4 +-
 .../db/IOstreams/Pstreams/UOPstream.H         |  12 +-
 .../polyMesh/globalMeshData/globalMeshData.C  |  49 ++-----
 .../mapDistributeBaseTemplates.C              |   6 +-
 src/OpenFOAM/meshes/polyMesh/polyMesh.H       |   1 +
 .../polyMesh/syncTools/syncToolsTemplates.C   |   6 +-
 .../vtk/output/foamVtkPatchWriterTemplates.C  |  22 +--
 .../fvMeshDistribute/fvMeshDistribute.C       |   1 -
 .../field/externalCoupled/externalCoupled.C   |  65 +++++----
 .../field/externalCoupled/externalCoupled.H   |  26 ++--
 .../externalCoupledTemplates.C                | 130 ++++++------------
 .../lagrangian/dataCloud/dataCloudTemplates.C |  24 ++--
 .../meshRefinement/meshRefinementTemplates.C  |  19 +--
 .../output/vtk/patch/foamVtkPatchMeshWriter.C |  40 +++---
 .../surface/isoSurface/isoSurfacePoint.C      |  11 +-
 29 files changed, 272 insertions(+), 595 deletions(-)
 rename applications/test/router/{Gather => }/Gather.H (82%)
 rename applications/test/router/{Gather => }/Gather.txx (54%)
 delete mode 100644 applications/test/router/Gather/GatherBase.H
 delete mode 100644 applications/test/router/Gather/GatherBase.txx

diff --git a/applications/test/IndirectList/Test-IndirectList.C b/applications/test/IndirectList/Test-IndirectList.C
index dae630976a..bf174fc570 100644
--- a/applications/test/IndirectList/Test-IndirectList.C
+++ b/applications/test/IndirectList/Test-IndirectList.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011 OpenFOAM Foundation
-    Copyright (C) 2017-2022 OpenCFD Ltd.
+    Copyright (C) 2017-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -156,29 +156,24 @@ int main(int argc, char *argv[])
         os.writeEntry("idl3", idl3);
     }
 
-    if (Pstream::parRun())
+    if (UPstream::parRun())
     {
-        if (Pstream::master())
+        if (UPstream::master())
         {
             Pout<< "full: " << flatOutput(idl3.values()) << nl
                 << "send: " << flatOutput(idl3) << endl;
 
-            for (const int proci : Pstream::subProcs())
+            for (const int proci : UPstream::subProcs())
             {
-                OPstream toSlave(Pstream::commsTypes::scheduled, proci);
-                toSlave << idl3;
+                OPstream::send(idl3, proci);
             }
         }
         else
         {
             // From master
-            IPstream fromMaster
-            (
-                Pstream::commsTypes::scheduled,
-                Pstream::masterNo()
-            );
+            List<label> recv;
 
-            List<label> recv(fromMaster);
+            IPstream::recv(recv, UPstream::masterNo());
 
             Pout<<"recv: " << flatOutput(recv) << endl;
         }
diff --git a/applications/test/parallel-nbx2/Test-parallel-nbx2.C b/applications/test/parallel-nbx2/Test-parallel-nbx2.C
index 27bb661e0d..bd3a4a224e 100644
--- a/applications/test/parallel-nbx2/Test-parallel-nbx2.C
+++ b/applications/test/parallel-nbx2/Test-parallel-nbx2.C
@@ -139,7 +139,7 @@ int main(int argc, char *argv[])
             {
                 recvBufs(proci).resize_nocopy(count);
 
-                // Non-blocking read
+                // Non-blocking read - MPI_Irecv()
                 UIPstream::read
                 (
                     recvRequests.emplace_back(),
@@ -155,9 +155,9 @@ int main(int argc, char *argv[])
             {
                 IPstream is
                 (
-                    UPstream::commsTypes::scheduled,
-                    probed.first,
-                    probed.second,
+                    UPstream::commsTypes::scheduled,  // ie, MPI_Recv()
+                    proci,
+                    count,  // bufSize
                     tag,
                     comm
                 );
diff --git a/applications/test/parallel/Test-parallel.C b/applications/test/parallel/Test-parallel.C
index 7479a49922..06d3ea16e9 100644
--- a/applications/test/parallel/Test-parallel.C
+++ b/applications/test/parallel/Test-parallel.C
@@ -131,13 +131,17 @@ void testTransfer(const T& input)
         for (const int proci : UPstream::subProcs())
         {
             Perr<< "master sending to proc:" << proci << endl;
-            OPstream::bsend(data, proci);
+            OPstream os(UPstream::commsTypes::blocking, proci);
+            os  << data;
         }
     }
     else
     {
-        Perr<< "proc sending to master" << endl;
-        OPstream::bsend(data, UPstream::masterNo());
+        {
+            Perr<< "proc sending to master" << endl;
+            OPstream os(UPstream::commsTypes::blocking, UPstream::masterNo());
+            os  << data;
+        }
 
         Perr<< "proc receiving from master" << endl;
         IPstream::recv(data, UPstream::masterNo());
@@ -165,13 +169,17 @@ void testTokenized(const T& data)
         for (const int proci : UPstream::subProcs())
         {
             Perr<< "master sending to proc:" << proci << endl;
-            OPstream::bsend(tok, proci);
+            OPstream os(UPstream::commsTypes::blocking, proci);
+            os  << tok;
         }
     }
     else
     {
-        Perr<< "proc sending to master" << endl;
-        OPstream::bsend(tok, UPstream::masterNo());
+        {
+            Perr<< "proc sending to master" << endl;
+            OPstream os(UPstream::commsTypes::blocking, UPstream::masterNo());
+            os  << tok;
+        }
 
         Perr<< "proc receiving from master" << endl;
         IPstream::recv(tok, UPstream::masterNo());
diff --git a/applications/test/router/Gather/Gather.H b/applications/test/router/Gather.H
similarity index 82%
rename from applications/test/router/Gather/Gather.H
rename to applications/test/router/Gather.H
index 9fefcac0be..af08cf8b04 100644
--- a/applications/test/router/Gather/Gather.H
+++ b/applications/test/router/Gather.H
@@ -30,16 +30,15 @@ Description
     Gather data from all processors onto all processors.
 
 SourceFiles
-    Gather.C
+    Gather.txx
 
 \*---------------------------------------------------------------------------*/
 
-#ifndef Gather_H
-#define Gather_H
+#ifndef Foam_TestGather_H
+#define Foam_TestGather_H
 
 #include "List.H"
 #include "labelList.H"
-#include "GatherBase.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
@@ -50,34 +49,34 @@ namespace Foam
                            Class Gather Declaration
 \*---------------------------------------------------------------------------*/
 
-template<class T0>
+template<class Type>
 class Gather
 :
-    public GatherBase,
-    public List<T0>
+    public List<Type>
 {
-    // Private data
+    // Private Data
 
         //- Number of processors (1 for sequential)
         label nProcs_;
 
-        //- Storage of type 0
-        //List<T0> data0_;
+        //- Storage
+        //List<Type> list_;
+
 
 public:
 
     // Constructors
 
         //- Construct from containers across processors
-        Gather(const T0&, const bool redistribute=true);
+        Gather(const Type& localData, const bool redistribute=true);
 
 
     // Member Functions
 
-//        const List<T0>& data0() const
-//        {
-//            return data0_;
-//        }
+        List<Type>& list() noexcept { return *this; }
+        const List<Type>& list() const noexcept { return *this; }
+    //  List<Type>& list() noexcept { return list_; }
+    //  const List<Type>& list() const noexcept { return list_; }
 };
 
 
diff --git a/applications/test/router/Gather/Gather.txx b/applications/test/router/Gather.txx
similarity index 54%
rename from applications/test/router/Gather/Gather.txx
rename to applications/test/router/Gather.txx
index cf3542edad..b8ac980ad9 100644
--- a/applications/test/router/Gather/Gather.txx
+++ b/applications/test/router/Gather.txx
@@ -30,96 +30,71 @@ License
 #include "IPstream.H"
 #include "OPstream.H"
 
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
-
-namespace Foam
-{
-
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
-// Construct from component
-template<class T0>
-Gather<T0>::Gather(const T0& localData, const bool redistribute)
+template<class Type>
+Foam::Gather<Type>::Gather(const Type& localData, const bool redistribute)
 :
-    List<T0>(0),
-    nProcs_(max(1, Pstream::nProcs()))
+    nProcs_(Foam::max(1, UPstream::nProcs()))
 {
-    this->setSize(nProcs_);
+    this->list().resize(nProcs_);
 
     //
     // Collect sizes on all processor
     //
 
-    if (Pstream::parRun())
+    if (UPstream::parRun())
     {
-        if (Pstream::master())
+        if (UPstream::master())
         {
-            auto outIter = this->begin();
-            *outIter = localData;
+            auto iter = this->list().begin();
+            *iter = localData;
 
             // Receive data
-            for (const int proci : Pstream::subProcs())
+            for (const int proci : UPstream::subProcs())
             {
-                IPstream fromSlave(Pstream::commsTypes::scheduled, proci);
-                fromSlave >> *(++outIter);
+                ++iter;
+                IPstream::recv(*iter, proci);
             }
 
             // Send data
-            for (const int proci : Pstream::subProcs())
+            for (const int proci : UPstream::subProcs())
             {
-                OPstream toSlave(Pstream::commsTypes::scheduled, proci);
-
                 if (redistribute)
                 {
-                    toSlave << *this;
+                    OPstream::send(*this, proci);
                 }
                 else
                 {
-                    // Dummy send just to balance sends/receives
-                    toSlave << 0;
+                    // Dummy send (to balance sends/receives)
+                    OPstream::send(label(0), proci);
                 }
             }
         }
         else
         {
-            // Slave: send my local data to master
-            {
-                OPstream toMaster
-                (
-                    Pstream::commsTypes::scheduled,
-                    Pstream::masterNo()
-                );
-                toMaster << localData;
-            }
+            // Send my local data to master
+            OPstream::send(localData, UPstream::masterNo());
 
             // Receive data from master
+            if (redistribute)
             {
-                IPstream fromMaster
-                (
-                    Pstream::commsTypes::scheduled,
-                    Pstream::masterNo()
-                );
-                if (redistribute)
-                {
-                    fromMaster >> *this;
-                }
-                else
-                {
-                    label dummy;
-                    fromMaster >> dummy;
-                }
+                IPstream::recv(*this, UPstream::masterNo());
+            }
+            else
+            {
+                // Dummy receive
+                label dummy;
+                IPstream::recv(dummy, UPstream::masterNo());
             }
         }
     }
     else
     {
-        this->operator[](0) = localData;
+        this->list().resize(1);
+        this->list()[0] = localData;
     }
 }
 
 
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
-
-} // End namespace Foam
-
 // ************************************************************************* //
diff --git a/applications/test/router/Gather/GatherBase.H b/applications/test/router/Gather/GatherBase.H
deleted file mode 100644
index 2fb0a4cdba..0000000000
--- a/applications/test/router/Gather/GatherBase.H
+++ /dev/null
@@ -1,102 +0,0 @@
-/*---------------------------------------------------------------------------*\
-  =========                 |
-  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
-   \\    /   O peration     |
-    \\  /    A nd           | www.openfoam.com
-     \\/     M anipulation  |
--------------------------------------------------------------------------------
-    Copyright (C) 2011-2015 OpenFOAM Foundation
--------------------------------------------------------------------------------
-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/>.
-
-Class
-    Foam::GatherBase
-
-Description
-
-SourceFiles
-    GatherBase.C
-
-\*---------------------------------------------------------------------------*/
-
-#ifndef GatherBase_H
-#define GatherBase_H
-
-#include "List.H"
-#include "labelList.H"
-
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
-
-namespace Foam
-{
-
-/*---------------------------------------------------------------------------*\
-                           Class GatherBase Declaration
-\*---------------------------------------------------------------------------*/
-
-class GatherBase
-{
-
-public:
-
-        //- Flatten: appends all elements of list into one single list.
-        //  Used to collapse 'Gathered' data.
-        template<class T>
-        static T flatten(const List<T>);
-
-        //- Flatten and offset 'Gathered' indices (into value) so they
-        //  remain valid with respect to values (after they have been flattened)
-        template<class DataType, class IndexType, class AddOp>
-        static IndexType offset
-        (
-            const List<DataType>& values,
-            const List<IndexType>& indices,
-            AddOp aop
-        );
-};
-
-
-template<class T>
-class AddOp
-{
-
-public:
-
-    T operator()
-    (
-        const T& x,
-        const label offset
-    ) const
-    {
-        return x + offset;
-    }
-};
-
-
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
-
-} // End namespace Foam
-
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
-
-#include "GatherBase.txx"
-
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
-
-#endif
-
-// ************************************************************************* //
diff --git a/applications/test/router/Gather/GatherBase.txx b/applications/test/router/Gather/GatherBase.txx
deleted file mode 100644
index 917a4308b1..0000000000
--- a/applications/test/router/Gather/GatherBase.txx
+++ /dev/null
@@ -1,115 +0,0 @@
-/*---------------------------------------------------------------------------*\
-  =========                 |
-  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
-   \\    /   O peration     |
-    \\  /    A nd           | www.openfoam.com
-     \\/     M anipulation  |
--------------------------------------------------------------------------------
-    Copyright (C) 2011-2015 OpenFOAM Foundation
--------------------------------------------------------------------------------
-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/>.
-
-\*---------------------------------------------------------------------------*/
-
-#include "GatherBase.H"
-
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
-
-namespace Foam
-{
-
-// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
-
-template<class Type>
-Type GatherBase::flatten(const List<Type> lst)
-{
-    label sum = 0;
-
-    forAll(lst, lstI)
-    {
-        sum += lst[lstI].size();
-    }
-
-    Type result(sum);
-
-    label index = 0;
-
-    forAll(lst, lstI)
-    {
-        const Type& sub = lst[lstI];
-
-        forAll(sub, subI)
-        {
-            result[index++] = sub[subI];
-        }
-    }
-
-    return result;
-}
-
-
-template<class DataType, class IndexType, class AddOp>
-IndexType GatherBase::offset
-(
-    const List<DataType>& values,
-    const List<IndexType>& indices,
-    AddOp aop
-)
-{
-    if (values.size() != indices.size())
-    {
-        FatalErrorInFunction
-            << "Input data and indices lists not equal size." << endl
-            << "data size:" << values.size()
-            << "  indices:" << indices.size()
-            << abort(FatalError);
-    }
-
-
-    label sum = 0;
-
-    forAll(indices, lstI)
-    {
-        sum += indices[lstI].size();
-    }
-
-    IndexType result(sum);
-
-    label index = 0;
-
-    label offset = 0;
-
-    forAll(indices, lstI)
-    {
-        const IndexType& sub = indices[lstI];
-
-        forAll(sub, subI)
-        {
-            result[index++] = aop(sub[subI], offset);
-        }
-        offset += values[lstI].size();
-    }
-
-    return result;
-}
-
-
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
-
-} // End namespace Foam
-
-// ************************************************************************* //
diff --git a/applications/test/treeComms/Test-treeComms.C b/applications/test/treeComms/Test-treeComms.C
index d31cc69694..832f1459f7 100644
--- a/applications/test/treeComms/Test-treeComms.C
+++ b/applications/test/treeComms/Test-treeComms.C
@@ -206,8 +206,8 @@ int main(int argc, char *argv[])
 
         for (const int proci : UPstream::subProcs())
         {
-            IPstream fromProc(UPstream::commsTypes::scheduled, proci);
-            labelList below(fromProc);
+            labelList below;
+            IPstream::recv(below, proci);
 
             printConnection(os, proci, below);
         }
@@ -222,13 +222,7 @@ int main(int argc, char *argv[])
     }
     else
     {
-        OPstream toMaster
-        (
-             Pstream::commsTypes::scheduled,
-             Pstream::masterNo()
-        );
-
-        toMaster << myComm.below();
+        OPstream::send(myComm.below(), UPstream::masterNo());
         // Pout<< flatOutput(myComm.allBelow()) << nl;
     }
 
diff --git a/applications/utilities/mesh/manipulation/createPatch/createPatch.C b/applications/utilities/mesh/manipulation/createPatch/createPatch.C
index 0eb33b5b9b..133d208d96 100644
--- a/applications/utilities/mesh/manipulation/createPatch/createPatch.C
+++ b/applications/utilities/mesh/manipulation/createPatch/createPatch.C
@@ -541,7 +541,7 @@ void syncPoints
     // Is there any coupled patch with transformation?
     bool hasTransformation = false;
 
-    if (Pstream::parRun())
+    if (UPstream::parRun())
     {
         const labelList& procPatches = mesh.globalData().processorPatches();
 
@@ -569,7 +569,12 @@ void syncPoints
                 }
 
                 // buffered send
-                OPstream::bsend(patchInfo, procPatch.neighbProcNo());
+                OPstream toNbr
+                (
+                    UPstream::commsTypes::blocking,
+                    procPatch.neighbProcNo()
+                );
+                toNbr << patchInfo;
             }
         }
 
@@ -583,10 +588,10 @@ void syncPoints
 
             if (pp.nPoints() && !procPatch.owner())
             {
-                pointField nbrPatchInfo;
-
                 // We do not know the number of points on the other side
                 // so cannot use UIPstream::read
+
+                pointField nbrPatchInfo;
                 IPstream::recv(nbrPatchInfo, procPatch.neighbProcNo());
 
                 // Null any value which is not on neighbouring processor
diff --git a/applications/utilities/mesh/manipulation/splitMeshRegions/splitMeshRegions.C b/applications/utilities/mesh/manipulation/splitMeshRegions/splitMeshRegions.C
index eb84cc3ff6..64c0b0ef34 100644
--- a/applications/utilities/mesh/manipulation/splitMeshRegions/splitMeshRegions.C
+++ b/applications/utilities/mesh/manipulation/splitMeshRegions/splitMeshRegions.C
@@ -374,9 +374,9 @@ void getInterfaceSizes
     }
 
 
-    if (Pstream::parRun())
+    if (UPstream::parRun())
     {
-        if (Pstream::master())
+        if (UPstream::master())
         {
             // Receive and add to my sizes
             for (const int proci : UPstream::subProcs())
@@ -420,8 +420,8 @@ void getInterfaceSizes
         }
         else
         {
-            // buffered send to master
-            OPstream::bsend(regionsToSize, UPstream::masterNo());
+            // send to master
+            OPstream::send(regionsToSize, UPstream::masterNo());
         }
     }
 
diff --git a/src/OpenFOAM/containers/Lists/SortableList/ParSortableList.C b/src/OpenFOAM/containers/Lists/SortableList/ParSortableList.C
index c6ce91be38..d5f8ad2cd9 100644
--- a/src/OpenFOAM/containers/Lists/SortableList/ParSortableList.C
+++ b/src/OpenFOAM/containers/Lists/SortableList/ParSortableList.C
@@ -115,8 +115,8 @@ void Foam::ParSortableList<Type>::checkAndSend
         }
 
         {
-            OPstream toSlave(Pstream::commsTypes::blocking, destProci);
-            toSlave << values << indices;
+            OPstream toProc(UPstream::commsTypes::blocking, destProci);
+            toProc << values << indices;
         }
     }
 }
@@ -309,9 +309,9 @@ void Foam::ParSortableList<Type>::sort()
                     Pout<< "Receiving from " << proci << endl;
                 }
 
-                IPstream fromSlave(Pstream::commsTypes::blocking, proci);
+                IPstream fromProc(UPstream::commsTypes::blocking, proci);
 
-                fromSlave >> recValues >> recIndices;
+                fromProc >> recValues >> recIndices;
 
                 if (debug & 2)
                 {
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
index 87844b933e..c71bb4b24c 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
@@ -89,7 +89,7 @@ public:
         {
             IPstream is
             (
-                UPstream::commsTypes::scheduled,
+                UPstream::commsTypes::scheduled,  // ie, MPI_Recv()
                 fromProcNo,
                 0,  // bufSize
                 tag,
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
index bd1f6953e3..87d5636759 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
@@ -93,29 +93,6 @@ public:
             os << value;
         }
 
-        //- Serialize a value and send (buffered/blocking mode).
-        //- Uses \c operator<< for serialization
-        template<class Type>
-        static void bsend
-        (
-            const Type& value,
-            const int toProcNo,
-            const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
-            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
-        )
-        {
-            OPstream::send
-            (
-                value,
-                UPstream::commsTypes::blocking,
-                toProcNo,
-                tag,
-                comm,
-                fmt
-            );
-        }
-
         //- Serialize a value and send (standard mode).
         //- Uses \c operator<< for serialization
         template<class Type>
@@ -131,7 +108,7 @@ public:
             OPstream::send
             (
                 value,
-                UPstream::commsTypes::scheduled,
+                UPstream::commsTypes::scheduled,  // ie, MPI_Send()
                 toProcNo,
                 tag,
                 comm,
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/OPstreams.C b/src/OpenFOAM/db/IOstreams/Pstreams/OPstreams.C
index fa913a868c..e956e8de74 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/OPstreams.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/OPstreams.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011 OpenFOAM Foundation
-    Copyright (C) 2022-2023 OpenCFD Ltd.
+    Copyright (C) 2022-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -86,6 +86,15 @@ Foam::OPstream::OPstream
 {}
 
 
+// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
+
+bool Foam::UOPstream::send()
+{
+    sendAtDestruct_ = false;
+    return bufferIPCsend();
+}
+
+
 // * * * * * * * * * * * * * * * * Destructor  * * * * * * * * * * * * * * * //
 
 Foam::UOPstream::~UOPstream()
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchangeConsensus.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchangeConsensus.C
index ccc9cb6c3f..de7ce5fa42 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchangeConsensus.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchangeConsensus.C
@@ -202,7 +202,7 @@ void exchangeConsensus
 
             UIPstream::read
             (
-                UPstream::commsTypes::blocking,
+                UPstream::commsTypes::scheduled,  // ie, MPI_Recv()
                 proci,
                 recvData.data_bytes(),
                 recvData.size_bytes(),
@@ -369,7 +369,7 @@ void exchangeConsensus
 
             UIPstream::read
             (
-                UPstream::commsTypes::blocking,
+                UPstream::commsTypes::scheduled,  // ie, MPI_Recv()
                 proci,
                 recvData.data_bytes(),
                 recvData.size_bytes(),
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
index 274e8aaedb..8519e03f81 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2014 OpenFOAM Foundation
-    Copyright (C) 2017-2023 OpenCFD Ltd.
+    Copyright (C) 2017-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -104,7 +104,7 @@ protected:
         const int comm_;
 
         //- Call bufferIPCsend on termination (in the destructor)
-        const bool sendAtDestruct_;
+        bool sendAtDestruct_;
 
         //- Reference to the send buffer data
         DynamicList<char>& sendBuf_;
@@ -309,7 +309,7 @@ class UOPstream
 {
     // Private Member Functions
 
-        //- Final buffer send, called by destructor
+        //- Buffer send, usually called by destructor
         bool bufferIPCsend();
 
 
@@ -349,6 +349,10 @@ public:
 
     // Member Functions
 
+        //- Send buffer contents now and not in destructor [advanced usage].
+        //- Returns true on success
+        bool send();
+
         //- Use all write methods from base class
         using UOPstreamBase::write;
 
@@ -470,7 +474,7 @@ class UOPBstream
 {
     // Private Member Functions
 
-        //- Final buffer send, called by destructor
+        //- Buffer send, usually called by destructor
         bool bufferIPCsend();
 
 
diff --git a/src/OpenFOAM/meshes/polyMesh/globalMeshData/globalMeshData.C b/src/OpenFOAM/meshes/polyMesh/globalMeshData/globalMeshData.C
index 6c9e7c46f8..c46241eceb 100644
--- a/src/OpenFOAM/meshes/polyMesh/globalMeshData/globalMeshData.C
+++ b/src/OpenFOAM/meshes/polyMesh/globalMeshData/globalMeshData.C
@@ -415,8 +415,8 @@ void Foam::globalMeshData::calcSharedEdges() const
     {
         if (UPstream::parRun())
         {
-            // buffered send local edges to master
-            OPstream::bsend(localShared, UPstream::masterNo());
+            // send local edges to master
+            OPstream::send(localShared, UPstream::masterNo());
         }
     }
 
@@ -442,7 +442,7 @@ void Foam::globalMeshData::calcSharedEdges() const
         {
             // My local edge is indeed a shared one. Go through all local edge
             // labels with this point combination.
-            const labelList& edgeLabels = iter();
+            const labelList& edgeLabels = iter.val();
 
             for (const label edgei : edgeLabels)
             {
@@ -470,11 +470,9 @@ void Foam::globalMeshData::calcSharedEdges() const
     {
         Pout<< "globalMeshData : nGlobalEdges_:" << nGlobalEdges_ << nl
             << "globalMeshData : sharedEdgeLabels:"
-            << sharedEdgeLabelsPtr_().size()
-            << nl
+            << sharedEdgeLabelsPtr_().size() << nl
             << "globalMeshData : sharedEdgeAddr:"
-            << sharedEdgeAddrPtr_().size()
-            << endl;
+            << sharedEdgeAddrPtr_().size() << endl;
     }
 }
 
@@ -1856,7 +1854,7 @@ Foam::pointField Foam::globalMeshData::sharedPoints() const
     const labelList& pointAddr = sharedPointAddr();
     const labelList& pointLabels = sharedPointLabels();
 
-    if (Pstream::master())
+    if (UPstream::master())
     {
         // Master:
         // insert my own data first
@@ -1868,9 +1866,9 @@ Foam::pointField Foam::globalMeshData::sharedPoints() const
         }
 
         // Receive data and insert
-        for (const int proci : Pstream::subProcs())
+        for (const int proci : UPstream::subProcs())
         {
-            IPstream fromProc(Pstream::commsTypes::blocking, proci);
+            IPstream fromProc(UPstream::commsTypes::scheduled, proci);
 
             labelList nbrSharedPointAddr;
             pointField nbrSharedPoints;
@@ -1886,13 +1884,13 @@ Foam::pointField Foam::globalMeshData::sharedPoints() const
     }
     else
     {
-        if (Pstream::parRun())
+        if (UPstream::parRun())
         {
             // Send address and points
             OPstream toMaster
             (
-                Pstream::commsTypes::blocking,
-                Pstream::masterNo()
+                UPstream::commsTypes::scheduled,
+                UPstream::masterNo()
             );
             toMaster
                 << pointAddr
@@ -2689,19 +2687,8 @@ void Foam::globalMeshData::updateMesh()
         Pout<< "globalMeshData : merge dist:" << tolDim << endl;
     }
 
-    // NOTE
-    // - revisit the dupComm hack
-    // - verify if it should be mesh_.comm() instead of worldComm
 
-    // *** Temporary hack to avoid problems with overlapping communication
-    // *** between these reductions and the calculation of deltaCoeffs
-    UPstream::communicator dupComm
-    (
-        UPstream::worldComm,
-        labelRange(UPstream::nProcs(UPstream::worldComm))
-    );
-
-    const label comm = dupComm.comm();
+    const label comm = mesh_.comm();
     const label oldWarnComm = UPstream::commWarn(comm);
 
     if (UPstream::is_parallel(comm))
@@ -2752,18 +2739,6 @@ void Foam::globalMeshData::updateMesh()
     // Restore communicator settings
     UPstream::commWarn(oldWarnComm);
 
-    // OLD CODE:
-    // FixedList<label, 3> totals;
-    // totals[0] = mesh_.nPoints();
-    // totals[1] = mesh_.nFaces();
-    // totals[2] = mesh_.nCells();
-    //
-    // reduce(totals, sumOp<label>(), UPstream::msgType(), comm);
-    //
-    // nTotalPoints_ = totals[0];
-    // nTotalFaces_ = totals[1];
-    // nTotalCells_ = totals[2];
-
     if (debug)
     {
         Info<< "globalMeshData : Total points/faces/cells : ("
diff --git a/src/OpenFOAM/meshes/polyMesh/mapPolyMesh/mapDistribute/mapDistributeBaseTemplates.C b/src/OpenFOAM/meshes/polyMesh/mapPolyMesh/mapDistribute/mapDistributeBaseTemplates.C
index a14b4cc2b1..85c89213d9 100644
--- a/src/OpenFOAM/meshes/polyMesh/mapPolyMesh/mapDistribute/mapDistributeBaseTemplates.C
+++ b/src/OpenFOAM/meshes/polyMesh/mapPolyMesh/mapDistribute/mapDistributeBaseTemplates.C
@@ -509,7 +509,8 @@ void Foam::mapDistributeBase::distribute
                 );
 
                 // buffered send
-                OPstream::bsend(subField, proci, tag, comm);
+                OPstream os(commsType, proci, 0, tag, comm);
+                os  << subField;
             }
         }
 
@@ -953,7 +954,8 @@ void Foam::mapDistributeBase::distribute
                 );
 
                 // buffered send
-                OPstream::bsend(subField, proci, tag, comm);
+                OPstream os(commsType, proci, 0, tag, comm);
+                os  << subField;
             }
         }
 
diff --git a/src/OpenFOAM/meshes/polyMesh/polyMesh.H b/src/OpenFOAM/meshes/polyMesh/polyMesh.H
index d4d6c0c15e..f1331d9734 100644
--- a/src/OpenFOAM/meshes/polyMesh/polyMesh.H
+++ b/src/OpenFOAM/meshes/polyMesh/polyMesh.H
@@ -55,6 +55,7 @@ SourceFiles
 #include "labelIOList.H"
 #include "polyBoundaryMesh.H"
 #include "boundBox.H"
+#include "globalIndex.H"
 #include "pointZoneMesh.H"
 #include "faceZoneMesh.H"
 #include "cellZoneMesh.H"
diff --git a/src/OpenFOAM/meshes/polyMesh/syncTools/syncToolsTemplates.C b/src/OpenFOAM/meshes/polyMesh/syncTools/syncToolsTemplates.C
index 4148c24427..457a9daff6 100644
--- a/src/OpenFOAM/meshes/polyMesh/syncTools/syncToolsTemplates.C
+++ b/src/OpenFOAM/meshes/polyMesh/syncTools/syncToolsTemplates.C
@@ -94,7 +94,7 @@ void Foam::syncTools::syncPointMap
     const globalMeshData& pd = mesh.globalData();
 
     // Values on shared points. Keyed on global shared index.
-    Map<T> sharedPointValues(0);
+    Map<T> sharedPointValues;
 
     if (pd.nGlobalPoints() > 0)
     {
@@ -205,7 +205,7 @@ void Foam::syncTools::syncPointMap
                     continue;
                 }
 
-                Map<T> nbrPatchInfo(0);
+                Map<T> nbrPatchInfo;
                 {
                     UIPstream fromNbr(nbrProci, pBufs);
                     fromNbr >> nbrPatchInfo;
@@ -472,7 +472,7 @@ void Foam::syncTools::syncEdgeMap
                     continue;
                 }
 
-                EdgeMap<T> nbrPatchInfo(0);
+                EdgeMap<T> nbrPatchInfo;
                 {
                     UIPstream fromNbr(nbrProci, pBufs);
                     fromNbr >> nbrPatchInfo;
diff --git a/src/conversion/vtk/output/foamVtkPatchWriterTemplates.C b/src/conversion/vtk/output/foamVtkPatchWriterTemplates.C
index 0219a3b835..4c22bf3d65 100644
--- a/src/conversion/vtk/output/foamVtkPatchWriterTemplates.C
+++ b/src/conversion/vtk/output/foamVtkPatchWriterTemplates.C
@@ -90,7 +90,7 @@ void Foam::vtk::patchWriter::write
             // Receive each patch field and write
             for (const int subproci : Pstream::subProcs())
             {
-                IPstream fromProc(Pstream::commsTypes::blocking, subproci);
+                IPstream fromProc(UPstream::commsTypes::scheduled, subproci);
 
                 for (label i=0; i < nPatches; ++i)
                 {
@@ -105,8 +105,8 @@ void Foam::vtk::patchWriter::write
             // Send each patch field
             OPstream toProc
             (
-                Pstream::commsTypes::blocking,
-                Pstream::masterNo()
+                UPstream::commsTypes::scheduled,
+                UPstream::masterNo()
             );
 
             for (const label patchId : patchIDs_)
@@ -188,7 +188,7 @@ void Foam::vtk::patchWriter::write
             // Receive each patch field and write
             for (const int subproci : Pstream::subProcs())
             {
-                IPstream fromProc(Pstream::commsTypes::blocking, subproci);
+                IPstream fromProc(UPstream::commsTypes::scheduled, subproci);
 
                 for (label i=0; i < nPatches; ++i)
                 {
@@ -203,8 +203,8 @@ void Foam::vtk::patchWriter::write
             // Send each patch field
             OPstream toProc
             (
-                Pstream::commsTypes::blocking,
-                Pstream::masterNo()
+                UPstream::commsTypes::scheduled,
+                UPstream::masterNo()
             );
 
             for (const label patchId : patchIDs_)
@@ -287,14 +287,14 @@ void Foam::vtk::patchWriter::write
         // Patch Ids are identical across all processes
         const label nPatches = patchIDs_.size();
 
-        if (Pstream::master())
+        if (UPstream::master())
         {
             Field<Type> recv;
 
             // Receive each patch field and write
-            for (const int subproci : Pstream::subProcs())
+            for (const int subproci : UPstream::subProcs())
             {
-                IPstream fromProc(Pstream::commsTypes::blocking, subproci);
+                IPstream fromProc(UPstream::commsTypes::scheduled, subproci);
 
                 for (label i=0; i < nPatches; ++i)
                 {
@@ -309,8 +309,8 @@ void Foam::vtk::patchWriter::write
             // Send each patch field
             OPstream toProc
             (
-                Pstream::commsTypes::blocking,
-                Pstream::masterNo()
+                UPstream::commsTypes::scheduled,
+                UPstream::masterNo()
             );
 
             for (const label patchId : patchIDs_)
diff --git a/src/dynamicMesh/fvMeshDistribute/fvMeshDistribute.C b/src/dynamicMesh/fvMeshDistribute/fvMeshDistribute.C
index f4db96fdb8..ffd40037f0 100644
--- a/src/dynamicMesh/fvMeshDistribute/fvMeshDistribute.C
+++ b/src/dynamicMesh/fvMeshDistribute/fvMeshDistribute.C
@@ -2171,7 +2171,6 @@ Foam::autoPtr<Foam::mapDistributePolyMesh> Foam::fvMeshDistribute::distribute
             }
 
             // Pstream for sending mesh and fields
-            //OPstream str(Pstream::commsTypes::blocking, recvProc);
             UOPstream str(recvProc, pBufs);
 
             // Mesh subsetting engine - subset the cells of the current domain.
diff --git a/src/functionObjects/field/externalCoupled/externalCoupled.C b/src/functionObjects/field/externalCoupled/externalCoupled.C
index 9506d85ac3..fb91a1c315 100644
--- a/src/functionObjects/field/externalCoupled/externalCoupled.C
+++ b/src/functionObjects/field/externalCoupled/externalCoupled.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2015-2023 OpenCFD Ltd.
+    Copyright (C) 2015-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -30,6 +30,7 @@ License
 #include "addToRunTimeSelectionTable.H"
 #include "OSspecific.H"
 #include "Fstream.H"
+#include "SpanStream.H"
 #include "volFields.H"
 #include "globalIndex.H"
 #include "fvMesh.H"
@@ -110,15 +111,15 @@ void Foam::functionObjects::externalCoupled::readColumns
     // Get sizes for all processors
     const globalIndex globalFaces(globalIndex::gatherOnly{}, nRows);
 
-    PstreamBuffers pBufs(Pstream::commsTypes::nonBlocking);
+    PstreamBuffers pBufs(UPstream::commsTypes::nonBlocking);
 
-    if (Pstream::master())
+    if (UPstream::master())
     {
+        // Read data from file and send to destination processor
+        auto& ifile = masterFilePtr();
         string line;
 
-        // Read data from file and send to destination processor
-
-        for (const int proci : Pstream::allProcs())
+        for (const int proci : UPstream::allProcs())
         {
             // Temporary storage
             List<scalarField> values(nColumns);
@@ -136,9 +137,9 @@ void Foam::functionObjects::externalCoupled::readColumns
                 // Get a line
                 do
                 {
-                    if (!masterFilePtr().good())
+                    if (!ifile.good())
                     {
-                        FatalIOErrorInFunction(masterFilePtr())
+                        FatalIOErrorInFunction(ifile)
                             << "Trying to read data for processor " << proci
                             << " row " << rowi
                             << ". Does your file have as many rows as there are"
@@ -146,15 +147,15 @@ void Foam::functionObjects::externalCoupled::readColumns
                             << ") ?" << exit(FatalIOError);
                     }
 
-                    masterFilePtr().getLine(line);
+                    ifile.getLine(line);
                 }
                 while (line.empty() || line[0] == '#');
 
-                IStringStream lineStr(line);
+                ISpanStream isstr(line);
 
                 for (label columni = 0; columni < nColumns; ++columni)
                 {
-                    lineStr >> values[columni][rowi];
+                    isstr >> values[columni][rowi];
                 }
             }
 
@@ -165,7 +166,7 @@ void Foam::functionObjects::externalCoupled::readColumns
     }
     pBufs.finishedScatters();
 
-    // Get scattered data from PstreamBuffers
+    // Get scattered data
     UIPstream fromMaster(UPstream::masterNo(), pBufs);
     fromMaster >> data;
 }
@@ -175,21 +176,21 @@ void Foam::functionObjects::externalCoupled::readLines
 (
     const label nRows,
     autoPtr<IFstream>& masterFilePtr,
-    OStringStream& lines
+    std::string& lines
 ) const
 {
     // Get sizes for all processors
     const globalIndex globalFaces(globalIndex::gatherOnly{}, nRows);
 
-    PstreamBuffers pBufs(Pstream::commsTypes::nonBlocking);
+    PstreamBuffers pBufs(UPstream::commsTypes::nonBlocking);
 
-    if (Pstream::master())
+    if (UPstream::master())
     {
+        // Read line from file and send to destination processor
+        auto& ifile = masterFilePtr();
         string line;
 
-        // Read line from file and send to destination processor
-
-        for (const int proci : Pstream::allProcs())
+        for (const int proci : UPstream::allProcs())
         {
             // Number of rows to read for processor proci
             const label procNRows = globalFaces.localSize(proci);
@@ -201,9 +202,9 @@ void Foam::functionObjects::externalCoupled::readLines
                 // Get a line
                 do
                 {
-                    if (!masterFilePtr().good())
+                    if (!ifile.good())
                     {
-                        FatalIOErrorInFunction(masterFilePtr())
+                        FatalIOErrorInFunction(ifile)
                             << "Trying to read data for processor " << proci
                             << " row " << rowi
                             << ". Does your file have as many rows as there are"
@@ -211,11 +212,11 @@ void Foam::functionObjects::externalCoupled::readLines
                             << ") ?" << exit(FatalIOError);
                     }
 
-                    masterFilePtr().getLine(line);
+                    ifile.getLine(line);
                 }
                 while (line.empty() || line[0] == '#');
 
-                // Send line to the destination processor
+                // Send line (without newline) to the destination processor
                 toProc << line;
             }
         }
@@ -223,12 +224,16 @@ void Foam::functionObjects::externalCoupled::readLines
 
     pBufs.finishedScatters();
 
-    // Get scattered data from PstreamBuffers
+    // Sizing is approximate (slightly too large)
+    lines.reserve(pBufs.recvDataCount(UPstream::masterNo()));
+
+    // Get scattered data
     UIPstream fromMaster(UPstream::masterNo(), pBufs);
     for (label rowi = 0; rowi < nRows; ++rowi)
     {
         string line(fromMaster);
-        lines << line.c_str() << nl;
+        lines += line;
+        lines += '\n';
     }
 }
 
@@ -253,9 +258,9 @@ void Foam::functionObjects::externalCoupled::writeGeometry
 
     autoPtr<OFstream> osPointsPtr;
     autoPtr<OFstream> osFacesPtr;
-    if (Pstream::master())
+    if (UPstream::master())
     {
-        mkDir(dir);
+        Foam::mkDir(dir);
         osPointsPtr.reset(new OFstream(dir/"patchPoints"));
         osFacesPtr.reset(new OFstream(dir/"patchFaces"));
 
@@ -278,7 +283,7 @@ void Foam::functionObjects::externalCoupled::writeGeometry
         (
             mesh.boundaryMesh().patchSet
             (
-                wordRes(one{}, groupName)
+                wordRes(Foam::one{}, groupName)
             ).sortedToc()
         );
 
@@ -303,13 +308,13 @@ void Foam::functionObjects::externalCoupled::writeGeometry
             List<faceList> collectedFaces(Pstream::nProcs());
             faceList& patchFaces = collectedFaces[proci];
             patchFaces = p.localFaces();
-            forAll(patchFaces, facei)
+            for (auto& f : patchFaces)
             {
-                inplaceRenumber(pointToGlobal, patchFaces[facei]);
+                inplaceRenumber(pointToGlobal, f);
             }
             Pstream::gatherList(collectedFaces);
 
-            if (Pstream::master())
+            if (UPstream::master())
             {
                 allPoints.clear();
                 allFaces.clear();
diff --git a/src/functionObjects/field/externalCoupled/externalCoupled.H b/src/functionObjects/field/externalCoupled/externalCoupled.H
index dde7ba1604..567b85a337 100644
--- a/src/functionObjects/field/externalCoupled/externalCoupled.H
+++ b/src/functionObjects/field/externalCoupled/externalCoupled.H
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2015-2020 OpenCFD Ltd.
+    Copyright (C) 2015-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -167,8 +167,8 @@ SourceFiles
 
 \*---------------------------------------------------------------------------*/
 
-#ifndef functionObjects_externalCoupled_H
-#define functionObjects_externalCoupled_H
+#ifndef Foam_functionObjects_externalCoupled_H
+#define Foam_functionObjects_externalCoupled_H
 
 #include "timeFunctionObject.H"
 #include "externalFileCoupler.H"
@@ -198,7 +198,6 @@ class externalCoupled
     public functionObjects::timeFunctionObject,
     public externalFileCoupler
 {
-
     // Private Member Data
 
         //- Calculation frequency
@@ -259,8 +258,9 @@ class externalCoupled
 
         void initCoupling();
 
-        //- Read (and distribute) scalar columns from stream. Every processor
-        //  gets nRows (= patch size) of these. Note: could make its argument
+        //- Read (and distribute) scalar columns from stream.
+        //- Every processor gets nRows (= patch size) of these.
+        //  Note: could make its argument
         //  ISstream& but then would need additional logic to construct valid
         //  stream on all processors.
         void readColumns
@@ -271,21 +271,17 @@ class externalCoupled
             List<scalarField>& data
         ) const;
 
-        //- Read (and distribute) lines from stream. Every processor
-        //  gets nRows (= patch size) of these. Data kept as stream (instead
-        //  of strings) for ease of interfacing to readData routines that take
-        //  an Istream.
+        //- Read (and distribute) lines from stream.
+        //- Every processor gets nRows (= patch size) of these
+        //- (newline terminated).
         void readLines
         (
             const label nRows,
             autoPtr<IFstream>& masterFilePtr,
-            OStringStream& data
+            //! [out] the nRows lines read (for this rank) - newline terminated
+            std::string& lines
         ) const;
 
-        //- Helper: append data from all processors onto master
-        template<class Type>
-        static tmp<Field<Type>> gatherAndCombine(const Field<Type>& fld);
-
         static void checkOrder(const wordList& regionNames);
 
         //- Perform the coupling with necessary initialization etc.
diff --git a/src/functionObjects/field/externalCoupled/externalCoupledTemplates.C b/src/functionObjects/field/externalCoupled/externalCoupledTemplates.C
index 34e99f6297..814cc122d2 100644
--- a/src/functionObjects/field/externalCoupled/externalCoupledTemplates.C
+++ b/src/functionObjects/field/externalCoupled/externalCoupledTemplates.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2015-2021 OpenCFD Ltd.
+    Copyright (C) 2015-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -33,6 +33,7 @@ License
 #include "mixedFvPatchFields.H"
 #include "fixedGradientFvPatchFields.H"
 #include "fixedValueFvPatchFields.H"
+#include "SpanStream.H"
 #include "StringStream.H"
 #include "globalIndex.H"
 
@@ -58,7 +59,7 @@ bool Foam::functionObjects::externalCoupled::readData
     // File only opened on master; contains data for all processors, for all
     // patchIDs.
     autoPtr<IFstream> masterFilePtr;
-    if (Pstream::master())
+    if (UPstream::master())
     {
         const fileName transferFile
         (
@@ -80,13 +81,13 @@ bool Foam::functionObjects::externalCoupled::readData
     }
 
 
-    const wordRes patchSelection(one{}, groupName);
+    const wordRes patchSelection(Foam::one{}, groupName);
 
     label nFound = 0;
 
     for (const fvMesh& mesh : meshes)
     {
-        const volFieldType* vfptr = mesh.findObject<volFieldType>(fieldName);
+        auto* vfptr = mesh.getObjectPtr<volFieldType>(fieldName);
 
         if (!vfptr)
         {
@@ -94,8 +95,7 @@ bool Foam::functionObjects::externalCoupled::readData
         }
         ++nFound;
 
-        typename volFieldType::Boundary& bf =
-            const_cast<volFieldType*>(vfptr)->boundaryFieldRef();
+        auto& bf = vfptr->boundaryFieldRef();
 
         // Get the patches
         const labelList patchIDs
@@ -117,16 +117,18 @@ bool Foam::functionObjects::externalCoupled::readData
                 );
 
                 // Read from master into local stream
-                OStringStream os;
+                std::string lines;
                 readLines
                 (
-                    bf[patchi].size(),      // number of lines to read
+                    bf[patchi].size(),  // number of lines to read
                     masterFilePtr,
-                    os
+                    lines
                 );
 
+                ISpanStream isstr(lines);
+
                 // Pass responsibility for all reading over to bc
-                pf.readData(IStringStream(os.str())());
+                pf.readData(isstr);
 
                 // Update the value from the read coefficient. Bypass any
                 // additional processing by derived type.
@@ -265,51 +267,6 @@ bool Foam::functionObjects::externalCoupled::readData
 }
 
 
-template<class Type>
-Foam::tmp<Foam::Field<Type>>
-Foam::functionObjects::externalCoupled::gatherAndCombine
-(
-    const Field<Type>& fld
-)
-{
-    // Collect values from all processors
-    List<Field<Type>> gatheredValues(Pstream::nProcs());
-    gatheredValues[Pstream::myProcNo()] = fld;
-    Pstream::gatherList(gatheredValues);
-
-
-    auto tresult = tmp<Field<Type>>::New();
-    auto& result = tresult.ref();
-
-    if (Pstream::master())
-    {
-        // Combine values into single field
-        label globalElemi = 0;
-
-        forAll(gatheredValues, lsti)
-        {
-            globalElemi += gatheredValues[lsti].size();
-        }
-
-        result.setSize(globalElemi);
-
-        globalElemi = 0;
-
-        forAll(gatheredValues, lsti)
-        {
-            const Field<Type>& sub = gatheredValues[lsti];
-
-            forAll(sub, elemi)
-            {
-                result[globalElemi++] = sub[elemi];
-            }
-        }
-    }
-
-    return tresult;
-}
-
-
 template<class Type>
 bool Foam::functionObjects::externalCoupled::writeData
 (
@@ -352,14 +309,14 @@ bool Foam::functionObjects::externalCoupled::writeData
     }
 
 
-    const wordRes patchSelection(one{}, groupName);
+    const wordRes patchSelection(Foam::one{}, groupName);
 
     bool headerDone = false;
     label nFound = 0;
 
     for (const fvMesh& mesh : meshes)
     {
-        const volFieldType* vfptr = mesh.findObject<volFieldType>(fieldName);
+        const auto* vfptr = mesh.getObjectPtr<volFieldType>(fieldName);
 
         if (!vfptr)
         {
@@ -367,8 +324,7 @@ bool Foam::functionObjects::externalCoupled::writeData
         }
         ++nFound;
 
-        const typename volFieldType::Boundary& bf =
-            vfptr->boundaryField();
+        const auto& bf = vfptr->boundaryField();
 
         // Get the patches
         const labelList patchIDs
@@ -379,7 +335,7 @@ bool Foam::functionObjects::externalCoupled::writeData
         // Handle column-wise writing of patch data. Supports most easy types
         for (const label patchi : patchIDs)
         {
-            const globalIndex globalFaces(bf[patchi].size());
+            // const globalIndex globalFaces(bf[patchi].size());
 
             if (isA<patchFieldType>(bf[patchi]))
             {
@@ -397,7 +353,7 @@ bool Foam::functionObjects::externalCoupled::writeData
 
                 // Collect contributions from all processors and output them on
                 // master
-                if (Pstream::master())
+                if (UPstream::master())
                 {
                     // Output master data first
                     if (!headerDone)
@@ -407,27 +363,17 @@ bool Foam::functionObjects::externalCoupled::writeData
                     }
                     masterFilePtr() << os.str().c_str();
 
-                    for (const int proci : Pstream::subProcs())
+                    for (const int proci : UPstream::subProcs())
                     {
-                        IPstream fromSlave
-                        (
-                            Pstream::commsTypes::scheduled,
-                            proci
-                        );
+                        string str;
+                        IPstream::recv(str, proci);
 
-                        string str(fromSlave);
                         masterFilePtr() << str.c_str();
                     }
                 }
                 else
                 {
-                    OPstream toMaster
-                    (
-                        Pstream::commsTypes::scheduled,
-                        Pstream::masterNo()
-                    );
-
-                    toMaster << os.str();
+                    OPstream::send(os.str(), UPstream::masterNo());
                 }
             }
             else if (isA<mixedFvPatchField<Type>>(bf[patchi]))
@@ -435,15 +381,21 @@ bool Foam::functionObjects::externalCoupled::writeData
                 const mixedFvPatchField<Type>& pf =
                     refCast<const mixedFvPatchField<Type>>(bf[patchi]);
 
-                Field<Type> value(gatherAndCombine(pf));
-                Field<Type> snGrad(gatherAndCombine(pf.snGrad()()));
-                Field<Type> refValue(gatherAndCombine(pf.refValue()));
-                Field<Type> refGrad(gatherAndCombine(pf.refGrad()));
-                scalarField valueFraction(gatherAndCombine(pf.valueFraction()));
+                const globalIndex glob
+                (
+                    globalIndex::gatherOnly{},
+                    pf.size()
+                );
 
-                if (Pstream::master())
+                Field<Type> value(glob.gather(pf));
+                Field<Type> snGrad(glob.gather(pf.snGrad()()));
+                Field<Type> refValue(glob.gather(pf.refValue()));
+                Field<Type> refGrad(glob.gather(pf.refGrad()));
+                scalarField valueFraction(glob.gather(pf.valueFraction()));
+
+                if (UPstream::master())
                 {
-                    forAll(refValue, facei)
+                    forAll(value, facei)
                     {
                         masterFilePtr()
                             << value[facei] << token::SPACE
@@ -457,9 +409,17 @@ bool Foam::functionObjects::externalCoupled::writeData
             else
             {
                 // Output the value and snGrad
-                Field<Type> value(gatherAndCombine(bf[patchi]));
-                Field<Type> snGrad(gatherAndCombine(bf[patchi].snGrad()()));
-                if (Pstream::master())
+
+                const globalIndex glob
+                (
+                    globalIndex::gatherOnly{},
+                    bf[patchi].size()
+                );
+
+                Field<Type> value(glob.gather(bf[patchi]));
+                Field<Type> snGrad(glob.gather(bf[patchi].snGrad()()));
+
+                if (UPstream::master())
                 {
                     forAll(value, facei)
                     {
diff --git a/src/functionObjects/lagrangian/dataCloud/dataCloudTemplates.C b/src/functionObjects/lagrangian/dataCloud/dataCloudTemplates.C
index 67319c08ef..d225a6ad2d 100644
--- a/src/functionObjects/lagrangian/dataCloud/dataCloudTemplates.C
+++ b/src/functionObjects/lagrangian/dataCloud/dataCloudTemplates.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2018-2022 OpenCFD Ltd.
+    Copyright (C) 2018-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -77,7 +77,7 @@ void Foam::functionObjects::dataCloud::writeListParallel
     const List<Type>& field
 )
 {
-    if (Pstream::master())
+    if (UPstream::master())
     {
         writeList(os, points, field);
 
@@ -85,9 +85,9 @@ void Foam::functionObjects::dataCloud::writeListParallel
         Field<Type> recvField;
 
         // Receive and write
-        for (const int proci : Pstream::subProcs())
+        for (const int proci : UPstream::subProcs())
         {
-            IPstream fromProc(Pstream::commsTypes::blocking, proci);
+            IPstream fromProc(UPstream::commsTypes::scheduled, proci);
 
             fromProc >> recvPoints >> recvField;
 
@@ -99,8 +99,8 @@ void Foam::functionObjects::dataCloud::writeListParallel
         // Send to master
         OPstream toMaster
         (
-            Pstream::commsTypes::blocking,
-            Pstream::masterNo()
+            UPstream::commsTypes::scheduled,
+            UPstream::masterNo()
         );
 
         toMaster
@@ -134,7 +134,7 @@ void Foam::functionObjects::dataCloud::writeListParallel
     const bitSet& selected
 )
 {
-    if (Pstream::master())
+    if (UPstream::master())
     {
         writeList(os, points, field, selected);
 
@@ -142,9 +142,9 @@ void Foam::functionObjects::dataCloud::writeListParallel
         Field<Type> recvField;
 
         // Receive and write
-        for (const int proci : Pstream::subProcs())
+        for (const int proci : UPstream::subProcs())
         {
-            IPstream fromProc(Pstream::commsTypes::blocking, proci);
+            IPstream fromProc(UPstream::commsTypes::scheduled, proci);
 
             fromProc >> recvPoints >> recvField;
 
@@ -156,8 +156,8 @@ void Foam::functionObjects::dataCloud::writeListParallel
         // Send to master
         OPstream toMaster
         (
-            Pstream::commsTypes::blocking,
-            Pstream::masterNo()
+            UPstream::commsTypes::scheduled,
+            UPstream::masterNo()
         );
 
         toMaster
@@ -195,7 +195,7 @@ bool Foam::functionObjects::dataCloud::writeField
 
     autoPtr<OFstream> osPtr;
 
-    if (Pstream::master())
+    if (UPstream::master())
     {
         osPtr.reset(new OFstream(outputName));
         osPtr->precision(precision_);
diff --git a/src/mesh/snappyHexMesh/meshRefinement/meshRefinementTemplates.C b/src/mesh/snappyHexMesh/meshRefinement/meshRefinementTemplates.C
index 324c6d2972..931e9ced31 100644
--- a/src/mesh/snappyHexMesh/meshRefinement/meshRefinementTemplates.C
+++ b/src/mesh/snappyHexMesh/meshRefinement/meshRefinementTemplates.C
@@ -166,23 +166,8 @@ void Foam::meshRefinement::collectAndPrint
 {
     const globalIndex globalPoints(points.size());
 
-    pointField allPoints;
-    globalPoints.gather
-    (
-        points,
-        allPoints,
-        UPstream::msgType(),
-        Pstream::commsTypes::blocking
-    );
-
-    List<T> allData;
-    globalPoints.gather
-    (
-        data,
-        allData,
-        UPstream::msgType(),
-        Pstream::commsTypes::blocking
-    );
+    pointField allPoints(globalPoints.gather(points));
+    List<T> allData(globalPoints.gather(data));
 
 
     scalarField magAllPoints(mag(allPoints-point(-0.317, 0.117, 0.501)));
diff --git a/src/meshTools/output/vtk/patch/foamVtkPatchMeshWriter.C b/src/meshTools/output/vtk/patch/foamVtkPatchMeshWriter.C
index 52e533adf5..296050fc18 100644
--- a/src/meshTools/output/vtk/patch/foamVtkPatchMeshWriter.C
+++ b/src/meshTools/output/vtk/patch/foamVtkPatchMeshWriter.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2016-2023 OpenCFD Ltd.
+    Copyright (C) 2016-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -87,7 +87,7 @@ void Foam::vtk::patchMeshWriter::writePoints()
 
     this->beginPoints(numberOfPoints_);
 
-    if (parallel_ ? Pstream::master() : true)
+    if (parallel_ ? UPstream::master() : true)
     {
         for (const label patchId : patchIDs_)
         {
@@ -103,14 +103,14 @@ void Foam::vtk::patchMeshWriter::writePoints()
         // Patch Ids are identical across all processes
         const label nPatches = patchIDs_.size();
 
-        if (Pstream::master())
+        if (UPstream::master())
         {
             pointField recv;
 
             // Receive each point field and write
-            for (const int subproci : Pstream::subProcs())
+            for (const int subproci : UPstream::subProcs())
             {
-                IPstream fromProc(Pstream::commsTypes::blocking, subproci);
+                IPstream fromProc(UPstream::commsTypes::scheduled, subproci);
 
                 for (label i=0; i < nPatches; ++i)
                 {
@@ -125,8 +125,8 @@ void Foam::vtk::patchMeshWriter::writePoints()
             // Send each point field
             OPstream toProc
             (
-                Pstream::commsTypes::blocking,
-                Pstream::masterNo()
+                UPstream::commsTypes::scheduled,
+                UPstream::masterNo()
             );
 
             for (const label patchId : patchIDs_)
@@ -523,7 +523,7 @@ void Foam::vtk::patchMeshWriter::writePatchIDs()
 
     this->beginDataArray<label>("patchID", nPolys);
 
-    if (parallel_ ? Pstream::master() : true)
+    if (parallel_ ? UPstream::master() : true)
     {
         for (const label patchId : patchIDs_)
         {
@@ -533,14 +533,14 @@ void Foam::vtk::patchMeshWriter::writePatchIDs()
 
     if (parallel_)
     {
-        if (Pstream::master())
+        if (UPstream::master())
         {
             labelList recv;
 
             // Receive each pair
-            for (const int subproci : Pstream::subProcs())
+            for (const int subproci : UPstream::subProcs())
             {
-                IPstream fromProc(Pstream::commsTypes::blocking, subproci);
+                IPstream fromProc(UPstream::commsTypes::scheduled, subproci);
 
                 fromProc >> recv;
 
@@ -559,8 +559,8 @@ void Foam::vtk::patchMeshWriter::writePatchIDs()
             // Send
             OPstream toProc
             (
-                Pstream::commsTypes::blocking,
-                Pstream::masterNo()
+                UPstream::commsTypes::scheduled,
+                UPstream::masterNo()
             );
 
             // Encode as [size, id] pairs
@@ -595,7 +595,7 @@ bool Foam::vtk::patchMeshWriter::writeProcIDs()
 
 bool Foam::vtk::patchMeshWriter::writeNeighIDs()
 {
-    if (!Pstream::parRun())
+    if (!UPstream::parRun())
     {
         // Skip in non-parallel
         return false;
@@ -626,7 +626,7 @@ bool Foam::vtk::patchMeshWriter::writeNeighIDs()
 
     bool good = false;
 
-    if (parallel_ ? Pstream::master() : true)
+    if (parallel_ ? UPstream::master() : true)
     {
         for (const label patchId : patchIDs_)
         {
@@ -642,14 +642,14 @@ bool Foam::vtk::patchMeshWriter::writeNeighIDs()
 
     if (parallel_)
     {
-        if (Pstream::master())
+        if (UPstream::master())
         {
             labelList recv;
 
             // Receive each pair
-            for (const int subproci : Pstream::subProcs())
+            for (const int subproci : UPstream::subProcs())
             {
-                IPstream fromProc(Pstream::commsTypes::blocking, subproci);
+                IPstream fromProc(UPstream::commsTypes::scheduled, subproci);
 
                 fromProc >> recv;
 
@@ -668,8 +668,8 @@ bool Foam::vtk::patchMeshWriter::writeNeighIDs()
             // Send
             OPstream toProc
             (
-                Pstream::commsTypes::blocking,
-                Pstream::masterNo()
+                UPstream::commsTypes::scheduled,
+                UPstream::masterNo()
             );
 
             // Encode as [size, id] pairs
diff --git a/src/sampling/surface/isoSurface/isoSurfacePoint.C b/src/sampling/surface/isoSurface/isoSurfacePoint.C
index 0c0d0ac5d8..0de39dbcd6 100644
--- a/src/sampling/surface/isoSurface/isoSurfacePoint.C
+++ b/src/sampling/surface/isoSurface/isoSurfacePoint.C
@@ -165,7 +165,12 @@ void Foam::isoSurfacePoint::syncUnseparatedPoints
                 }
 
                 // buffered send
-                OPstream::bsend(patchInfo, procPatch.neighbProcNo());
+                OPstream toNbr
+                (
+                    UPstream::commsTypes::blocking,
+                    procPatch.neighbProcNo()
+                );
+                toNbr << patchInfo;
             }
         }
 
@@ -177,10 +182,10 @@ void Foam::isoSurfacePoint::syncUnseparatedPoints
 
             if (pp.nPoints() && collocatedPatch(pp))
             {
-                pointField nbrPatchInfo;
-
                 // We do not know the number of points on the other side
                 // so cannot use UIPstream::read
+
+                pointField nbrPatchInfo;
                 IPstream::recv(nbrPatchInfo, procPatch.neighbProcNo());
 
                 const labelList& meshPts = procPatch.meshPoints();