diff --git a/applications/test/parallel-chunks/Test-parallel-chunks.C b/applications/test/parallel-chunks/Test-parallel-chunks.C
index 1dc4cb7380..36a6cc63c8 100644
--- a/applications/test/parallel-chunks/Test-parallel-chunks.C
+++ b/applications/test/parallel-chunks/Test-parallel-chunks.C
@@ -210,7 +210,7 @@ void broadcast_chunks
             << "Contiguous data only." << sizeof(T) << Foam::abort(FatalError);
     }
 
-    if (UPstream::maxCommsSize <= int(sizeof(T)))
+    if (UPstream::maxCommsSize <= 0)
     {
         // Do in one go
         Info<< "send " << sendData.size() << " elements in one go" << endl;
@@ -230,48 +230,61 @@ void broadcast_chunks
     // guaranteed that some processor's sending size is some other
     // processor's receive size. Also we can ignore any local comms.
 
-    // We need to send bytes so the number of iterations:
+    // We need to send chunks so the number of iterations:
     //  maxChunkSize                        iterations
     //  ------------                        ----------
     //  0                                   0
     //  1..maxChunkSize                     1
     //  maxChunkSize+1..2*maxChunkSize      2
-    //      etc.
+    //  ...
 
-    const label maxChunkSize(UPstream::maxCommsSize/sizeof(T));
+    const label maxChunkSize
+    (
+        max
+        (
+            static_cast<label>(1),
+            static_cast<label>(UPstream::maxCommsSize/sizeof(T))
+        )
+    );
 
-    label nIter(0);
+    label nChunks(0);
     {
-        label nSendMax = 0;
+        // Get max send count (elements)
         // forAll(sendBufs, proci)
         // {
         //     if (proci != Pstream::myProcNo(comm))
         //     {
-        //         nSendMax = max(nSendMax, sendBufs[proci].size());
+        //         nChunks = max(nChunks, sendBufs[proci].size());
         //     }
         // }
-        nSendMax = sendSize;
+        nChunks = sendSize;
 
-        if (nSendMax)
+        // Convert from send count (elements) to number of chunks.
+        // Can normally calculate with (count-1), but add some safety
+        if (nChunks)
         {
-            nIter = 1 + ((nSendMax-1)/maxChunkSize);
+            nChunks = 1 + (nChunks/maxChunkSize);
         }
-        reduce(nIter, maxOp<label>(), tag, comm);
+        reduce(nChunks, maxOp<label>(), tag, comm);
 
         Info
-            << "send " << nSendMax << " elements ("
-            << (nSendMax*sizeof(T)) << " bytes) in " << nIter
-            << " iterations of " << maxChunkSize << " chunks ("
+            << "send " << sendSize << " elements ("
+            << (sendSize*sizeof(T)) << " bytes) in " << nChunks
+            << " chunks of " << maxChunkSize << " elements ("
             << (maxChunkSize*sizeof(T)) << " bytes) for maxCommsSize:"
             << Pstream::maxCommsSize
             << endl;
     }
 
+    // stress-test with shortened sendSize
+    // will produce useless loops, but no calls
+    // sendSize /= 2;
+
     label nSend(0);
     label startSend(0);
     char* charPtrSend;
 
-    for (label iter = 0; iter < nIter; ++iter)
+    for (label iter = 0; iter < nChunks; ++iter)
     {
         nSend = min
         (
@@ -297,6 +310,8 @@ void broadcast_chunks
             startSend += nSend;
         }
     }
+
+    Info<< "final: " << startSend << endl;
 }
 
 
@@ -305,8 +320,9 @@ void broadcast_chunks
 int main(int argc, char *argv[])
 {
     argList::noCheckProcessorDirectories();
+    argList::addOption("comms-size", "int", "override Pstream::maxCommsSize");
+
     #include "setRootCase.H"
-    #include "createTime.H"
 
     if (!Pstream::parRun())
     {
@@ -322,6 +338,9 @@ int main(int argc, char *argv[])
     broadcast_chunks<labelList, label>(input1);
 
     Pstream::maxCommsSize = 33;
+
+    args.readIfPresent("comms-size", Pstream::maxCommsSize);
+
     broadcast_chunks<labelList, label>(input1);
 
     // Mostly the same with PstreamBuffers
diff --git a/applications/utilities/parallelProcessing/redistributePar/redistributePar.C b/applications/utilities/parallelProcessing/redistributePar/redistributePar.C
index cca7e2d2c0..8c49938be0 100644
--- a/applications/utilities/parallelProcessing/redistributePar/redistributePar.C
+++ b/applications/utilities/parallelProcessing/redistributePar/redistributePar.C
@@ -999,16 +999,16 @@ void correctCoupledBoundaryConditions(fvMesh& mesh)
             const lduSchedule& patchSchedule =
                 fld.mesh().globalData().patchSchedule();
 
-            forAll(patchSchedule, patchEvali)
+            for (const auto& schedEval : patchSchedule)
             {
-                const label patchi = patchSchedule[patchEvali].patch;
+                const label patchi = schedEval.patch;
                 const auto& fvp = mesh.boundary()[patchi];
                 auto& pfld = bfld[patchi];
 
                 const auto* ppPtr = isA<CoupledPatchType>(fvp);
                 if (ppPtr && ppPtr->coupled())
                 {
-                    if (patchSchedule[patchEvali].init)
+                    if (schedEval.init)
                     {
                         pfld.initEvaluate(Pstream::commsTypes::scheduled);
                     }
diff --git a/applications/utilities/preProcessing/mapFieldsPar/MapVolFields.H b/applications/utilities/preProcessing/mapFieldsPar/MapVolFields.H
index 6a3c1ffcc3..f2b787754d 100644
--- a/applications/utilities/preProcessing/mapFieldsPar/MapVolFields.H
+++ b/applications/utilities/preProcessing/mapFieldsPar/MapVolFields.H
@@ -95,9 +95,10 @@ void evaluateConstraintTypes(GeometricField<Type, fvPatchField, volMesh>& fld)
         const lduSchedule& patchSchedule =
             fld.mesh().globalData().patchSchedule();
 
-        forAll(patchSchedule, patchEvali)
+        for (const auto& schedEval : patchSchedule)
         {
-            label patchi = patchSchedule[patchEvali].patch;
+            const label patchi = schedEval.patch;
+
             fvPatchField<Type>& tgtField = fldBf[patchi];
 
             if
@@ -106,7 +107,7 @@ void evaluateConstraintTypes(GeometricField<Type, fvPatchField, volMesh>& fld)
              && polyPatch::constraintType(tgtField.patch().patch().type())
             )
             {
-                if (patchSchedule[patchEvali].init)
+                if (schedEval.init)
                 {
                     tgtField.initEvaluate(Pstream::commsTypes::scheduled);
                 }
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchange.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchange.C
index 359ca29081..7ffb09e3d1 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchange.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchange.C
@@ -224,7 +224,7 @@ void Foam::Pstream::exchange
             }
         }
 
-        if (UPstream::maxCommsSize <= int(sizeof(T)))
+        if (UPstream::maxCommsSize <= 0)
         {
             // Do the exchanging in one go
             exchangeContainer<Container, T>
@@ -244,38 +244,41 @@ void Foam::Pstream::exchange
             // guaranteed that some processor's sending size is some other
             // processor's receive size. Also we can ignore any local comms.
 
-            // We need to send bytes so the number of iterations:
+            // We need to send chunks so the number of iterations:
             //  maxChunkSize                        iterations
             //  ------------                        ----------
             //  0                                   0
             //  1..maxChunkSize                     1
             //  maxChunkSize+1..2*maxChunkSize      2
-            //      etc.
+            //  ...
 
-            const label maxChunkSize(UPstream::maxCommsSize/sizeof(T));
+            const label maxChunkSize
+            (
+                max
+                (
+                    static_cast<label>(1),
+                    static_cast<label>(UPstream::maxCommsSize/sizeof(T))
+                )
+            );
 
-            label nIter(0);
+            label nChunks(0);
             {
-                label nSendMax = 0;
+                // Get max send count (elements)
                 forAll(sendBufs, proci)
                 {
                     if (proci != Pstream::myProcNo(comm))
                     {
-                        nSendMax = max(nSendMax, sendBufs[proci].size());
+                        nChunks = max(nChunks, sendBufs[proci].size());
                     }
                 }
 
-                if (nSendMax)
+                // Convert from send count (elements) to number of chunks.
+                // Can normally calculate with (count-1), but add some safety
+                if (nChunks)
                 {
-                    nIter = 1 + ((nSendMax-1)/maxChunkSize);
+                    nChunks = 1 + (nChunks/maxChunkSize);
                 }
-                reduce(nIter, maxOp<label>(), tag, comm);
-
-                /// Info<< "send " << nSendMax << " elements ("
-                ///     << (nSendMax*sizeof(T)) << " bytes) in " << nIter
-                ///     << " iterations of " << maxChunkSize << " chunks ("
-                ///     << (maxChunkSize*sizeof(T)) << " bytes) maxCommsSize:"
-                ///     << Pstream::maxCommsSize << endl;
+                reduce(nChunks, maxOp<label>(), tag, comm);
             }
 
             labelList nRecv(sendBufs.size());
@@ -286,7 +289,7 @@ void Foam::Pstream::exchange
             List<const char*> charPtrSend(sendBufs.size());
             List<char*> charPtrRecv(sendBufs.size());
 
-            for (label iter = 0; iter < nIter; ++iter)
+            for (label iter = 0; iter < nChunks; ++iter)
             {
                 forAll(sendBufs, proci)
                 {
diff --git a/src/OpenFOAM/fields/GeometricFields/GeometricField/GeometricBoundaryField.C b/src/OpenFOAM/fields/GeometricFields/GeometricField/GeometricBoundaryField.C
index 06b3d473d4..87d48080c4 100644
--- a/src/OpenFOAM/fields/GeometricFields/GeometricField/GeometricBoundaryField.C
+++ b/src/OpenFOAM/fields/GeometricFields/GeometricField/GeometricBoundaryField.C
@@ -440,16 +440,18 @@ void Foam::GeometricField<Type, PatchField, GeoMesh>::Boundary::evaluate()
         const lduSchedule& patchSchedule =
             bmesh_.mesh().globalData().patchSchedule();
 
-        forAll(patchSchedule, patchEvali)
+        for (const auto& schedEval : patchSchedule)
         {
-            if (patchSchedule[patchEvali].init)
+            const label patchi = schedEval.patch;
+
+            if (schedEval.init)
             {
-                this->operator[](patchSchedule[patchEvali].patch)
+                this->operator[](patchi)
                     .initEvaluate(Pstream::commsTypes::scheduled);
             }
             else
             {
-                this->operator[](patchSchedule[patchEvali].patch)
+                this->operator[](patchi)
                     .evaluate(Pstream::commsTypes::scheduled);
             }
         }
diff --git a/src/OpenFOAM/matrices/LduMatrix/LduMatrix/LduMatrixUpdateMatrixInterfaces.C b/src/OpenFOAM/matrices/LduMatrix/LduMatrix/LduMatrixUpdateMatrixInterfaces.C
index d549f2e73a..64dd444721 100644
--- a/src/OpenFOAM/matrices/LduMatrix/LduMatrix/LduMatrixUpdateMatrixInterfaces.C
+++ b/src/OpenFOAM/matrices/LduMatrix/LduMatrix/LduMatrixUpdateMatrixInterfaces.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2019 OpenCFD Ltd.
+    Copyright (C) 2019-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -148,13 +148,13 @@ void Foam::LduMatrix<Type, DType, LUType>::updateMatrixInterfaces
         const lduSchedule& patchSchedule = this->patchSchedule();
 
         // Loop over all the "normal" interfaces relating to standard patches
-        forAll(patchSchedule, i)
+        for (const auto& schedEval : patchSchedule)
         {
-            label interfacei = patchSchedule[i].patch;
+            const label interfacei = schedEval.patch;
 
             if (interfaces_.set(interfacei))
             {
-                if (patchSchedule[i].init)
+                if (schedEval.init)
                 {
                     interfaces_[interfacei].initInterfaceMatrixUpdate
                     (
diff --git a/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/GAMGProcAgglomerations/GAMGProcAgglomeration/GAMGProcAgglomeration.C b/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/GAMGProcAgglomerations/GAMGProcAgglomeration/GAMGProcAgglomeration.C
index 47681853c0..7d311fcd7f 100644
--- a/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/GAMGProcAgglomerations/GAMGProcAgglomeration/GAMGProcAgglomeration.C
+++ b/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/GAMGProcAgglomerations/GAMGProcAgglomeration/GAMGProcAgglomeration.C
@@ -126,17 +126,16 @@ Foam::labelListList Foam::GAMGProcAgglomeration::globalCellCells
     const lduAddressing& addr = mesh.lduAddr();
     lduInterfacePtrsList interfaces = mesh.interfaces();
 
-    const label myProcID = Pstream::myProcNo(mesh.comm());
+    const label myProcID = UPstream::myProcNo(mesh.comm());
 
-    globalIndex globalNumbering
+    const globalIndex globalNumbering
     (
         addr.size(),
-        Pstream::msgType(),
         mesh.comm(),
-        Pstream::parRun()
+        UPstream::parRun()
     );
 
-    labelList globalIndices
+    const labelList globalIndices
     (
         identity
         (
@@ -163,9 +162,9 @@ Foam::labelListList Foam::GAMGProcAgglomeration::globalCellCells
             }
         }
 
-        if (Pstream::parRun())
+        if (UPstream::parRun())
         {
-            Pstream::waitRequests(nReq);
+            UPstream::waitRequests(nReq);
         }
 
         forAll(interfaces, inti)
diff --git a/src/OpenFOAM/meshes/pointMesh/pointBoundaryMesh/pointBoundaryMesh.C b/src/OpenFOAM/meshes/pointMesh/pointBoundaryMesh/pointBoundaryMesh.C
index ef3bf29267..7f7ea57cd7 100644
--- a/src/OpenFOAM/meshes/pointMesh/pointBoundaryMesh/pointBoundaryMesh.C
+++ b/src/OpenFOAM/meshes/pointMesh/pointBoundaryMesh/pointBoundaryMesh.C
@@ -93,8 +93,8 @@ void Foam::pointBoundaryMesh::calcGeometry()
 
     if
     (
-        Pstream::defaultCommsType == Pstream::commsTypes::blocking
-     || Pstream::defaultCommsType == Pstream::commsTypes::nonBlocking
+        pBufs.commsType() == Pstream::commsTypes::blocking
+     || pBufs.commsType() == Pstream::commsTypes::nonBlocking
     )
     {
         forAll(*this, patchi)
@@ -109,18 +109,18 @@ void Foam::pointBoundaryMesh::calcGeometry()
             operator[](patchi).calcGeometry(pBufs);
         }
     }
-    else if (Pstream::defaultCommsType == Pstream::commsTypes::scheduled)
+    else if (pBufs.commsType() == Pstream::commsTypes::scheduled)
     {
         const lduSchedule& patchSchedule = mesh().globalData().patchSchedule();
 
         // Dummy.
         pBufs.finishedSends();
 
-        forAll(patchSchedule, patchEvali)
+        for (const auto& schedEval : patchSchedule)
         {
-            label patchi = patchSchedule[patchEvali].patch;
+            const label patchi = schedEval.patch;
 
-            if (patchSchedule[patchEvali].init)
+            if (schedEval.init)
             {
                 operator[](patchi).initGeometry(pBufs);
             }
@@ -139,8 +139,8 @@ void Foam::pointBoundaryMesh::movePoints(const pointField& p)
 
     if
     (
-        Pstream::defaultCommsType == Pstream::commsTypes::blocking
-     || Pstream::defaultCommsType == Pstream::commsTypes::nonBlocking
+        pBufs.commsType() == Pstream::commsTypes::blocking
+     || pBufs.commsType() == Pstream::commsTypes::nonBlocking
     )
     {
         forAll(*this, patchi)
@@ -155,18 +155,18 @@ void Foam::pointBoundaryMesh::movePoints(const pointField& p)
             operator[](patchi).movePoints(pBufs, p);
         }
     }
-    else if (Pstream::defaultCommsType == Pstream::commsTypes::scheduled)
+    else if (pBufs.commsType() == Pstream::commsTypes::scheduled)
     {
         const lduSchedule& patchSchedule = mesh().globalData().patchSchedule();
 
         // Dummy.
         pBufs.finishedSends();
 
-        forAll(patchSchedule, patchEvali)
+        for (const auto& schedEval : patchSchedule)
         {
-            label patchi = patchSchedule[patchEvali].patch;
+            const label patchi = schedEval.patch;
 
-            if (patchSchedule[patchEvali].init)
+            if (schedEval.init)
             {
                 operator[](patchi).initMovePoints(pBufs, p);
             }
@@ -185,8 +185,8 @@ void Foam::pointBoundaryMesh::updateMesh()
 
     if
     (
-        Pstream::defaultCommsType == Pstream::commsTypes::blocking
-     || Pstream::defaultCommsType == Pstream::commsTypes::nonBlocking
+        pBufs.commsType() == Pstream::commsTypes::blocking
+     || pBufs.commsType() == Pstream::commsTypes::nonBlocking
     )
     {
         forAll(*this, patchi)
@@ -201,18 +201,18 @@ void Foam::pointBoundaryMesh::updateMesh()
             operator[](patchi).updateMesh(pBufs);
         }
     }
-    else if (Pstream::defaultCommsType == Pstream::commsTypes::scheduled)
+    else if (pBufs.commsType() == Pstream::commsTypes::scheduled)
     {
         const lduSchedule& patchSchedule = mesh().globalData().patchSchedule();
 
         // Dummy.
         pBufs.finishedSends();
 
-        forAll(patchSchedule, patchEvali)
+        for (const auto& schedEval : patchSchedule)
         {
-            label patchi = patchSchedule[patchEvali].patch;
+            const label patchi = schedEval.patch;
 
-            if (patchSchedule[patchEvali].init)
+            if (schedEval.init)
             {
                 operator[](patchi).initUpdateMesh(pBufs);
             }
diff --git a/src/OpenFOAM/meshes/polyMesh/polyBoundaryMesh/polyBoundaryMesh.C b/src/OpenFOAM/meshes/polyMesh/polyBoundaryMesh/polyBoundaryMesh.C
index 64431fabe5..6c52daaa22 100644
--- a/src/OpenFOAM/meshes/polyMesh/polyBoundaryMesh/polyBoundaryMesh.C
+++ b/src/OpenFOAM/meshes/polyMesh/polyBoundaryMesh/polyBoundaryMesh.C
@@ -267,8 +267,8 @@ void Foam::polyBoundaryMesh::calcGeometry()
 
     if
     (
-        Pstream::defaultCommsType == Pstream::commsTypes::blocking
-     || Pstream::defaultCommsType == Pstream::commsTypes::nonBlocking
+        pBufs.commsType() == Pstream::commsTypes::blocking
+     || pBufs.commsType() == Pstream::commsTypes::nonBlocking
     )
     {
         forAll(*this, patchi)
@@ -283,18 +283,18 @@ void Foam::polyBoundaryMesh::calcGeometry()
             operator[](patchi).calcGeometry(pBufs);
         }
     }
-    else if (Pstream::defaultCommsType == Pstream::commsTypes::scheduled)
+    else if (pBufs.commsType() == Pstream::commsTypes::scheduled)
     {
         const lduSchedule& patchSchedule = mesh().globalData().patchSchedule();
 
         // Dummy.
         pBufs.finishedSends();
 
-        for (const auto& patchEval : patchSchedule)
+        for (const auto& schedEval : patchSchedule)
         {
-            const label patchi = patchEval.patch;
+            const label patchi = schedEval.patch;
 
-            if (patchEval.init)
+            if (schedEval.init)
             {
                 operator[](patchi).initGeometry(pBufs);
             }
@@ -1120,8 +1120,8 @@ void Foam::polyBoundaryMesh::movePoints(const pointField& p)
 
     if
     (
-        Pstream::defaultCommsType == Pstream::commsTypes::blocking
-     || Pstream::defaultCommsType == Pstream::commsTypes::nonBlocking
+        pBufs.commsType() == Pstream::commsTypes::blocking
+     || pBufs.commsType() == Pstream::commsTypes::nonBlocking
     )
     {
         forAll(*this, patchi)
@@ -1136,18 +1136,18 @@ void Foam::polyBoundaryMesh::movePoints(const pointField& p)
             operator[](patchi).movePoints(pBufs, p);
         }
     }
-    else if (Pstream::defaultCommsType == Pstream::commsTypes::scheduled)
+    else if (pBufs.commsType() == Pstream::commsTypes::scheduled)
     {
         const lduSchedule& patchSchedule = mesh().globalData().patchSchedule();
 
         // Dummy.
         pBufs.finishedSends();
 
-        for (const auto& patchEval : patchSchedule)
+        for (const auto& schedEval : patchSchedule)
         {
-            const label patchi = patchEval.patch;
+            const label patchi = schedEval.patch;
 
-            if (patchEval.init)
+            if (schedEval.init)
             {
                 operator[](patchi).initMovePoints(pBufs, p);
             }
@@ -1170,8 +1170,8 @@ void Foam::polyBoundaryMesh::updateMesh()
 
     if
     (
-        Pstream::defaultCommsType == Pstream::commsTypes::blocking
-     || Pstream::defaultCommsType == Pstream::commsTypes::nonBlocking
+        pBufs.commsType() == Pstream::commsTypes::blocking
+     || pBufs.commsType() == Pstream::commsTypes::nonBlocking
     )
     {
         forAll(*this, patchi)
@@ -1186,18 +1186,18 @@ void Foam::polyBoundaryMesh::updateMesh()
             operator[](patchi).updateMesh(pBufs);
         }
     }
-    else if (Pstream::defaultCommsType == Pstream::commsTypes::scheduled)
+    else if (pBufs.commsType() == Pstream::commsTypes::scheduled)
     {
         const lduSchedule& patchSchedule = mesh().globalData().patchSchedule();
 
         // Dummy.
         pBufs.finishedSends();
 
-        for (const auto& patchEval : patchSchedule)
+        for (const auto& schedEval : patchSchedule)
         {
-            const label patchi = patchEval.patch;
+            const label patchi = schedEval.patch;
 
-            if (patchEval.init)
+            if (schedEval.init)
             {
                 operator[](patchi).initUpdateMesh(pBufs);
             }
diff --git a/src/OpenFOAM/parallel/globalIndex/globalIndex.C b/src/OpenFOAM/parallel/globalIndex/globalIndex.C
index 9604f38c85..969e7602e0 100644
--- a/src/OpenFOAM/parallel/globalIndex/globalIndex.C
+++ b/src/OpenFOAM/parallel/globalIndex/globalIndex.C
@@ -188,12 +188,6 @@ void Foam::globalIndex::bin
 }
 
 
-void Foam::globalIndex::reset(const label localSize)
-{
-    reset(localSize, Pstream::msgType(), UPstream::worldComm, true);
-}
-
-
 void Foam::globalIndex::reset
 (
     const label localSize,
@@ -209,25 +203,27 @@ void Foam::globalIndex::reset
 void Foam::globalIndex::reset
 (
     const label localSize,
-    const int tag,
     const label comm,
     const bool parallel
 )
 {
+    labelList localLens;
+
     const label len = Pstream::nProcs(comm);
 
     if (len)
     {
-        // Seed with localSize, zero elsewhere (for non-parallel branch)
-        // NB: can consider UPstream::listGatherValues
-
-        labelList localLens(len, Zero);
-        localLens[Pstream::myProcNo(comm)] = localSize;
-
-        if (parallel)
+        if (parallel && UPstream::parRun())
         {
-            Pstream::gatherList(localLens, tag, comm);
-            Pstream::scatterList(localLens, tag, comm);
+            localLens = UPstream::listGatherValues(localSize, comm);
+            Pstream::broadcast(localLens, comm);
+        }
+        else
+        {
+            // Non-parallel branch: use localSize on-proc, zero elsewhere
+
+            localLens.resize(len, Zero);
+            localLens[Pstream::myProcNo(comm)] = localSize;
         }
 
         reset(localLens, true);  // checkOverflow = true
diff --git a/src/OpenFOAM/parallel/globalIndex/globalIndex.H b/src/OpenFOAM/parallel/globalIndex/globalIndex.H
index b46866bf57..a32c2d08ee 100644
--- a/src/OpenFOAM/parallel/globalIndex/globalIndex.H
+++ b/src/OpenFOAM/parallel/globalIndex/globalIndex.H
@@ -127,9 +127,14 @@ public:
             enum accessType accType
         );
 
-        //- Construct from local size.
-        //  Communication with default communicator and message tag.
-        inline explicit globalIndex(const label localSize);
+        //- Construct from local size, using gather/broadcast
+        //- with default/specified communicator if parallel.
+        inline explicit globalIndex
+        (
+            const label localSize,
+            const label comm = UPstream::worldComm,   //!< communicator
+            const bool parallel = UPstream::parRun()  //!< use parallel comms
+        );
 
         //- Construct by gathering local sizes without rescattering.
         //- This 'one-sided' globalIndex will be empty on non-master processes.
@@ -148,18 +153,7 @@ public:
         (
             const label localSize,
             const globalIndex::gatherNone,
-            const label comm = -1   //!< dummy communicator
-        );
-
-        //- Construct from local size.
-        //  Communication with given communicator and message tag,
-        //  unless parallel == false
-        inline globalIndex
-        (
-            const label localSize,
-            const int tag,          //!< message tag
-            const label comm,       //!< communicator
-            const bool parallel     //!< use parallel comms
+            const label comm = -1   //!< no communicator needed
         );
 
         //- Construct from Istream.
@@ -211,9 +205,14 @@ public:
         //- Write-access to the offsets, for changing after construction
         inline labelList& offsets() noexcept;
 
-        //- Reset from local size.
-        //  Does communication with default communicator and message tag.
-        void reset(const label localSize);
+        //- Reset from local size, using gather/broadcast
+        //- with default/specified communicator if parallel.
+        void reset
+        (
+            const label localSize,
+            const label comm = UPstream::worldComm,   //!< communicator
+            const bool parallel = UPstream::parRun()  //!< use parallel comms
+        );
 
         //- Reset by gathering local sizes without rescattering.
         //- This 'one-sided' globalIndex will be empty on non-master processes.
@@ -226,23 +225,12 @@ public:
             const label comm = UPstream::worldComm  //!< communicator
         );
 
-        //- Reset from local size.
-        //  Does communication with given communicator and message tag,
-        //  unless parallel == false
-        void reset
-        (
-            const label localSize,
-            const int tag,          //!< message tag
-            const label comm,       //!< communicator
-            const bool parallel     //!< use parallel comms
-        );
-
         //- Reset from list of local sizes,
         //- with optional check for label overflow.
         //- No communication required
         void reset
         (
-            const labelUList& sizes,
+            const labelUList& localLens,
             const bool checkOverflow = false
         );
 
@@ -328,15 +316,6 @@ public:
             inline label whichProcID(const label i) const;
 
 
-    // Housekeeping
-
-            //- Same as localStart
-            label offset(const label proci) const
-            {
-                return localStart(proci);
-            }
-
-
     // Iteration
 
         //- Forward input iterator with const access
@@ -818,6 +797,41 @@ public:
 
         friend Istream& operator>>(Istream& is, globalIndex& gi);
         friend Ostream& operator<<(Ostream& os, const globalIndex& gi);
+
+
+    // Housekeeping
+
+        //- Construct from local size, using gather/broadcast
+        //- with default/specified communicator if parallel.
+        FOAM_DEPRECATED_FOR(2022-03, "construct without message tag")
+        globalIndex
+        (
+            const label localSize,
+            const int tag,          // message tag (unused)
+            const label comm,       // communicator
+            const bool parallel     // use parallel comms
+        )
+        {
+            reset(localSize, comm, parallel);
+        }
+
+        //- Reset from local size, using gather/broadcast
+        //- with default/specified communicator if parallel.
+        FOAM_DEPRECATED_FOR(2022-03, "reset without message tag")
+        void reset
+        (
+            const label localSize,
+            const int tag,          // message tag (unused)
+            const label comm,       // communicator
+            const bool parallel     // use parallel comms
+        )
+        {
+            reset(localSize, comm, parallel);
+        }
+
+        //- Prefer localStart() to avoid confusing with offsets()
+        FOAM_DEPRECATED_FOR(2022-02, "use localStart()")
+        label offset(const label proci) const { return localStart(proci); }
 };
 
 
diff --git a/src/OpenFOAM/parallel/globalIndex/globalIndexI.H b/src/OpenFOAM/parallel/globalIndex/globalIndexI.H
index 99cd557486..4adb07cc0e 100644
--- a/src/OpenFOAM/parallel/globalIndex/globalIndexI.H
+++ b/src/OpenFOAM/parallel/globalIndex/globalIndexI.H
@@ -77,9 +77,14 @@ inline Foam::globalIndex::globalIndex
 }
 
 
-inline Foam::globalIndex::globalIndex(const label localSize)
+inline Foam::globalIndex::globalIndex
+(
+    const label localSize,
+    const label comm,
+    const bool parallel
+)
 {
-    reset(localSize);
+    reset(localSize, comm, parallel);
 }
 
 
@@ -109,18 +114,6 @@ inline Foam::globalIndex::globalIndex
 }
 
 
-inline Foam::globalIndex::globalIndex
-(
-    const label localSize,
-    const int tag,
-    const label comm,
-    const bool parallel
-)
-{
-    reset(localSize, tag, comm, parallel);
-}
-
-
 // * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
 
 inline bool Foam::globalIndex::empty() const
diff --git a/src/Pstream/mpi/UPstream.C b/src/Pstream/mpi/UPstream.C
index 8b2763bdd2..a1cd9c391f 100644
--- a/src/Pstream/mpi/UPstream.C
+++ b/src/Pstream/mpi/UPstream.C
@@ -296,7 +296,7 @@ bool Foam::UPstream::init(int& argc, char**& argv, const bool needsThread)
         wordList worlds(numprocs);
         worlds[Pstream::myProcNo()] = world;
         Pstream::gatherList(worlds);
-        Pstream::scatterList(worlds);
+        Pstream::broadcast(worlds);
 
         // Compact
         if (Pstream::master())
@@ -315,8 +315,8 @@ bool Foam::UPstream::init(int& argc, char**& argv, const bool needsThread)
                 worldIDs_[proci] = allWorlds_.find(world);
             }
         }
-        Pstream::scatter(allWorlds_);
-        Pstream::scatter(worldIDs_);
+        Pstream::broadcast(allWorlds_);
+        Pstream::broadcast(worldIDs_);
 
         DynamicList<label> subRanks;
         forAll(worlds, proci)
diff --git a/src/Pstream/mpi/allReduce.H b/src/Pstream/mpi/allReduce.H
index 11639312e3..9b4c3fc857 100644
--- a/src/Pstream/mpi/allReduce.H
+++ b/src/Pstream/mpi/allReduce.H
@@ -54,8 +54,9 @@ namespace Foam
 namespace PstreamDetail
 {
 
+// MPI_Bcast, using root=0
 template<class Type>
-void allBroadcast
+void broadcast0
 (
     Type* values,
     int count,
@@ -63,6 +64,18 @@ void allBroadcast
     const label communicator
 );
 
+// MPI_Reduce, using root=0
+template<class Type>
+void reduce0
+(
+    Type* values,
+    int count,
+    MPI_Datatype datatype,
+    MPI_Op optype,
+    const label communicator
+);
+
+// MPI_Allreduce
 template<class Type>
 void allReduce
 (
@@ -73,6 +86,8 @@ void allReduce
     const label communicator
 );
 
+
+// MPI_Iallreduce
 template<class Type>
 void iallReduce
 (
diff --git a/src/Pstream/mpi/allReduceTemplates.C b/src/Pstream/mpi/allReduceTemplates.C
index 1649bd4b76..c370d88135 100644
--- a/src/Pstream/mpi/allReduceTemplates.C
+++ b/src/Pstream/mpi/allReduceTemplates.C
@@ -33,7 +33,7 @@ License
 // * * * * * * * * * * * * * * * Global Functions  * * * * * * * * * * * * * //
 
 template<class Type>
-void Foam::PstreamDetail::allBroadcast
+void Foam::PstreamDetail::broadcast0
 (
     Type* values,
     int count,
@@ -62,6 +62,55 @@ void Foam::PstreamDetail::allBroadcast
 }
 
 
+template<class Type>
+void Foam::PstreamDetail::reduce0
+(
+    Type* values,
+    int count,
+    MPI_Datatype datatype,
+    MPI_Op optype,
+    const label communicator
+)
+{
+    if (!UPstream::parRun())
+    {
+        return;
+    }
+
+    if (UPstream::warnComm != -1 && communicator != UPstream::warnComm)
+    {
+        Pout<< "** reducing:";
+        if (count == 1)
+        {
+            Pout<< (*values);
+        }
+        else
+        {
+            Pout<< UList<Type>(values, count);
+        }
+        Pout<< " with comm:" << communicator
+            << " warnComm:" << UPstream::warnComm << endl;
+        error::printStack(Pout);
+    }
+
+    profilingPstream::beginTiming();
+
+    // const int retval =
+    MPI_Reduce
+    (
+        MPI_IN_PLACE,
+        values,
+        count,
+        datatype,
+        optype,
+        0,  // (root process) is master == UPstream::masterNo()
+        PstreamGlobals::MPICommunicators_[communicator]
+    );
+
+    profilingPstream::addReduceTime();
+}
+
+
 template<class Type>
 void Foam::PstreamDetail::allReduce
 (
diff --git a/src/dynamicMesh/motionSmoother/motionSmootherAlgo.C b/src/dynamicMesh/motionSmoother/motionSmootherAlgo.C
index e30bfde962..7356791427 100644
--- a/src/dynamicMesh/motionSmoother/motionSmootherAlgo.C
+++ b/src/dynamicMesh/motionSmoother/motionSmootherAlgo.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2018 OpenCFD Ltd.
+    Copyright (C) 2018-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -423,16 +423,16 @@ void Foam::motionSmootherAlgo::setDisplacementPatchFields
     // fixedValue bc's first.
     labelHashSet adaptPatchSet(patchIDs);
 
-    const lduSchedule& patchSchedule = displacement.mesh().globalData().
-        patchSchedule();
+    const lduSchedule& patchSchedule =
+        displacement.mesh().globalData().patchSchedule();
 
-    forAll(patchSchedule, patchEvalI)
+    for (const auto& schedEval : patchSchedule)
     {
-        const label patchi = patchSchedule[patchEvalI].patch;
+        const label patchi = schedEval.patch;
 
         if (!adaptPatchSet.found(patchi))
         {
-            if (patchSchedule[patchEvalI].init)
+            if (schedEval.init)
             {
                 displacementBf[patchi]
                     .initEvaluate(Pstream::commsTypes::scheduled);
@@ -575,17 +575,16 @@ void Foam::motionSmootherAlgo::correctBoundaryConditions
 
     const lduSchedule& patchSchedule = mesh_.globalData().patchSchedule();
 
-    pointVectorField::Boundary& displacementBf =
-        displacement.boundaryFieldRef();
+    auto& displacementBf = displacement.boundaryFieldRef();
 
     // 1. evaluate on adaptPatches
-    forAll(patchSchedule, patchEvalI)
+    for (const auto& schedEval : patchSchedule)
     {
-        const label patchi = patchSchedule[patchEvalI].patch;
+        const label patchi = schedEval.patch;
 
         if (adaptPatchSet.found(patchi))
         {
-            if (patchSchedule[patchEvalI].init)
+            if (schedEval.init)
             {
                 displacementBf[patchi]
                     .initEvaluate(Pstream::commsTypes::blocking);
@@ -600,13 +599,13 @@ void Foam::motionSmootherAlgo::correctBoundaryConditions
 
 
     // 2. evaluate on non-AdaptPatches
-    forAll(patchSchedule, patchEvalI)
+    for (const auto& schedEval : patchSchedule)
     {
-        const label patchi = patchSchedule[patchEvalI].patch;
+        const label patchi = schedEval.patch;
 
         if (!adaptPatchSet.found(patchi))
         {
-            if (patchSchedule[patchEvalI].init)
+            if (schedEval.init)
             {
                 displacementBf[patchi]
                     .initEvaluate(Pstream::commsTypes::blocking);
diff --git a/src/functionObjects/field/mapFields/mapFieldsTemplates.C b/src/functionObjects/field/mapFields/mapFieldsTemplates.C
index 2d1f66322a..54d268270e 100644
--- a/src/functionObjects/field/mapFields/mapFieldsTemplates.C
+++ b/src/functionObjects/field/mapFields/mapFieldsTemplates.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2016-2020 OpenCFD Ltd.
+    Copyright (C) 2016-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -91,9 +91,10 @@ void Foam::functionObjects::mapFields::evaluateConstraintTypes
         const lduSchedule& patchSchedule =
             fld.mesh().globalData().patchSchedule();
 
-        forAll(patchSchedule, patchEvali)
+        for (const auto& schedEval : patchSchedule)
         {
-            label patchi = patchSchedule[patchEvali].patch;
+            const label patchi = schedEval.patch;
+
             fvPatchField<Type>& tgtField = fldBf[patchi];
 
             if
@@ -102,7 +103,7 @@ void Foam::functionObjects::mapFields::evaluateConstraintTypes
              && polyPatch::constraintType(tgtField.patch().patch().type())
             )
             {
-                if (patchSchedule[patchEvali].init)
+                if (schedEval.init)
                 {
                     tgtField.initEvaluate(Pstream::commsTypes::scheduled);
                 }
diff --git a/src/parallel/decompose/decompositionMethods/decompositionMethod/decompositionMethod.C b/src/parallel/decompose/decompositionMethods/decompositionMethod/decompositionMethod.C
index b801e146b2..c591103428 100644
--- a/src/parallel/decompose/decompositionMethods/decompositionMethod/decompositionMethod.C
+++ b/src/parallel/decompose/decompositionMethods/decompositionMethod/decompositionMethod.C
@@ -480,13 +480,7 @@ void Foam::decompositionMethod::calcCellCells
     // Create global cell numbers
     // ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-    const globalIndex globalAgglom
-    (
-        nLocalCoarse,
-        Pstream::msgType(),
-        Pstream::worldComm,
-        parallel
-    );
+    const globalIndex globalAgglom(nLocalCoarse, Pstream::worldComm, parallel);
 
 
     // Get agglomerate owner on other side of coupled faces
@@ -679,13 +673,7 @@ void Foam::decompositionMethod::calcCellCells
     // Create global cell numbers
     // ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-    const globalIndex globalAgglom
-    (
-        nLocalCoarse,
-        Pstream::msgType(),
-        Pstream::worldComm,
-        parallel
-    );
+    const globalIndex globalAgglom(nLocalCoarse, Pstream::worldComm, parallel);
 
 
     // Get agglomerate owner on other side of coupled faces