ENH: reduce overhead and clearer data ownership for OFstreamCollator

- local data to be written is now transferable into the
  OFstreamCollator. This avoids making a full copy when threading is
  active.

- use plain lists for managing proc data
  * storage: List<List<char>> instead of PtrList<List<char>>
  * views:   List<stdFoam::span<char>> instead of PtrList<SubList<char>>

- use gather/write (unthreaded) as backstop if the output is too big
  to fit in the buffer size. Emit warning instead of FatalError

applications/test/checkIOspeed/Make/files

@@ -0,0 +1,3 @@
+Test-checkIOspeed.cxx
+
+EXE = $(FOAM_USER_APPBIN)/Test-checkIOspeed

applications/test/checkIOspeed/Make/options

@@ -0,0 +1,7 @@
+EXE_INC = \
+    -I$(LIB_SRC)/finiteVolume/lnInclude \
+    -I$(LIB_SRC)/mesh/blockMesh/lnInclude
+
+EXE_LIBS = \
+    -lfiniteVolume \
+    -lblockMesh

applications/test/checkIOspeed/Test-checkIOspeed.cxx

@@ -0,0 +1,400 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2024 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Application
+    Test-checkIOspeed
+
+Description
+    Simple test of file writing, including timings
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "profiling.H"
+#include "clockTime.H"
+
+#include "fileName.H"
+#include "fileOperation.H"
+#include "IOstreams.H"
+#include "OSspecific.H"
+#include "globalIndex.H"
+#include "volFields.H"
+#include "IOField.H"
+#include "PDRblock.H"
+
+// Not really great since CoherentMesh only works with reading!
+#ifdef USE_COHERENT
+#include "OFCstream.H"
+#include "SliceStreamRepo.H"
+#endif
+
+#include <numeric>
+
+using namespace Foam;
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+// Main program:
+
+int main(int argc, char *argv[])
+{
+    argList::addNote("Rewrites fields multiple times");
+
+    argList::noFunctionObjects();  // Disallow function objects
+    argList::noCheckProcessorDirectories();
+
+    argList::addVerboseOption("additional verbosity");
+    argList::addOption
+    (
+        "output",
+        "N",
+        "Begin output iteration (default: 10000)"
+    );
+    argList::addOption
+    (
+        "count",
+        "N",
+        "Number of writes (default: 1)"
+    );
+    argList::addOption
+    (
+        "fields",
+        "N",
+        "Number of fields to write (default: 1)"
+    );
+    argList::addOption
+    (
+        "global",
+        "N",
+        "Global field size"
+    );
+    argList::addOption
+    (
+        "local",
+        "N",
+        "Local fields size (default: 1000)"
+    );
+    argList::addOption
+    (
+        "mesh",
+        "(nx ny nz)",
+        "Create with a mesh"
+    );
+    argList::addOption
+    (
+        "exclude",
+        "(int ... )",
+        "zero-sized on ranks with specified modulo"
+    );
+
+    argList::addBoolOption("coherent", "Force coherent output");
+
+    #include "setRootCase.H"
+    #include "createTime.H"
+
+    const label firstOutput = args.getOrDefault("output", 10000);
+    const label nOutput = args.getOrDefault("count", 1);
+    const label nFields = args.getOrDefault("fields", 1);
+    labelVector meshCells(0, 0, 0);
+
+    const int verbose = args.verbose();
+
+    const bool useCoherent = args.found("coherent");
+
+    labelList excludes;
+    args.readListIfPresent("exclude", excludes);
+
+    bool writeOnProc = true;
+
+    const label myProci = UPstream::myProcNo();
+
+    for (const label excl : excludes)
+    {
+        if (excl > 1 && myProci > 0 && (myProci % excl) == 0)
+        {
+            writeOnProc = false;
+            break;
+        }
+    }
+
+    const label nProcsEff =
+        returnReduce((writeOnProc ? 1 : 0), sumOp<label>());
+
+    Info<< "Output " << nProcsEff
+        << "/" << UPstream::nProcs()
+        << " ranks" << nl;
+
+
+    if (args.readIfPresent("mesh", meshCells))
+    {
+        if (!writeOnProc)
+        {
+            meshCells = Zero;
+        }
+
+        PDRblock block(boundBox(point::zero, point::one), meshCells);
+
+        // Advance time
+        // - coherent currently still needs to read the mesh itself!
+        runTime.setTime(firstOutput, firstOutput);
+
+        IOobject meshIO
+        (
+            polyMesh::defaultRegion,
+            runTime.timeName(),
+            runTime,
+            IOobject::NO_READ,
+            IOobject::NO_WRITE,
+            IOobject::NO_REGISTER
+        );
+
+        autoPtr<polyMesh> pmeshPtr(block.innerMesh(meshIO));
+
+        fvMesh mesh
+        (
+            meshIO,
+            pointField(pmeshPtr->points()),
+            faceList(pmeshPtr->faces()),
+            labelList(pmeshPtr->faceOwner()),
+            labelList(pmeshPtr->faceNeighbour())
+        );
+
+        pmeshPtr.reset(nullptr);
+
+        const label fieldSize = mesh.nCells();
+
+        const globalIndex giCells(fieldSize);
+
+        // Create fields
+        Info<< nl << "Create " << nFields << " fields" << nl
+            << "field-size:" << fieldSize
+            << " total-size:" << giCells.totalSize() << nl;
+
+        // Dimensioned field (no proc boundaries)
+
+        PtrList<volScalarField::Internal> fields(nFields);
+
+        {
+            IOobject io
+            (
+                "field",
+                runTime.timeName(),
+                runTime,
+                IOobject::NO_READ,
+                IOobject::NO_WRITE,
+                IOobject::NO_REGISTER
+            );
+
+            forAll(fields, fieldi)
+            {
+                io.resetHeader("field" + Foam::name(fieldi));
+
+                fields.set
+                (
+                    fieldi,
+                    new volScalarField::Internal(io, mesh, dimless)
+                );
+
+                auto& fld = fields[fieldi];
+                std::iota(fld.begin(), fld.end(), scalar(0));
+            }
+        }
+
+        IOstreamOption streamOpt(IOstreamOption::BINARY);
+
+        if (useCoherent)
+        {
+            #ifdef USE_COHERENT
+            streamOpt.format(IOstreamOption::COHERENT);
+            runTime.writeFormat(IOstreamOption::COHERENT);
+
+            mesh.writeObject(streamOpt, true);
+
+            Info<< nl
+                << "Specified -coherent (instance: "
+                << mesh.pointsInstance() << ")" << endl;
+
+            const auto& coherent = CoherentMesh::New(mesh);
+
+            Info<< "    points  = "
+                << coherent.globalPointOffsets().totalSize() << nl
+                << "    cells   = "
+                << coherent.globalCellOffsets().totalSize() << nl
+                << "    patches = "
+                << coherent.nNonProcessorPatches() << nl;
+
+            #else
+            Info<< "Warning: -coherent ignored" << nl;
+            #endif
+        }
+
+        Info<< nl
+            << "Writing " << nOutput << " times starting at "
+            << firstOutput << nl;
+
+        clockTime timing;
+
+        if (verbose) Info<< "Time:";
+
+        for
+        (
+            label timeIndex = firstOutput, count = 0;
+            count < nOutput;
+            ++timeIndex, ++count
+        )
+        {
+            runTime.setTime(timeIndex, timeIndex);
+            if (verbose) Info<< ' ' << runTime.timeName() << flush;
+            runTime.writeNow();
+
+            for (const auto& fld : fields)
+            {
+                fld.regIOobject::writeObject(streamOpt, writeOnProc);
+            }
+        }
+
+        if (useCoherent)
+        {
+            #ifdef USE_COHERENT
+            SliceStreamRepo::closeInstance();
+            #endif
+        }
+
+        if (verbose) Info<< nl;
+        Info<< nl << "Writing took "
+            << timing.timeIncrement() << "s" << endl;
+
+        Info<< nl
+            << "Cleanup newly generated files with" << nl << nl
+            << "    foamListTimes -rm -time "
+            << firstOutput << ":" << nl
+            << "    foamListTimes -processor -rm -time "
+            << firstOutput << ":" << nl;
+    }
+    else
+    {
+        label fieldSize = 1000;
+
+        if (args.readIfPresent("global", fieldSize))
+        {
+            fieldSize /= nProcsEff;
+        }
+        else
+        {
+            args.readIfPresent("local", fieldSize);
+        }
+
+        if (!writeOnProc)
+        {
+            fieldSize = 0;
+        }
+
+        const globalIndex giCells(fieldSize);
+
+        // Create fields
+        Info<< nl << "Create " << nFields << " fields" << nl
+            << "field-size:" << fieldSize
+            << " total-size:" << giCells.totalSize() << nl;
+
+
+        PtrList<IOField<scalar>> fields(nFields);
+
+        {
+            IOobject io
+            (
+                "field",
+                runTime.timeName(),
+                runTime,
+                IOobject::NO_READ,
+                IOobject::NO_WRITE,
+                IOobject::NO_REGISTER
+            );
+
+            forAll(fields, fieldi)
+            {
+                io.resetHeader("field" + Foam::name(fieldi));
+
+                fields.set
+                (
+                    fieldi,
+                    new IOField<scalar>(io, fieldSize)
+                );
+
+                auto& fld = fields[fieldi];
+                std::iota(fld.begin(), fld.end(), scalar(0));
+            }
+        }
+
+        IOstreamOption streamOpt(IOstreamOption::BINARY);
+
+        if (useCoherent)
+        {
+            Info<< "Warning: -coherent ignored" << nl;
+        }
+
+        Info<< nl
+            << "Writing " << nOutput << " times starting at "
+            << firstOutput << nl;
+
+        clockTime timing;
+
+        if (verbose) Info<< "Time:";
+
+        for
+        (
+            label timeIndex = firstOutput, count = 0;
+            count < nOutput;
+            ++timeIndex, ++count
+        )
+        {
+            runTime.setTime(timeIndex, timeIndex);
+            if (verbose) Info<< ' ' << runTime.timeName() << flush;
+            runTime.writeNow();
+
+            for (const auto& fld : fields)
+            {
+                fld.regIOobject::writeObject(streamOpt, writeOnProc);
+            }
+        }
+
+        if (verbose) Info<< nl;
+        Info<< nl << "Writing took "
+            << timing.timeIncrement() << "s" << endl;
+
+        Info<< nl
+            << "Cleanup newly generated files with" << nl << nl
+            << "    foamListTimes -rm -time "
+            << firstOutput << ":" << nl
+            << "    foamListTimes -processor -rm -time "
+            << firstOutput << ":" << nl;
+    }
+
+
+    Info<< "\nEnd\n" << endl;
+    return 0;
+}
+
+
+// ************************************************************************* //

applications/test/fileHandler-writing/Make/files

@@ -1,3 +1,3 @@
-Test-fileHandler-writing.C
+Test-fileHandler-writing.cxx
 
 EXE = $(FOAM_USER_APPBIN)/Test-fileHandler-writing

applications/test/fileHandler-writing/Test-fileHandler-writing.cxx

@@ -56,8 +56,18 @@ int main(int argc, char *argv[])
 
     argList::noFunctionObjects();  // Disallow function objects
     argList::addVerboseOption("additional verbosity");
-    argList::addOption("output", "Begin output iteration (default: 10000)");
-    argList::addOption("count", "Number of writes (default: 1)");
+    argList::addOption
+    (
+        "output",
+        "N",
+        "Begin output iteration (default: 10000)"
+    );
+    argList::addOption
+    (
+        "count",
+        "N",
+        "Number of writes (default: 1)"
+    );
 
     #include "setRootCase.H"
     #include "createTime.H"

etc/controlDict

@@ -114,7 +114,7 @@ OptimisationSwitches
     //  (sized with magnitude of value) is large enough to hold all
     //  outstanding writes so will not try to initialise the Pstream with
     //  threading support.
-    //  Default: 1e9
+    //  Default: 0
     maxThreadFileBufferSize 0;
 
     //- masterUncollated: non-blocking buffer size.

src/OpenFOAM/db/IOobjects/decomposedBlockData/decomposedBlockData.H

@@ -135,23 +135,15 @@ protected:
 
     // Protected Member Functions
 
-        //- Helper: determine number of processors whose recvSizes fits
-        //- into maxBufferSize
-        static label calcNumProcs
-        (
-            const label comm,
-            const off_t maxBufferSize,
-            const labelUList& recvSizes,
-            const label startProci
-        );
-
-        //- Read data into *this. ISstream is only valid on master.
+        //- Read data (on master) and transmit.
         static bool readBlocks
         (
             const label comm,
+            // [in] The input stream (only valid on master)
             autoPtr<ISstream>& isPtr,
-            List<char>& contentChars,
-            const UPstream::commsTypes commsType
+            // [out] The processor local data
+            List<char>& localData,
+            const UPstream::commsTypes commsType  /* unused */
         );
 
         //- Helper: skip a block of (binary) character data
@@ -277,6 +269,19 @@ public:
         }
 
         //- Helper: write block of (binary) character content
+        //  Housekeeping
+        static std::streamoff writeBlockEntry
+        (
+            OSstream& os,
+            const label blocki,
+            const stdFoam::span<char>& s
+        )
+        {
+            return writeBlockEntry(os, blocki, s.data(), s.size());
+        }
+
+        //- Helper: write block of (binary) character content
+        //  Housekeeping
         static std::streamoff writeBlockEntry
         (
             OSstream& os,
@@ -307,41 +312,37 @@ public:
         );
 
         //- Read master header information (into headerIO) and return
-        //- data in stream. Note: isPtr is only valid on master.
+        //- data in stream.
         static autoPtr<ISstream> readBlocks
         (
             const label comm,
             const fileName& fName,
+            //! [in] The input stream (only valid on master)
             autoPtr<ISstream>& isPtr,
+            //! [out] header information
             IOobject& headerIO,
-            const UPstream::commsTypes commsType
+            const UPstream::commsTypes commsType  /* unused */
         );
 
-        //- Helper: gather single label. Note: using native Pstream.
-        //  datas sized with num procs but undefined contents on
-        //  slaves
-        static void gather
-        (
-            const label comm,
-            const label data,
-            labelList& datas
-        );
-
-        //- Helper: gather data from (subset of) slaves.
+        //- Helper: gather data from (subset of) sub-ranks.
+        //  In non-blocking mode it sets up send/recv for non-empty content.
+        //  In blocking/scheduled mode it uses MPI_Gatherv to collect data.
         //
         //  Returns:
-        //  - recvData : received data
+        //  - recvData : the received data
         //  - recvOffsets : offset in data. recvOffsets is nProcs+1
-        static void gatherSlaveData
+        static void gatherProcData
         (
             const label comm,
-            const UList<char>& data,
-            const labelUList& recvSizes,
+            const UList<char>& localData,   //!< [in] required on all procs
+            const labelUList& recvSizes,    //!< [in] only required on master
 
-            const labelRange& fromProcs,
+            const labelRange& whichProcs,   //!< [in] required on all procs
 
-            List<int>& recvOffsets,
-            DynamicList<char>& recvData
+            List<int>& recvOffsets,         //!< [out] only relevant on master
+            DynamicList<char>& recvData,    //!< [out] only relevant on master
+
+            const UPstream::commsTypes commsType
         );
 
         //- Write *this. Ostream only valid on master.
@@ -349,19 +350,98 @@ public:
         static bool writeBlocks
         (
             const label comm,
+
+            //! [in] output stream (relevant on master)
             autoPtr<OSstream>& osPtr,
+            //! [out] start offsets to each block (relevant on master),
+            //! ignored if List::null() type
             List<std::streamoff>& blockOffset,
 
-            const UList<char>& masterData,
+            const UList<char>& localData,   //!< [in] required on all procs
+            const labelUList& recvSizes,    //!< [in] only required on master
 
-            const labelUList& recvSizes,
-
-            // Optional slave data (on master)
-            const UPtrList<SubList<char>>& slaveData,
+            //! Optional proc data (only written on master)
+            //! but \b must also be symmetrically defined (empty/non-empty)
+            //! on all ranks
+            const UList<stdFoam::span<char>>& procData,
 
             const UPstream::commsTypes commsType,
             const bool syncReturnState = true
         );
+
+
+    // Housekeeping
+
+        //- Write *this. Ostream only valid on master.
+        //  Returns offsets of processor blocks in blockOffset
+        FOAM_DEPRECATED_FOR(2023-09, "write with char span instead")
+        static bool writeBlocks
+        (
+            const label comm,
+            autoPtr<OSstream>& osPtr,
+            List<std::streamoff>& blockOffset,
+
+            const UList<char>& localData, // [in] required on all procs
+            const labelUList& recvSizes,  // [in] only required on master
+
+            // Optional proc data (only written on master)
+            // but \b must also be symmetrically defined (empty/non-empty)
+            // on all ranks
+            const UPtrList<SubList<char>>& procData,
+
+            const UPstream::commsTypes commsType,
+            const bool syncReturnState = true
+        )
+        {
+            // Transcribe to span<char>
+            List<stdFoam::span<char>> spans(procData.size());
+            forAll(procData, proci)
+            {
+                if (procData.test(proci))
+                {
+                    spans[proci] = stdFoam::span<char>
+                    (
+                        const_cast<char*>(procData[proci].cdata()),
+                        procData[proci].size()
+                    );
+                }
+            }
+
+            return decomposedBlockData::writeBlocks
+            (
+                comm,
+                osPtr,
+                blockOffset,
+                localData,
+                recvSizes,
+                spans,
+                commsType,
+                syncReturnState
+            );
+        }
+
+        //- Deprecated(2023-09) - consider UPstream::listGatherValue
+        //  The only difference is that this gather also resizes the output
+        //  on the non-master procs
+        //  \deprecated(2023-09) - consider UPstream::listGatherValue
+        FOAM_DEPRECATED_FOR(2023-09, "consider UPstream::listGatherValue()")
+        static void gather
+        (
+            const label comm,
+            const label localValue,
+            labelList& allValues
+        )
+        {
+            allValues.resize_nocopy(UPstream::nProcs(comm));
+
+            UPstream::mpiGather
+            (
+                reinterpret_cast<const char*>(&localValue),
+                allValues.data_bytes(),
+                sizeof(label),  // The send/recv size per rank
+                comm
+            );
+        }
 };
 
 

src/OpenFOAM/db/IOstreams/Fstreams/masterOFstream.C

@@ -29,7 +29,7 @@ License
 #include "masterOFstream.H"
 #include "OFstream.H"
 #include "OSspecific.H"
-#include "PstreamBuffers.H"
+#include "Pstream.H"
 #include "masterUncollatedFileOperation.H"
 
 // * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
@@ -38,10 +38,10 @@ void Foam::masterOFstream::checkWrite
 (
     const fileName& fName,
     const char* str,
-    std::streamsize len
+    const std::streamsize len
 )
 {
-    if (!len)
+    if (!str || !len)
     {
         // Can probably skip all of this if there is nothing to write
         return;
@@ -63,9 +63,7 @@ void Foam::masterOFstream::checkWrite
             << exit(FatalIOError);
     }
 
-    // Use writeRaw() instead of writeQuoted(string,false) to output
-    // characters directly.
-
+    // Write characters directly to std::ostream
     os.writeRaw(str, len);
 
     if (!os.good())
@@ -77,97 +75,159 @@ void Foam::masterOFstream::checkWrite
 }
 
 
-void Foam::masterOFstream::checkWrite
-(
-    const fileName& fName,
-    const std::string& s
-)
-{
-    checkWrite(fName, s.data(), s.length());
-}
-
-
 void Foam::masterOFstream::commit()
 {
+    // Take ownership of serialized content, without copying or reallocation
+    DynamicList<char> charData(OCharStream::release());
+
     if (UPstream::parRun())
     {
+        // Ignore content if not writing (reduces communication)
+        if (!writeOnProc_)
+        {
+            charData.clear();
+        }
+
         List<fileName> filePaths(UPstream::nProcs(comm_));
         filePaths[UPstream::myProcNo(comm_)] = pathName_;
         Pstream::gatherList(filePaths, UPstream::msgType(), comm_);
 
+        // Test for identical output paths
         bool uniform =
         (
             UPstream::master(comm_)
-         && fileOperation::uniformFile(filePaths)
+          ? fileOperation::uniformFile(filePaths)
+          : true
         );
 
         Pstream::broadcast(uniform, comm_);
 
         if (uniform)
         {
+            // Identical file paths - write on master
             if (UPstream::master(comm_) && writeOnProc_)
             {
-                checkWrite(pathName_, this->str());
+                checkWrite(pathName_, charData);
             }
-
-            this->reset();
             return;
         }
 
         // Different files
-        PstreamBuffers pBufs(comm_, UPstream::commsTypes::nonBlocking);
+        // ---------------
+        // Current strategy is to setup all non-blocking send/recv
+        // using the probed message size to establish the recv size
+        // (to avoid an additional communication of the sizes).
+        //
+        // For ranks with writeOnProc=false, the message size is 0.
 
-        if (!UPstream::master(comm_))
+        // An alternative approach would be to gather recv sizes
+        // to avoid zero-sized messages and/or use double buffering
+        // to recv into a buffer and write.
+        //
+        // const labelList recvSizes
+        // (
+        //     UPstream::listGatherValues<label>
+        //     (
+        //         (UPstream::is_subrank(comm_) ? charData.size() : label(0)),
+        //         comm_
+        //     )
+        // );
+
+        const label startOfRequests = UPstream::nRequests();
+
+        // Some unique tag for this read/write/probe grouping
+        const int messageTag = UPstream::msgType() + 256;
+
+        if (UPstream::is_subrank(comm_))
         {
-            if (writeOnProc_)
-            {
-                // Send buffer to master
-                string s(this->str());
-
-                UOPstream os(UPstream::masterNo(), pBufs);
-                os.write(s.data(), s.length());
-            }
-            this->reset();  // Done with contents
+            // Send to master. When (!writeOnProc_) it is zero-sized.
+            UOPstream::write
+            (
+                UPstream::commsTypes::nonBlocking,
+                UPstream::masterNo(),
+                charData.cdata_bytes(),
+                charData.size_bytes(),
+                messageTag,
+                comm_
+            );
         }
-
-        pBufs.finishedGathers();
-
-
-        if (UPstream::master(comm_))
+        else if (UPstream::master(comm_))
         {
+            // The receive slots
+            List<List<char>> procBuffers(UPstream::nProcs(comm_));
+
+            const auto recvProcs = UPstream::subProcs(comm_);
+
+            for (const int proci : recvProcs)
+            {
+                auto& procSlice = procBuffers[proci];
+
+                // Probe the message size
+                std::pair<int, int> probed =
+                    UPstream::probeMessage
+                    (
+                        UPstream::commsTypes::blocking,
+                        proci,
+                        messageTag,
+                        comm_
+                    );
+
+                procSlice.resize_nocopy(probed.second);
+
+                // Receive content (can also be zero-sized)
+                UIPstream::read
+                (
+                    UPstream::commsTypes::nonBlocking,
+                    proci,
+                    procSlice.data_bytes(),
+                    procSlice.size_bytes(),
+                    messageTag,
+                    comm_
+                );
+            }
+
             if (writeOnProc_)
             {
-                // Write master data
-                checkWrite(filePaths[UPstream::masterNo()], this->str());
+                // Write non-empty master data
+                checkWrite(pathName_, charData);
             }
-            this->reset();  // Done with contents
 
-
-            // Allocate large enough to read without resizing
-            List<char> buf(pBufs.maxRecvCount());
-
-            for (const int proci : UPstream::subProcs(comm_))
+            // Poll for completed receive requests and dispatch
+            DynamicList<int> indices(recvProcs.size());
+            while
+            (
+                UPstream::waitSomeRequests
+                (
+                    startOfRequests,
+                    recvProcs.size(),
+                   &indices
+                )
+            )
             {
-                const std::streamsize count(pBufs.recvDataCount(proci));
-
-                if (count)
+                for (const int idx : indices)
                 {
-                    UIPstream is(proci, pBufs);
+                    const int proci = recvProcs[idx];
+                    auto& procSlice = procBuffers[proci];
 
-                    is.read(buf.data(), count);
-                    checkWrite(filePaths[proci], buf.cdata(), count);
+                    if (!procSlice.empty())
+                    {
+                        // Write non-empty sub-proc data
+                        checkWrite(filePaths[proci], procSlice);
+                    }
+
+                    // Eager cleanup?
+                    // TBD: procSlice.clear();
                 }
             }
         }
+
+        UPstream::waitRequests(startOfRequests);
     }
     else
     {
-        checkWrite(pathName_, this->str());
-        this->reset();
+        // Write (non-empty) data
+        checkWrite(pathName_, charData);
     }
-
-    // This method is only called once (internally)
-    // so no need to clear/flush old buffered data
 }
 
 
@@ -183,7 +243,7 @@ Foam::masterOFstream::masterOFstream
     const bool writeOnProc
 )
 :
-    OStringStream(streamOpt),
+    OCharStream(streamOpt),
     pathName_(pathName),
     atomic_(atomic),
     compression_(streamOpt.compression()),

src/OpenFOAM/db/IOstreams/Fstreams/masterOFstream.H

@@ -41,7 +41,7 @@ SourceFiles
 #ifndef Foam_masterOFstream_H
 #define Foam_masterOFstream_H
 
-#include "StringStream.H"
+#include "SpanStream.H"
 #include "UPstream.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
@@ -55,7 +55,7 @@ namespace Foam
 
 class masterOFstream
 :
-    public OStringStream
+    public OCharStream
 {
     // Private Data
 
@@ -85,13 +85,20 @@ class masterOFstream
         (
             const fileName& fName,
             const char* str,
-            std::streamsize len
+            const std::streamsize len
         );
 
         //- Open file with checking and write append contents
-        void checkWrite(const fileName& fName, const std::string& s);
+        void checkWrite
+        (
+            const fileName& fName,
+            const UList<char>& charData
+        )
+        {
+            checkWrite(fName, charData.cdata(), charData.size_bytes());
+        }
 
-        //- Commit buffered information, including parallel gather as required
+        //- Commit buffered information, including communication as required
         void commit();
 
 

src/OpenFOAM/global/fileOperations/collatedFileOperation/OFstreamCollator.C

@@ -47,9 +47,9 @@ bool Foam::OFstreamCollator::writeFile
     const label comm,
     const word& objectType,
     const fileName& fName,
-    const string& masterData,
+    const UList<char>& localData,
     const labelUList& recvSizes,
-    const UPtrList<SubList<char>>& slaveData,    // optional slave data
+    const UList<stdFoam::span<char>>& procData,  // optional proc data
     IOstreamOption streamOpt,
     IOstreamOption::atomicType atomic,
     IOstreamOption::appendType append,
@@ -58,18 +58,14 @@ bool Foam::OFstreamCollator::writeFile
 {
     if (debug)
     {
-        Pout<< "OFstreamCollator : Writing master " << label(masterData.size())
+        Pout<< "OFstreamCollator : Writing local " << localData.size()
             << " bytes to " << fName << " using comm " << comm
-            << " and " << slaveData.size() << " sub-ranks" << endl;
+            << " and " << procData.size() << " sub-ranks" << endl;
 
-        forAll(slaveData, proci)
+        forAll(procData, proci)
         {
-            if (slaveData.set(proci))
-            {
-                Pout<< "    " << proci
-                    << " size:" << slaveData[proci].size()
-                    << endl;
-            }
+            Pout<< "    " << proci << " size:"
+                << label(procData[proci].size()) << nl;
         }
     }
 
@@ -104,32 +100,27 @@ bool Foam::OFstreamCollator::writeFile
     // for some mpi so default is non-blocking.
     const UPstream::commsTypes myCommsType
     (
+        mag
         (
             fileOperations::masterUncollatedFileOperation::
-                maxMasterFileBufferSize == 0
-        )
+            maxMasterFileBufferSize
+        ) < 1
       ? UPstream::commsTypes::scheduled
       : UPstream::commsTypes::nonBlocking
     );
 
 
-    UList<char> slice
-    (
-        const_cast<char*>(masterData.data()),
-        label(masterData.size())
-    );
-
-    List<std::streamoff> blockOffset;
+    List<std::streamoff> blockOffsets;  // Optional
     decomposedBlockData::writeBlocks
     (
         comm,
         osPtr,
-        blockOffset,
-        slice,
+        blockOffsets,  // or List<std::streamoff>::null()
+        localData,
         recvSizes,
-        slaveData,
+        procData,
         myCommsType,
-        false       // do not reduce return state
+        false       // do not sync return state
     );
 
     if (osPtr && !osPtr->good())
@@ -140,17 +131,18 @@ bool Foam::OFstreamCollator::writeFile
 
     if (debug)
     {
-        Pout<< "OFstreamCollator : Finished writing " << masterData.size()
-            << " bytes";
+        Pout<< "OFstreamCollator : Finished writing "
+            << localData.size() << " bytes";
+
         if (UPstream::master(comm))
         {
-            off_t sum = 0;
+            off_t total = 0;
             for (const label recv : recvSizes)
             {
-                sum += recv;
+                total += recv;
             }
             // Use std::to_string to display long int
-            Pout<< " (overall " << std::to_string(sum) << ')';
+            Pout<< " (overall " << std::to_string(total) << ')';
         }
         Pout<< " to " << fName
             << " using comm " << comm << endl;
@@ -167,13 +159,16 @@ void* Foam::OFstreamCollator::writeAll(void *threadarg)
     // Consume stack
     while (true)
     {
-        writeData* ptr = nullptr;
+        std::unique_ptr<writeData> ptr;
 
         {
             std::lock_guard<std::mutex> guard(handler.mutex_);
+
             if (handler.objects_.size())
             {
-                ptr = handler.objects_.pop();
+                // FIFO
+                ptr.reset(handler.objects_.front());
+                handler.objects_.pop_front();
             }
         }
 
@@ -181,51 +176,39 @@ void* Foam::OFstreamCollator::writeAll(void *threadarg)
         {
             break;
         }
-        else
+
+        writeData& obj = *ptr;
+
+        // Obtain spans from storage
+        List<stdFoam::span<char>> procData(obj.procData_.size());
+        forAll(procData, proci)
         {
-            // Convert storage to pointers
-            PtrList<SubList<char>> slaveData;
-            if (ptr->slaveData_.size())
-            {
-                slaveData.resize(ptr->slaveData_.size());
-                forAll(slaveData, proci)
-                {
-                    if (ptr->slaveData_.set(proci))
-                    {
-                        slaveData.set
-                        (
-                            proci,
-                            new SubList<char>
-                            (
-                                ptr->slaveData_[proci],
-                                ptr->sizes_[proci]
-                            )
-                        );
-                    }
-                }
-            }
-
-            bool ok = writeFile
+            procData[proci] = stdFoam::span<char>
             (
-                ptr->comm_,
-                ptr->objectType_,
-                ptr->pathName_,
-                ptr->data_,
-                ptr->sizes_,
-                slaveData,
-                ptr->streamOpt_,
-                ptr->atomic_,
-                ptr->append_,
-                ptr->headerEntries_
+                const_cast<char*>(obj.procData_[proci].cdata()),
+                obj.procData_[proci].size()
             );
-            if (!ok)
-            {
-                FatalIOErrorInFunction(ptr->pathName_)
-                    << "Failed writing " << ptr->pathName_
-                    << exit(FatalIOError);
-            }
+        }
 
-            delete ptr;
+        bool ok = writeFile
+        (
+            obj.comm_,
+            obj.objectType_,
+            obj.pathName_,
+            obj.localData_,
+            obj.sizes_,
+            procData,
+            obj.streamOpt_,
+            obj.atomic_,
+            obj.append_,
+            obj.headerEntries_
+        );
+
+        if (!ok)
+        {
+            FatalIOErrorInFunction(obj.pathName_)
+                << "Failed writing " << obj.pathName_
+                << exit(FatalIOError);
         }
         //sleep(1);
     }
@@ -248,14 +231,14 @@ void Foam::OFstreamCollator::waitForBufferSpace(const off_t wantedSize) const
 {
     while (true)
     {
-        // Count files to be written
+        // The pending output size(s)
         off_t totalSize = 0;
 
         {
             std::lock_guard<std::mutex> guard(mutex_);
-            forAllConstIters(objects_, iter)
+            for (const writeData* ptr : objects_)
             {
-                totalSize += iter()->size();
+                if (ptr) totalSize += ptr->size();
             }
         }
 
@@ -287,17 +270,7 @@ void Foam::OFstreamCollator::waitForBufferSpace(const off_t wantedSize) const
 
 Foam::OFstreamCollator::OFstreamCollator(const off_t maxBufferSize)
 :
-    maxBufferSize_(maxBufferSize),
-    threadRunning_(false),
-    localComm_(UPstream::worldComm),
-    threadComm_
-    (
-        UPstream::allocateCommunicator
-        (
-            localComm_,
-            labelRange(UPstream::nProcs(localComm_))
-        )
-    )
+    OFstreamCollator(maxBufferSize, UPstream::worldComm)
 {}
 
 
@@ -312,6 +285,7 @@ Foam::OFstreamCollator::OFstreamCollator
     localComm_(comm),
     threadComm_
     (
+        // dupComm
         UPstream::allocateCommunicator
         (
             localComm_,
@@ -345,7 +319,7 @@ bool Foam::OFstreamCollator::write
 (
     const word& objectType,
     const fileName& fName,
-    const string& data,
+    DynamicList<char>&& localData,
     IOstreamOption streamOpt,
     IOstreamOption::atomicType atomic,
     IOstreamOption::appendType append,
@@ -355,78 +329,109 @@ bool Foam::OFstreamCollator::write
 {
     // Determine (on master) sizes to receive. Note: do NOT use thread
     // communicator
-    labelList recvSizes;
-    decomposedBlockData::gather(localComm_, label(data.size()), recvSizes);
+    const labelList recvSizes
+    (
+        UPstream::listGatherValues<label>(localData.size(), localComm_)
+    );
 
     off_t totalSize = 0;
     label maxLocalSize = 0;
+
+    if (UPstream::master(localComm_))
     {
-        if (UPstream::master(localComm_))
+        for (const label recvSize : recvSizes)
         {
-            for (const label recvSize : recvSizes)
-            {
-                totalSize += recvSize;
-                maxLocalSize = max(maxLocalSize, recvSize);
-            }
+            totalSize += recvSize;
+            maxLocalSize = max(maxLocalSize, recvSize);
         }
-        Pstream::broadcasts(localComm_, totalSize, maxLocalSize);
     }
+    Pstream::broadcasts(localComm_, totalSize, maxLocalSize);
+
+
+    // Determine how things will be gathered and written...
+
+    enum class dispatchModes { GATHER_WRITE, PREFETCH_THREADED, FULL_THREADED };
+
+    dispatchModes dispatch(dispatchModes::GATHER_WRITE);
 
     if (!useThread || maxBufferSize_ == 0 || maxLocalSize > maxBufferSize_)
+    {
+        dispatch = dispatchModes::GATHER_WRITE;
+    }
+    else if (totalSize <= maxBufferSize_)
+    {
+        // Total size can be stored locally
+        // - gather all data now and only do the writing in the thread
+
+        dispatch = dispatchModes::PREFETCH_THREADED;
+    }
+    else
+    {
+        // Gather data and write in the thread
+
+        dispatch = dispatchModes::FULL_THREADED;
+
+        if (!UPstream::haveThreads())
+        {
+            WarningInFunction
+                << "MPI not initialized with thread support." << nl
+                << "    maxThreadFileBufferSize = 0 to disable threading" << nl
+                << " or maxThreadFileBufferSize > " << totalSize
+                << " to collate before threaded writing." << nl << nl;
+
+            dispatch = dispatchModes::GATHER_WRITE;
+        }
+    }
+
+
+    // -----------
+    // Dispatching
+    // -----------
+
+    if (dispatch == dispatchModes::GATHER_WRITE)
     {
         if (debug)
         {
-            Pout<< "OFstreamCollator : non-thread gather and write of " << fName
-                << " using local comm " << localComm_ << endl;
+            Pout<< "OFstreamCollator : non-thread gather/write "
+                << "(local comm: " << localComm_ << ") of "
+                << fName << endl;
         }
+
         // Direct collating and writing (so master blocks until all written!)
-        const PtrList<SubList<char>> dummySlaveData;
         return writeFile
         (
             localComm_,
             objectType,
             fName,
-            data,
+            localData,
             recvSizes,
-            dummySlaveData,
+            UList<stdFoam::span<char>>(),  // dummy proc data
             streamOpt,
             atomic,
             append,
             headerEntries
         );
     }
-    else if (totalSize <= maxBufferSize_)
+    else if (dispatch == dispatchModes::PREFETCH_THREADED)
     {
-        // Total size can be stored locally so receive all data now and only
-        // do the writing in the thread
-
         if (debug)
         {
             Pout<< "OFstreamCollator : non-thread gather; thread write of "
                 << fName << endl;
         }
 
-        if (Pstream::master(localComm_))
+        if (UPstream::master(localComm_))
         {
             waitForBufferSpace(totalSize);
         }
 
-
-        // Receive in chunks of labelMax (2^31-1) since this is the maximum
-        // size that a List can be
-
-        autoPtr<writeData> fileAndDataPtr
+        std::unique_ptr<writeData> fileAndDataPtr
         (
             new writeData
             (
                 threadComm_,        // Note: comm not actually used anymore
                 objectType,
                 fName,
-                (
-                    Pstream::master(localComm_)
-                  ? data            // Only used on master
-                  : string::null
-                ),
                 recvSizes,
                 streamOpt,
                 atomic,
@@ -434,63 +439,81 @@ bool Foam::OFstreamCollator::write
                 headerEntries
             )
         );
-        writeData& fileAndData = fileAndDataPtr();
+        auto& fileAndData = *fileAndDataPtr;
 
-        PtrList<List<char>>& slaveData = fileAndData.slaveData_;
+        List<List<char>>& procData = fileAndData.procData_;
+        if (UPstream::master(localComm_))
+        {
+            // Move in local data (master only!)
+            fileAndData.transfer(localData);
 
-        UList<char> slice(const_cast<char*>(data.data()), label(data.size()));
+            // Storage for receive data
+            procData.resize(UPstream::nProcs(localComm_));
+
+            for (const int proci : UPstream::subProcs(localComm_))
+            {
+                procData[proci].resize(recvSizes[proci]);
+            }
+        }
+        else if (UPstream::is_subrank(localComm_))
+        {
+            // Requires a size for decomposedBlockData::writeBlocks() logic
+            procData.resize(UPstream::nProcs(localComm_));
+        }
 
-        slaveData.setSize(recvSizes.size());
 
         // Gather all data onto master. Is done in local communicator since
-        // not in write thread. Note that we do not store in contiguous
-        // buffer since that would limit to 2G chars.
+        // not in write thread.
         const label startOfRequests = UPstream::nRequests();
-        if (Pstream::master(localComm_))
+        if (UPstream::master(localComm_))
         {
-            for (label proci = 1; proci < slaveData.size(); proci++)
+            for (const int proci : UPstream::subProcs(localComm_))
             {
-                slaveData.set(proci, new List<char>(recvSizes[proci]));
+                List<char>& procSlice = procData[proci];
+                if (procSlice.empty()) continue;
+
                 UIPstream::read
                 (
                     UPstream::commsTypes::nonBlocking,
                     proci,
-                    slaveData[proci].data(),
-                    slaveData[proci].size_bytes(),
-                    Pstream::msgType(),
+                    procSlice.data_bytes(),
+                    procSlice.size_bytes(),
+                    UPstream::msgType(),
                     localComm_
                 );
             }
         }
-        else
+        else if (UPstream::is_subrank(localComm_) && !localData.empty())
         {
             if
             (
                !UOPstream::write
                 (
                     UPstream::commsTypes::nonBlocking,
-                    0,
-                    slice.cdata(),
-                    slice.size_bytes(),
-                    Pstream::msgType(),
+                    UPstream::masterNo(),
+                    localData.cdata_bytes(),
+                    localData.size_bytes(),
+                    UPstream::msgType(),
                     localComm_
                 )
             )
             {
                 FatalErrorInFunction
-                    << "Cannot send outgoing message. "
-                    << "to:" << 0 << " nBytes:"
-                    << label(slice.size_bytes())
+                    << "Cannot send outgoing message (size: "
+                    << localData.size() << ") to master" << nl
                     << Foam::abort(FatalError);
             }
         }
         UPstream::waitRequests(startOfRequests);
 
+        // The localData has been moved (master) or communicated
+        localData.clearStorage();
+
         {
             std::lock_guard<std::mutex> guard(mutex_);
 
-            // Append to thread buffer
-            objects_.push(fileAndDataPtr.ptr());
+            // Append to thread buffer (as FIFO), take ownership
+            objects_.push_back(fileAndDataPtr.release());
 
             // Start thread if not running
             if (!threadRunning_)
@@ -517,49 +540,46 @@ bool Foam::OFstreamCollator::write
 
         return true;
     }
-    else
+    else if (dispatch == dispatchModes::FULL_THREADED)
     {
         if (debug)
         {
-            Pout<< "OFstreamCollator : thread gather and write of " << fName
-                << " using communicator " << threadComm_ << endl;
+            Pout<< "OFstreamCollator : thread gather and write "
+                << "(thread comm: " << threadComm_
+                << ") of " << fName << endl;
         }
 
-        if (!UPstream::haveThreads())
+        if (UPstream::master(localComm_))
         {
-            FatalErrorInFunction
-                << "mpi does not seem to have thread support."
-                << " Make sure to set buffer size 'maxThreadFileBufferSize'"
-                << " to at least " << totalSize
-                << " to be able to do the collating before threading."
-                << exit(FatalError);
+            waitForBufferSpace(localData.size());
         }
 
-        if (Pstream::master(localComm_))
-        {
-            waitForBufferSpace(data.size());
-        }
+        std::unique_ptr<writeData> fileAndDataPtr
+        (
+            new writeData
+            (
+                threadComm_,
+                objectType,
+                fName,
+                recvSizes,
+                streamOpt,
+                atomic,
+                append,
+                headerEntries
+            )
+        );
+
+        // Move in local data (all procs)
+        fileAndDataPtr->transfer(localData);
 
         {
             std::lock_guard<std::mutex> guard(mutex_);
 
-            // Push all file info on buffer. Note that no slave data provided
+            // Append to thread buffer (as FIFO), take ownership
+            objects_.push_back(fileAndDataPtr.release());
+
+            // Note: no proc data provided
             // so it will trigger communication inside the thread
-            objects_.push
-            (
-                new writeData
-                (
-                    threadComm_,
-                    objectType,
-                    fName,
-                    data,
-                    recvSizes,
-                    streamOpt,
-                    atomic,
-                    append,
-                    headerEntries
-                )
-            );
 
             if (!threadRunning_)
             {
@@ -584,6 +604,12 @@ bool Foam::OFstreamCollator::write
 
         return true;
     }
+
+    FatalErrorInFunction
+        << "Unknown dispatch mode: " << int(dispatch)
+        << " - programming error?" << abort(FatalError);
+
+    return false;
 }
 
 
@@ -591,7 +617,7 @@ void Foam::OFstreamCollator::waitAll()
 {
     // Wait for all buffer space to be available i.e. wait for all jobs
     // to finish
-    if (Pstream::master(localComm_))
+    if (UPstream::master(localComm_))
     {
         if (debug)
         {

src/OpenFOAM/global/fileOperations/collatedFileOperation/OFstreamCollator.H

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2017-2018 OpenFOAM Foundation
-    Copyright (C) 2021-2022 OpenCFD Ltd.
+    Copyright (C) 2019-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -51,14 +51,14 @@ SourceFiles
 #ifndef Foam_OFstreamCollator_H
 #define Foam_OFstreamCollator_H
 
-#include <thread>
-#include <mutex>
 #include "IOstream.H"
-#include "labelList.H"
-#include "FIFOStack.H"
-#include "SubList.H"
+#include "List.H"
+#include "CircularBuffer.H"  // As FIFO
 #include "dictionary.H"
 
+#include <mutex>
+#include <thread>
+
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 namespace Foam
@@ -72,26 +72,33 @@ class OFstreamCollator
 {
     // Private Class
 
+        //- Holds data to be written
         struct writeData
         {
             const label comm_;
             const word objectType_;
             const fileName pathName_;
-            const string data_;
+            DynamicList<char> localData_;
             const labelList sizes_;
-            PtrList<List<char>> slaveData_;
+            List<List<char>> procData_;
             const IOstreamOption streamOpt_;
             IOstreamOption::atomicType atomic_;
             IOstreamOption::appendType append_;
             const dictionary headerEntries_;
 
+            writeData() = delete;                      // No default construct
+            writeData(const writeData&) = delete;      // No copy construct
+            writeData(writeData&&) = delete;           // No move construct
+            void operator=(const writeData&) = delete; // No copy assign
+            void operator=(writeData&&) = delete;      // No move assign
+
+            //- Construct without local data
             writeData
             (
                 const label comm,
                 const word& objectType,
                 const fileName& pathName,
-                const string& data,
-                const labelList& sizes,
+                const labelUList& sizes,
                 IOstreamOption streamOpt,
                 IOstreamOption::atomicType atomic,
                 IOstreamOption::appendType append,
@@ -101,27 +108,30 @@ class OFstreamCollator
                 comm_(comm),
                 objectType_(objectType),
                 pathName_(pathName),
-                data_(data),
+                localData_(),
                 sizes_(sizes),
-                slaveData_(),
+                procData_(),
                 streamOpt_(streamOpt),
                 atomic_(atomic),
                 append_(append),
                 headerEntries_(headerEntries)
             {}
 
-            //- The (approximate) size of master + any optional slave data
+            //- Move reset local data
+            void transfer(DynamicList<char>& localData)
+            {
+                localData_.transfer(localData);
+            }
+
+            //- The (approximate) size of local + any optional proc data
             off_t size() const
             {
-                off_t totalSize = data_.size();
-                forAll(slaveData_, i)
+                off_t total = localData_.size();
+                for (const auto& data : procData_)
                 {
-                    if (slaveData_.set(i))
-                    {
-                        totalSize += slaveData_[i].size();
-                    }
+                    total += data.size();
                 }
-                return totalSize;
+                return total;
             }
         };
 
@@ -135,8 +145,8 @@ class OFstreamCollator
 
         std::unique_ptr<std::thread> thread_;
 
-        //- Stack of files to write + contents
-        FIFOStack<writeData*> objects_;
+        //- FIFO of files to write and their contents
+        CircularBuffer<writeData*> objects_;
 
         //- Whether thread is running (and not exited)
         bool threadRunning_;
@@ -156,9 +166,9 @@ class OFstreamCollator
             const label comm,
             const word& objectType,
             const fileName& fName,
-            const string& masterData,
+            const UList<char>& localData,
             const labelUList& recvSizes,
-            const UPtrList<SubList<char>>& slaveData,
+            const UList<stdFoam::span<char>>& procData,
             IOstreamOption streamOpt,
             IOstreamOption::atomicType atomic,
             IOstreamOption::appendType append,
@@ -181,7 +191,8 @@ public:
 
     // Constructors
 
-        //- Construct from buffer size. 0 = do not use thread
+        //- Construct from buffer size (0 = do not use thread)
+        //- and with worldComm
         explicit OFstreamCollator(const off_t maxBufferSize);
 
         //- Construct from buffer size (0 = do not use thread)
@@ -195,14 +206,15 @@ public:
 
     // Member Functions
 
-        //- Write file with contents.
-        //  Blocks until writethread has space available
+        //- Write file with contents, possibly taking ownership of the
+        //- content.
+        //  Blocks until write-thread has space available
         //  (total file sizes < maxBufferSize)
         bool write
         (
             const word& objectType,
-            const fileName&,
-            const string& data,
+            const fileName& fName,
+            DynamicList<char>&& localData,
             IOstreamOption streamOpt,
             IOstreamOption::atomicType atomic,
             IOstreamOption::appendType append,
@@ -210,6 +222,37 @@ public:
             const dictionary& headerEntries = dictionary::null
         );
 
+        //- Write file with contents.
+        FOAM_DEPRECATED_FOR(2023-09, "use write with movable content")
+        bool write
+        (
+            const word& objectType,
+            const fileName& fName,
+            const std::string& s,
+            IOstreamOption streamOpt,
+            IOstreamOption::atomicType atomic,
+            IOstreamOption::appendType append,
+            const bool useThread = true,
+            const dictionary& headerEntries = dictionary::null
+        )
+        {
+            DynamicList<char> charData;
+            charData.setCapacity(s.size());
+            std::copy(s.begin(), s.end(), charData.begin());
+
+            return write
+            (
+                objectType,
+                fName,
+                std::move(charData),
+                streamOpt,
+                atomic,
+                append,
+                useThread,
+                headerEntries
+            );
+        }
+
         //- Wait for all thread actions to have finished
         void waitAll();
 };

src/OpenFOAM/global/fileOperations/collatedFileOperation/collatedFileOperation.C

@@ -59,7 +59,7 @@ namespace fileOperations
 
     float collatedFileOperation::maxThreadFileBufferSize
     (
-        debug::floatOptimisationSwitch("maxThreadFileBufferSize", 1e9)
+        debug::floatOptimisationSwitch("maxThreadFileBufferSize", 0)
     );
     registerOptSwitch
     (
@@ -90,26 +90,41 @@ void Foam::fileOperations::collatedFileOperation::printBanner
     DetailInfo
         << "I/O    : " << this->type();
 
-    if (maxThreadFileBufferSize == 0)
-    {
-        DetailInfo
-            << " [unthreaded] (maxThreadFileBufferSize = 0)." << nl
-            << "         Writing may be slow for large file sizes."
-            << endl;
-    }
-    else
+    if (mag(maxThreadFileBufferSize) > 1)
     {
+        // FUTURE: deprecate or remove threading?
         DetailInfo
             << " [threaded] (maxThreadFileBufferSize = "
             << maxThreadFileBufferSize << ")." << nl
             << "         Requires buffer large enough to collect all data"
-               " or thread support" << nl
-            << "         enabled in MPI. If MPI thread support cannot be"
-               " enabled, deactivate" << nl
-            << "         threading by setting maxThreadFileBufferSize"
-               " to 0 in" << nl
+               " or MPI thread support." << nl
+            << "         To avoid MPI threading [slow], set"
+               " (maxThreadFileBufferSize = 0) in" << nl
             << "         OpenFOAM etc/controlDict" << endl;
     }
+    else
+    {
+        DetailInfo
+            << " [unthreaded] (maxThreadFileBufferSize = 0)." << nl;
+
+        if (mag(maxMasterFileBufferSize) < 1)
+        {
+            DetailInfo
+                << "         With scheduled transfer" << nl;
+        }
+        else if (maxMasterFileBufferSize >= 1)
+        {
+            DetailInfo
+                << "         With non-blocking transfer,"
+                " buffer-size = " << maxMasterFileBufferSize << nl;
+        }
+        else
+        {
+            DetailInfo
+                << "         With non-blocking transfer,"
+                   " minimal buffer size" << nl;
+        }
+    }
 
     if (withRanks)
     {
@@ -455,7 +470,7 @@ bool Foam::fileOperations::collatedFileOperation::writeObject
         {
             // Re-check static maxThreadFileBufferSize variable to see
             // if needs to use threading
-            const bool useThread = (maxThreadFileBufferSize != 0);
+            const bool useThread = (mag(maxThreadFileBufferSize) > 1);
 
             if (debug)
             {

src/OpenFOAM/global/fileOperations/collatedFileOperation/threadedCollatedOFstream.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2017-2018 OpenFOAM Foundation
-    Copyright (C) 2020-2022 OpenCFD Ltd.
+    Copyright (C) 2020-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -41,7 +41,7 @@ Foam::threadedCollatedOFstream::threadedCollatedOFstream
     const bool useThread
 )
 :
-    OStringStream(streamOpt),
+    OCharStream(streamOpt),
     writer_(writer),
     pathName_(pathName),
     atomic_(atomic),
@@ -74,11 +74,22 @@ Foam::threadedCollatedOFstream::threadedCollatedOFstream
 
 Foam::threadedCollatedOFstream::~threadedCollatedOFstream()
 {
+    commit();
+}
+
+
+// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
+
+void Foam::threadedCollatedOFstream::commit()
+{
+    // Take ownership of serialized content, without copying or reallocation
+    DynamicList<char> charData(OCharStream::release());
+
     writer_.write
     (
         decomposedBlockData::typeName,
         pathName_,
-        str(),
+        std::move(charData),
         IOstreamOption(IOstreamOption::BINARY, version(), compression_),
         atomic_,
         IOstreamOption::NON_APPEND,

src/OpenFOAM/global/fileOperations/collatedFileOperation/threadedCollatedOFstream.H

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2017-2018 OpenFOAM Foundation
-    Copyright (C) 2021-2022 OpenCFD Ltd.
+    Copyright (C) 2021-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -39,7 +39,7 @@ SourceFiles
 #define Foam_threadedCollatedOFstream_H
 
 #include "dictionary.H"
-#include "StringStream.H"
+#include "SpanStream.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
@@ -55,7 +55,7 @@ class OFstreamCollator;
 
 class threadedCollatedOFstream
 :
-    public OStringStream
+    public OCharStream
 {
     // Private Data
 
@@ -78,6 +78,11 @@ class threadedCollatedOFstream
         dictionary headerEntries_;
 
 
+    // Private Member Functions
+
+        //- Commit buffered information
+        void commit();
+
 public:
 
     // Constructors
@@ -102,12 +107,14 @@ public:
         );
 
 
-    //- Destructor
+    //- Destructor - commits buffered information to file
     ~threadedCollatedOFstream();
 
 
     // Member Functions
 
+        // -> using OCharStream::rewind
+
         //- Define the header entries for the data block(s)
         void setHeaderEntries(const dictionary& dict);
 };