diff --git a/etc/controlDict b/etc/controlDict
index ea88edf7d7..d605346302 100644
--- a/etc/controlDict
+++ b/etc/controlDict
@@ -1,7 +1,7 @@
 /*--------------------------------*- C++ -*----------------------------------*\
 | =========                 |                                                 |
 | \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
-|  \\    /   O peration     | Version:  v2212                                 |
+|  \\    /   O peration     | Version:  v2306                                 |
 |   \\  /    A nd           | Website:  www.openfoam.com                      |
 |    \\/     M anipulation  |                                                 |
 \*---------------------------------------------------------------------------*/
@@ -146,6 +146,10 @@ OptimisationSwitches
     // global reduction, even if multi-pass is not needed)
     maxCommsSize    0;
 
+    // Optional (quite experimental) feature in lduMatrixUpdate
+    // to poll (processor) interfaces for individual readiness
+    // instead of waiting for all to complete first.
+    nPollProcInterfaces 0;
 
     // Trap floating point exception.
     // Can override with FOAM_SIGFPE env variable (true|false)
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
index eb4a485340..820ef017d7 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
@@ -499,56 +499,69 @@ public:
         );
 
 
-        // Non-blocking comms
+    // Requests (non-blocking comms)
 
-            //- Number of outstanding requests
-            static label nRequests() noexcept;
+        //- Number of outstanding requests (on the internal list of requests)
+        static label nRequests() noexcept;
 
-            //- Truncate outstanding requests to given length, which is
-            //- expected to be in the range 0 to nRequests.
-            //  A no-op for out-of-range values.
-            static void resetRequests(const label n);
+        //- Truncate outstanding requests to given length, which is
+        //- expected to be in the range [0 to nRequests()].
+        //  A no-op for out-of-range values.
+        static void resetRequests(const label n);
 
-            //- Wait until all requests (from position onwards) have finished.
-            //  A no-op if parRun() == false, if there are no pending requests
-            //  or if the start is out-of-range (0 to nRequests)
-            static void waitRequests(const label pos);
+        //- Wait until all requests (from position onwards) have finished.
+        //- Corresponds to MPI_Waitall()
+        //  A no-op if parRun() == false,
+        //  if the position is out-of-range [0 to nRequests()],
+        //  or the internal list of requests is empty.
+        //
+        //  \param pos starting position within the internal list of requests
+        static void waitRequests(const label pos);
 
-            //- Wait until all requests have finished.
-            //  A no-op if parRun() == false or the list is empty
-            static void waitRequests(UList<UPstream::Request>& requests);
+        //- Wait until all requests have finished.
+        //- Corresponds to MPI_Waitall()
+        //  A no-op if parRun() == false, or the list is empty.
+        static void waitRequests(UList<UPstream::Request>& requests);
 
-            //- Wait until any request has finished and return its index.
-            //  Returns -1 if parRun() == false, or the list is empty,
-            //  or if all the requests have already been handled
-            static label waitAnyRequest(UList<UPstream::Request>& requests);
+        //- Wait until any request has finished and return its index.
+        //- Corresponds to MPI_Waitany()
+        //  Returns -1 if parRun() == false, or the list is empty,
+        //  or if all the requests have already been handled
+        static label waitAnyRequest(UList<UPstream::Request>& requests);
 
-            //- Wait until request i has finished.
-            //  A no-op if parRun() == false,
-            //  there are no pending requests,
-            //  or if the index is out-of-range (0 to nRequests)
-            static void waitRequest(const label i);
+        //- Wait until request i has finished.
+        //- Corresponds to MPI_Wait()
+        //  A no-op if parRun() == false,
+        //  there are no pending requests,
+        //  or if the index is out-of-range (0 to nRequests)
+        static void waitRequest(const label i);
 
-            //- Wait until specified request has finished.
-            //  A no-op if parRun() == false or for a null-request
-            static void waitRequest(UPstream::Request& req);
+        //- Wait until specified request has finished.
+        //- Corresponds to MPI_Wait()
+        //  A no-op if parRun() == false or for a null-request
+        static void waitRequest(UPstream::Request& req);
 
-            //- Non-blocking comms: has request i finished?
-            //  A no-op and returns true if parRun() == false,
-            //  there are no pending requests,
-            //  or if the index is out-of-range (0 to nRequests)
-            static bool finishedRequest(const label i);
+        //- Non-blocking comms: has request i finished?
+        //- Corresponds to MPI_Test()
+        //  A no-op and returns true if parRun() == false,
+        //  there are no pending requests,
+        //  or if the index is out-of-range (0 to nRequests)
+        static bool finishedRequest(const label i);
 
-            //- Non-blocking comms: has request finished?
-            //  A no-op and returns true if parRun() == false
-            //  or for a null-request
-            static bool finishedRequest(UPstream::Request& req);
+        //- Non-blocking comms: has request finished?
+        //- Corresponds to MPI_Test()
+        //  A no-op and returns true if parRun() == false
+        //  or for a null-request
+        static bool finishedRequest(UPstream::Request& req);
 
-            //- Non-blocking comms: have all requests finished?
-            //  A no-op and returns true if parRun() == false or list is empty
-            static bool finishedRequests(UList<UPstream::Request>& requests);
+        //- Non-blocking comms: have all requests finished?
+        //- Corresponds to MPI_Testall()
+        //  A no-op and returns true if parRun() == false or list is empty
+        static bool finishedRequests(UList<UPstream::Request>& requests);
 
 
+    // General
+
         //- Set as parallel run on/off.
         //  \return the previous value
         static bool parRun(const bool on) noexcept
diff --git a/src/Pstream/mpi/PstreamGlobals.C b/src/Pstream/mpi/PstreamGlobals.C
index 601d371331..5fd81abae9 100644
--- a/src/Pstream/mpi/PstreamGlobals.C
+++ b/src/Pstream/mpi/PstreamGlobals.C
@@ -36,7 +36,6 @@ Foam::DynamicList<MPI_Comm> Foam::PstreamGlobals::MPICommunicators_;
 Foam::DynamicList<MPI_Group> Foam::PstreamGlobals::MPIGroups_;
 
 Foam::DynamicList<MPI_Request> Foam::PstreamGlobals::outstandingRequests_;
-Foam::DynamicList<Foam::label> Foam::PstreamGlobals::freedRequests_;
 
 
 // * * * * * * * * * * * * * * * Global Functions  * * * * * * * * * * * * * //
diff --git a/src/Pstream/mpi/PstreamGlobals.H b/src/Pstream/mpi/PstreamGlobals.H
index 1edb623a1a..276f189328 100644
--- a/src/Pstream/mpi/PstreamGlobals.H
+++ b/src/Pstream/mpi/PstreamGlobals.H
@@ -29,7 +29,7 @@ Namespace
 
 Description
     Global functions and variables for working with parallel streams,
-    but principally for mpi
+    but principally for MPI.
 
 SourceFiles
     PstreamGlobals.C
@@ -40,6 +40,7 @@ SourceFiles
 #define Foam_PstreamGlobals_H
 
 #include "DynamicList.H"
+#include "UPstream.H"  // for UPstream::Request
 #include <mpi.h>
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
@@ -71,7 +72,6 @@ extern DynamicList<MPI_Group> MPIGroups_;
 
 //- Outstanding non-blocking operations.
 extern DynamicList<MPI_Request> outstandingRequests_;
-extern DynamicList<label> freedRequests_;
 
 
 // * * * * * * * * * * * * * * * Global Functions  * * * * * * * * * * * * * //
@@ -79,28 +79,50 @@ extern DynamicList<label> freedRequests_;
 //- Fatal if comm is outside the allocated range
 void checkCommunicator(const label comm, const label toProcNo);
 
-//- Push request onto list of outstanding requests,
-//- optionally reusing previously freed request locations
-//
-//  \return index of request within outstandingRequests_
-inline label push_request(MPI_Request request)
+
+//- Reset UPstream::Request to null and/or the index of the outstanding
+//- request to -1.
+//  Does not affect the stack of outstanding requests.
+inline void reset_request
+(
+    UPstream::Request* requestPtr,
+    label* requestIdx = nullptr
+)
 {
-    while (!freedRequests_.empty())
+    if (requestPtr) *requestPtr = UPstream::Request(MPI_REQUEST_NULL);
+    if (requestIdx) *requestIdx = -1;
+}
+
+
+//- Transcribe MPI_Request to UPstream::Request
+//- (does not affect the stack of outstanding requests)
+//- or else push onto list of outstanding requests
+//- and (optionally) record its location
+inline void push_request
+(
+    MPI_Request request,
+    UPstream::Request* requestPtr = nullptr,
+    label* requestIdx = nullptr
+)
+{
+    if (requestPtr)
     {
-        const label index = freedRequests_.back();
-        freedRequests_.pop_back();
+        // Transcribe as UPstream::Request
+        *requestPtr = UPstream::Request(request);
 
-        if (index < outstandingRequests_.size())
-        {
-            outstandingRequests_[index] = request;
-            return index;
-        }
+        // Not on stack of outstanding requests
+        if (requestIdx) *requestIdx = -1;
     }
+    else
+    {
+        if (requestIdx)
+        {
+            // Its index into outstanding requests
+            *requestIdx = PstreamGlobals::outstandingRequests_.size();
+        }
 
-    const label index = outstandingRequests_.size();
-    outstandingRequests_.push_back(request);
-
-    return index;
+        PstreamGlobals::outstandingRequests_.push_back(request);
+    }
 }
 
 
diff --git a/src/Pstream/mpi/UIPstreamRead.C b/src/Pstream/mpi/UIPstreamRead.C
index 37cb441414..20fed25c8d 100644
--- a/src/Pstream/mpi/UIPstreamRead.C
+++ b/src/Pstream/mpi/UIPstreamRead.C
@@ -105,6 +105,8 @@ Foam::label Foam::UIPstream::read
     UPstream::Request* req
 )
 {
+    PstreamGlobals::reset_request(req);
+
     if (debug)
     {
         Pout<< "UIPstream::read : starting read from:" << fromProcNo
@@ -205,26 +207,18 @@ Foam::label Foam::UIPstream::read
             return 0;
         }
 
-        profilingPstream::addRequestTime();
-
         if (debug)
         {
             Pout<< "UIPstream::read : started read from:" << fromProcNo
                 << " tag:" << tag << " read size:" << label(bufSize)
                 << " commsType:" << UPstream::commsTypeNames[commsType]
-                <<
+                << " request:" <<
                 (req ? label(-1) : PstreamGlobals::outstandingRequests_.size())
                 << Foam::endl;
         }
 
-        if (req)
-        {
-            *req = UPstream::Request(request);
-        }
-        else
-        {
-            PstreamGlobals::outstandingRequests_.push_back(request);
-        }
+        PstreamGlobals::push_request(request, req);
+        profilingPstream::addRequestTime();
 
         // Assume the message is completely received.
         return bufSize;
diff --git a/src/Pstream/mpi/UOPstreamWrite.C b/src/Pstream/mpi/UOPstreamWrite.C
index c14404d077..944b09ed71 100644
--- a/src/Pstream/mpi/UOPstreamWrite.C
+++ b/src/Pstream/mpi/UOPstreamWrite.C
@@ -60,6 +60,8 @@ bool Foam::UOPstream::write
     const UPstream::sendModes sendMode
 )
 {
+    PstreamGlobals::reset_request(req);
+
     if (debug)
     {
         Pout<< "UOPstream::write : starting write to:" << toProcNo
@@ -79,7 +81,6 @@ bool Foam::UOPstream::write
         error::printStack(Pout);
     }
 
-
     PstreamGlobals::checkCommunicator(communicator, toProcNo);
 
 
@@ -179,27 +180,18 @@ bool Foam::UOPstream::write
             );
         }
 
-        profilingPstream::addRequestTime();
-
         if (debug)
         {
             Pout<< "UOPstream::write : started write to:" << toProcNo
                 << " tag:" << tag << " size:" << label(bufSize)
                 << " commType:" << UPstream::commsTypeNames[commsType]
-                << " request:"
-                <<
+                << " request:" <<
                 (req ? label(-1) : PstreamGlobals::outstandingRequests_.size())
                 << Foam::endl;
         }
 
-        if (req)
-        {
-            *req = UPstream::Request(request);
-        }
-        else
-        {
-            PstreamGlobals::outstandingRequests_.push_back(request);
-        }
+        PstreamGlobals::push_request(request, req);
+        profilingPstream::addRequestTime();
     }
     else
     {
diff --git a/src/Pstream/mpi/UPstream.C b/src/Pstream/mpi/UPstream.C
index bbd879d806..4555fd62ed 100644
--- a/src/Pstream/mpi/UPstream.C
+++ b/src/Pstream/mpi/UPstream.C
@@ -442,7 +442,6 @@ void Foam::UPstream::shutdown(int errNo)
         }
 
         PstreamGlobals::outstandingRequests_.clear();
-        PstreamGlobals::freedRequests_.clear();
 
         if (nOutstanding)
         {
diff --git a/src/Pstream/mpi/UPstreamRequest.C b/src/Pstream/mpi/UPstreamRequest.C
index 603b638a85..e719bf56d3 100644
--- a/src/Pstream/mpi/UPstreamRequest.C
+++ b/src/Pstream/mpi/UPstreamRequest.C
@@ -149,7 +149,7 @@ void Foam::UPstream::waitRequests(UList<UPstream::Request>& requests)
 
     if (!count)
     {
-        // Early exit: non-NULL requests found
+        // Early exit: has NULL requests only
         return;
     }
 
@@ -212,9 +212,18 @@ Foam::label Foam::UPstream::waitAnyRequest(UList<UPstream::Request>& requests)
     }
 
     // Transcribe MPI_Request back into UPstream::Request
-    while (--count >= 0)
+    // - do in reverse order - see note in finishedRequests()
     {
-        requests[count] = UPstream::Request(waitRequests[count]);
+        for (label i = count-1; i >= 0; --i)
+        {
+            requests[i] = UPstream::Request(waitRequests[i]);
+        }
+
+        // Trailing portion
+        for (label i = count; i < requests.size(); ++i)
+        {
+            requests[i] = UPstream::Request(MPI_REQUEST_NULL);
+        }
     }
 
     return index;
@@ -286,9 +295,6 @@ void Foam::UPstream::waitRequest(const label i)
         return;
     }
 
-    // Push index onto free cache (for later reuse)
-    PstreamGlobals::freedRequests_.push_back(i);
-
     auto& request = PstreamGlobals::outstandingRequests_[i];
 
     // No-op for null request
@@ -452,7 +458,7 @@ bool Foam::UPstream::finishedRequests(UList<UPstream::Request>& requests)
 
     if (!count)
     {
-        // Early exit: non-NULL requests found
+        // Early exit: has NULL requests only
         return true;
     }
 
@@ -476,9 +482,15 @@ bool Foam::UPstream::finishedRequests(UList<UPstream::Request>& requests)
         // This is uglier that we'd like, but much better than allocating
         // and freeing a scratch buffer each time we query things.
 
-        while (--count >= 0)
+        for (label i = count-1; i >= 0; --i)
         {
-            requests[count] = UPstream::Request(waitRequests[count]);
+            requests[i] = UPstream::Request(waitRequests[i]);
+        }
+
+        // Trailing portion
+        for (label i = count; i < requests.size(); ++i)
+        {
+            requests[i] = UPstream::Request(MPI_REQUEST_NULL);
         }
     }
 
diff --git a/src/Pstream/mpi/UPstreamWrappingTemplates.C b/src/Pstream/mpi/UPstreamWrappingTemplates.C
index 65e85bb8ea..f19507e857 100644
--- a/src/Pstream/mpi/UPstreamWrappingTemplates.C
+++ b/src/Pstream/mpi/UPstreamWrappingTemplates.C
@@ -125,11 +125,10 @@ void Foam::PstreamDetail::allReduce
     label* requestID
 )
 {
+    PstreamGlobals::reset_request(req, requestID);
+
     if (!UPstream::parRun())
     {
-        // No requests generated
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
         return;
     }
 
@@ -189,16 +188,8 @@ void Foam::PstreamDetail::allReduce
                 << Foam::abort(FatalError);
         }
 
-        if (req)
-        {
-            *req = UPstream::Request(request);
-            if (requestID) *requestID = -1;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::push_request(request);
-        }
 
+        PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
 #endif
@@ -207,9 +198,6 @@ void Foam::PstreamDetail::allReduce
     {
         profilingPstream::beginTiming();
 
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
-
         if
         (
             MPI_Allreduce
@@ -246,6 +234,8 @@ void Foam::PstreamDetail::allToAll
     label* requestID
 )
 {
+    PstreamGlobals::reset_request(req, requestID);
+
     const bool immediate = (req || requestID);
 
     const label numProc = UPstream::nProcs(comm);
@@ -280,10 +270,6 @@ void Foam::PstreamDetail::allToAll
     if (!UPstream::parRun() || numProc < 2)
     {
         recvData.deepCopy(sendData);
-
-        // No requests generated
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
         return;
     }
 
@@ -321,16 +307,7 @@ void Foam::PstreamDetail::allToAll
                 << Foam::abort(FatalError);
         }
 
-        if (req)
-        {
-            *req = UPstream::Request(request);
-            if (requestID) *requestID = -1;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::push_request(request);
-        }
-
+        PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
 #endif
@@ -339,9 +316,6 @@ void Foam::PstreamDetail::allToAll
     {
         profilingPstream::beginTiming();
 
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
-
         if
         (
             MPI_Alltoall
@@ -387,6 +361,8 @@ void Foam::PstreamDetail::allToAllv
     label* requestID
 )
 {
+    PstreamGlobals::reset_request(req, requestID);
+
     const bool immediate = (req || requestID);
 
     const label np = UPstream::nProcs(comm);
@@ -440,9 +416,6 @@ void Foam::PstreamDetail::allToAllv
             recvCounts[0]*sizeof(Type)
         );
 
-        // No requests generated
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
         return;
     }
 
@@ -481,16 +454,7 @@ void Foam::PstreamDetail::allToAllv
                 << Foam::abort(FatalError);
         }
 
-        if (req)
-        {
-            *req = UPstream::Request(request);
-            if (requestID) *requestID = -1;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::push_request(request);
-        }
-
+        PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
 #endif
@@ -499,9 +463,6 @@ void Foam::PstreamDetail::allToAllv
     {
         profilingPstream::beginTiming();
 
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
-
         if
         (
             MPI_Alltoallv
@@ -910,13 +871,11 @@ void Foam::PstreamDetail::gather
     label* requestID
 )
 {
+    PstreamGlobals::reset_request(req, requestID);
+
     if (!UPstream::parRun())
     {
         std::memmove(recvData, sendData, recvCount*sizeof(Type));
-
-        // No requests generated
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
         return;
     }
 
@@ -976,16 +935,7 @@ void Foam::PstreamDetail::gather
                 << Foam::abort(FatalError);
         }
 
-        if (req)
-        {
-            *req = UPstream::Request(request);
-            if (requestID) *requestID = -1;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::push_request(request);
-        }
-
+        PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
 #endif
@@ -994,9 +944,6 @@ void Foam::PstreamDetail::gather
     {
         profilingPstream::beginTiming();
 
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
-
         if
         (
             MPI_Gather
@@ -1040,13 +987,11 @@ void Foam::PstreamDetail::scatter
     label* requestID
 )
 {
+    PstreamGlobals::reset_request(req, requestID);
+
     if (!UPstream::parRun())
     {
         std::memmove(recvData, sendData, recvCount*sizeof(Type));
-
-        // No requests generated
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
         return;
     }
 
@@ -1106,16 +1051,7 @@ void Foam::PstreamDetail::scatter
                 << Foam::abort(FatalError);
         }
 
-        if (req)
-        {
-            *req = UPstream::Request(request);
-            if (requestID) *requestID = -1;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::push_request(request);
-        }
-
+        PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
 #endif
@@ -1124,9 +1060,6 @@ void Foam::PstreamDetail::scatter
     {
         profilingPstream::beginTiming();
 
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
-
         if
         (
             MPI_Scatter
@@ -1171,14 +1104,12 @@ void Foam::PstreamDetail::gatherv
     label* requestID
 )
 {
+    PstreamGlobals::reset_request(req, requestID);
+
     if (!UPstream::parRun())
     {
         // recvCounts[0] may be invalid - use sendCount instead
         std::memmove(recvData, sendData, sendCount*sizeof(Type));
-
-        // No requests generated
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
         return;
     }
 
@@ -1262,16 +1193,7 @@ void Foam::PstreamDetail::gatherv
                 << Foam::abort(FatalError);
         }
 
-        if (req)
-        {
-            *req = UPstream::Request(request);
-            if (requestID) *requestID = -1;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::push_request(request);
-        }
-
+        PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
 #endif
@@ -1280,9 +1202,6 @@ void Foam::PstreamDetail::gatherv
     {
         profilingPstream::beginTiming();
 
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
-
         if
         (
             MPI_Gatherv
@@ -1328,13 +1247,11 @@ void Foam::PstreamDetail::scatterv
     label* requestID
 )
 {
+    PstreamGlobals::reset_request(req, requestID);
+
     if (!UPstream::parRun())
     {
         std::memmove(recvData, sendData, recvCount*sizeof(Type));
-
-        // No requests generated
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
         return;
     }
 
@@ -1412,16 +1329,7 @@ void Foam::PstreamDetail::scatterv
                 << Foam::abort(FatalError);
         }
 
-        if (req)
-        {
-            *req = UPstream::Request(request);
-            if (requestID) *requestID = -1;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::push_request(request);
-        }
-
+        PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
 #endif
@@ -1430,9 +1338,6 @@ void Foam::PstreamDetail::scatterv
     {
         profilingPstream::beginTiming();
 
-        if (req) req->reset();
-        if (requestID) *requestID = -1;
-
         if
         (
             MPI_Scatterv