diff --git a/applications/test/parallel-comm2/Make/files b/applications/test/parallel-comm2/Make/files
new file mode 100644
index 0000000000..a6841ec2a1
--- /dev/null
+++ b/applications/test/parallel-comm2/Make/files
@@ -0,0 +1,3 @@
+Test-parallel-comm2.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-parallel-comm2
diff --git a/applications/test/parallel-comm2/Make/options b/applications/test/parallel-comm2/Make/options
new file mode 100644
index 0000000000..18e6fe47af
--- /dev/null
+++ b/applications/test/parallel-comm2/Make/options
@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */
diff --git a/applications/test/parallel-comm2/Test-parallel-comm2.C b/applications/test/parallel-comm2/Test-parallel-comm2.C
new file mode 100644
index 0000000000..934a480c20
--- /dev/null
+++ b/applications/test/parallel-comm2/Test-parallel-comm2.C
@@ -0,0 +1,148 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2022 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-parallel-comm2
+
+Description
+    Basic communicator tests
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "Time.H"
+#include "IPstream.H"
+#include "OPstream.H"
+#include "Pair.H"
+#include "Tuple2.H"
+#include "IOstreams.H"
+#include "PstreamReduceOps.H"
+
+using namespace Foam;
+
+void rankInfo(const label comm)
+{
+    const int ranki = UPstream::myProcNo(comm);
+
+    Pout<< "comm:" << comm
+        << "(parent:" << UPstream::parent(comm) << ')'
+        << " rank:" << ranki
+        << "(sub:" << UPstream::is_subrank(comm)
+        << ") nProcs:" << UPstream::nProcs(comm)
+        << " baseProcNo:" << UPstream::baseProcNo(comm, ranki);
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+int main(int argc, char *argv[])
+{
+    argList::noBanner();
+    argList::noCheckProcessorDirectories();
+    argList::addBoolOption("verbose", "Set debug level");
+
+    // Capture manually. We need values before proper startup
+    int nVerbose = 0;
+    for (int argi = 1; argi < argc; ++argi)
+    {
+        if (strcmp(argv[argi], "-verbose") == 0)
+        {
+            ++nVerbose;
+        }
+    }
+
+    UPstream::debug = nVerbose;
+
+    #include "setRootCase.H"
+
+    Info<< nl
+        << "nProcs = " << UPstream::nProcs()
+        << " with " << UPstream::nComms() << " predefined comm(s)" << nl;
+
+    Info<< nl;
+
+    //- Process IDs within a given communicator
+    Info<< "procIDs: "
+        << flatOutput(UPstream::procID(UPstream::worldComm)) << endl;
+
+    rankInfo(UPstream::worldComm);
+    Pout<< endl;
+
+    const int myProci = UPstream::myProcNo(UPstream::worldComm);
+    int localRanki = myProci;
+
+    labelList subRanks;
+    UPstream::communicator newComm;
+
+    #if 1
+    // With first ranks
+    subRanks = identity(UPstream::nProcs(UPstream::worldComm) / 2);
+
+    newComm.reset(UPstream::worldComm, subRanks);
+    localRanki = UPstream::myProcNo(newComm);
+
+    Pout.prefix() =
+    (
+        '[' + Foam::name(myProci) + " a:" + Foam::name(localRanki) + "] "
+    );
+
+    Info<< "procIDs: "
+        << flatOutput(UPstream::procID(newComm)) << endl;
+
+    rankInfo(newComm);
+    Pout<< endl;
+    #endif
+
+    #if 1
+    // With every other rank
+    subRanks = identity(UPstream::nProcs(UPstream::worldComm));
+
+    for (label& val : subRanks)
+    {
+        if (val % 2) val = -1;
+    }
+
+    newComm.reset(UPstream::worldComm, subRanks);
+    localRanki = UPstream::myProcNo(newComm);
+
+    Pout.prefix() =
+    (
+        '[' + Foam::name(myProci) + " b:" + Foam::name(localRanki) + "] "
+    );
+
+    Info<< "procIDs: "
+        << flatOutput(UPstream::procID(newComm)) << endl;
+
+    rankInfo(newComm);
+    Pout<< endl;
+    #endif
+
+    Info<< "\nEnd\n" << endl;
+
+    return 0;
+}
+
+
+// ************************************************************************* //
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
index d35520f1e2..d4ef48da5e 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
@@ -179,24 +179,40 @@ Foam::label Foam::UPstream::allocateCommunicator
     // Initialise; overwritten by allocatePstreamCommunicator
     myProcNo_[index] = 0;
 
-    const label numSubRanks = subRanks.size();
+    // The selected sub-ranks.
+    // - transcribe from label to int. Treat negative values as 'ignore'
+    // - enforce incremental order (so index is rank in next communicator)
 
-    // Convert from label to int
-    procIDs_[index].resize_nocopy(numSubRanks);
-    forAll(procIDs_[index], i)
+    auto& procIds = procIDs_[index];
+    procIds.resize_nocopy(subRanks.size());
+
+    label numSubRanks = 0;
+    bool monotonicOrder = true;
+    for (const label subRanki : subRanks)
     {
-        procIDs_[index][i] = subRanks[i];
-
-        // Enforce incremental order (so index is rank in next communicator)
-        if (i >= 1 && subRanks[i] <= subRanks[i-1])
+        if (subRanki < 0)
         {
-            FatalErrorInFunction
-                << "subranks not sorted : " << subRanks
-                << " when allocating subcommunicator from parent "
-                << parentIndex
-                << Foam::abort(FatalError);
+            continue;
         }
+        if (monotonicOrder && numSubRanks)
+        {
+            monotonicOrder = (procIds[numSubRanks-1] < subRanki);
+        }
+
+        procIds[numSubRanks] = subRanki;
+        ++numSubRanks;
     }
+
+    if (!monotonicOrder)
+    {
+        auto last = procIds.begin() + numSubRanks;
+        std::sort(procIds.begin(), last);
+        last = std::unique(procIds.begin(), last);
+        numSubRanks = label(last - procIds.begin());
+    }
+
+    procIds.resize(numSubRanks);
+
     parentComm_[index] = parentIndex;
 
     // Size but do not fill structure - this is done on-the-fly
@@ -206,6 +222,20 @@ Foam::label Foam::UPstream::allocateCommunicator
     if (doPstream && parRun())
     {
         allocatePstreamCommunicator(parentIndex, index);
+
+        // Could 'remember' locations of uninvolved ranks
+        /// if (myProcNo_[index] < 0 && parentIndex >= 0)
+        /// {
+        ///     // As global rank
+        ///     myProcNo_[index] = -(myProcNo_[worldComm]+1);
+        ///
+        /// OR:
+        ///     // As parent rank number
+        ///     if (myProcNo_[parentIndex] >= 0)
+        ///     {
+        ///         myProcNo_[index] = -(myProcNo_[parentIndex]+1);
+        ///     }
+        /// }
     }
 
     return index;
@@ -226,9 +256,9 @@ void Foam::UPstream::freeCommunicator
 
     if (debug)
     {
-        Pout<< "Communicators : Freeing communicator " << communicator << endl
-            << "    parent   : " << parentComm_[communicator] << endl
-            << "    myProcNo : " << myProcNo_[communicator] << endl
+        Pout<< "Communicators : Freeing communicator " << communicator
+            << " parent: " << parentComm_[communicator]
+            << " myProcNo: " << myProcNo_[communicator]
             << endl;
     }
 
@@ -252,7 +282,7 @@ void Foam::UPstream::freeCommunicators(const bool doPstream)
 {
     forAll(myProcNo_, communicator)
     {
-        if (myProcNo_[communicator] != -1)
+        if (myProcNo_[communicator] >= 0)
         {
             freeCommunicator(communicator, doPstream);
         }
@@ -262,7 +292,7 @@ void Foam::UPstream::freeCommunicators(const bool doPstream)
 
 int Foam::UPstream::baseProcNo(label comm, int procID)
 {
-    while (parent(comm) >= 0)
+    while (parent(comm) >= 0 && procID >= 0)
     {
         const auto& parentRanks = UPstream::procID(comm);
         procID = parentRanks[procID];
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
index 04ac5f6065..2ad8042b09 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
@@ -250,7 +250,7 @@ private:
 
 public:
 
-    // Declare name of the class and its debug switch
+    //- Declare name of the class and its debug switch
     ClassName("UPstream");
 
 
@@ -313,11 +313,16 @@ public:
 
     // Member Functions
 
-        //- Allocate a new communicator
+        //- Allocate a new communicator with subRanks of parent communicator
         static label allocateCommunicator
         (
+            //! The parent communicator
             const label parent,
+
+            //! The sub-ranks of parent to use (ignore negative values)
             const labelUList& subRanks,
+
+            //! Call allocatePstreamCommunicator
             const bool doPstream = true
         );
 
@@ -332,6 +337,7 @@ public:
         //- Free all communicators
         static void freeCommunicators(const bool doPstream);
 
+
         //- Wrapper class for allocating/freeing communicators
         class communicator
         {
@@ -339,38 +345,65 @@ public:
 
         public:
 
-            //- Default construct (a placeholder communicator)
-            communicator() : comm_(-1) {}
-
-            //- Move construct
-            communicator(communicator&&) = default;
-
-            //- Move assignment
-            communicator& operator=(communicator&&) = default;
-
             //- No copy construct
             communicator(const communicator&) = delete;
 
             //- No copy assignment
             void operator=(const communicator&) = delete;
 
-            //- Allocate a communicator from given parent
+            //- Default construct (a placeholder communicator)
+            communicator() noexcept : comm_(-1) {}
+
+            //- Move construct, takes ownership
+            communicator(communicator&& c) : comm_(c.comm_) { c.comm_ = -1; }
+
+            //- Allocate a communicator based on given parent
             communicator
             (
+                //! The parent communicator
                 const label parent,
+
+                //! The sub-ranks of parent to use (ignore negative values)
                 const labelUList& subRanks,
-                const bool doPstream
+
+                //! Call allocatePstreamCommunicator
+                const bool doPstream = true
             )
             :
                 comm_(allocateCommunicator(parent, subRanks, doPstream))
             {}
 
             //- Free allocated communicator and group
-            ~communicator()
+            ~communicator() { freeCommunicator(comm_); }
+
+            //- True if communicator is non-negative (ie, was allocated)
+            bool good() const noexcept { return (comm_ >= 0); }
+
+            //- The communicator label
+            label comm() const noexcept { return comm_; }
+
+            //- Free allocated communicator and group
+            void reset() { freeCommunicator(comm_); comm_ = -1; }
+
+            //- Allocate with subRanks of parent communicator
+            void reset(label parent, const labelUList& subRanks)
             {
                 freeCommunicator(comm_);
+                comm_ = allocateCommunicator(parent, subRanks);
             }
 
+            //- Take ownership, free allocated communicator and group.
+            void reset(communicator&& c)
+            {
+                if (comm_ != c.comm_) freeCommunicator(comm_);
+                comm_ = c.comm_;
+                c.comm_ = -1;
+            }
+
+            //- Move assignment, takes ownership
+            void operator=(communicator&& c) { reset(std::move(c)); }
+
+            //- Cast to label - the same as comm()
             operator label() const noexcept { return comm_; }
         };
 
@@ -468,12 +501,18 @@ public:
             return 0;
         }
 
-        //- Am I the master process
+        //- Am I the master rank
         static bool master(const label communicator = worldComm)
         {
             return myProcNo_[communicator] == masterNo();
         }
 
+        //- Is this process a sub-rank on the communicator
+        static bool is_subrank(const label communicator = worldComm)
+        {
+            return myProcNo_[communicator] > masterNo();
+        }
+
         //- Number of this process (starting from masterNo() = 0)
         static int myProcNo(const label communicator = worldComm)
         {
diff --git a/src/Pstream/mpi/PstreamGlobals.C b/src/Pstream/mpi/PstreamGlobals.C
index a5c9f5c87f..f6b09aa4cd 100644
--- a/src/Pstream/mpi/PstreamGlobals.C
+++ b/src/Pstream/mpi/PstreamGlobals.C
@@ -40,17 +40,15 @@ Foam::DynamicList<MPI_Comm> Foam::PstreamGlobals::MPICommunicators_;
 Foam::DynamicList<MPI_Group> Foam::PstreamGlobals::MPIGroups_;
 
 
+// * * * * * * * * * * * * * * * Global Functions  * * * * * * * * * * * * * //
+
 void Foam::PstreamGlobals::checkCommunicator
 (
     const label comm,
     const label toProcNo
 )
 {
-    if
-    (
-        comm < 0
-     || comm >= PstreamGlobals::MPICommunicators_.size()
-    )
+    if (comm < 0 || comm >= PstreamGlobals::MPICommunicators_.size())
     {
         FatalErrorInFunction
             << "toProcNo:" << toProcNo << " : illegal communicator "
diff --git a/src/Pstream/mpi/PstreamGlobals.H b/src/Pstream/mpi/PstreamGlobals.H
index c472379e34..33287c6c9a 100644
--- a/src/Pstream/mpi/PstreamGlobals.H
+++ b/src/Pstream/mpi/PstreamGlobals.H
@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2013-2015 OpenFOAM Foundation
+    Copyright (C) 2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -65,9 +66,35 @@ extern DynamicList<MPI_Comm> MPICommunicators_;
 // Groups associated with the currrent communicators.
 extern DynamicList<MPI_Group> MPIGroups_;
 
+
+// * * * * * * * * * * * * * * * Global Functions  * * * * * * * * * * * * * //
+
 //- Fatal if comm is outside the allocated range
 void checkCommunicator(const label comm, const label toProcNo);
 
+//- Reuse previously freed request locations or push request onto list
+//- of outstanding requests.
+//
+//  \return index of request within outstandingRequests_
+inline label push_request(MPI_Request request)
+{
+    label index;
+
+    if (freedRequests_.size())
+    {
+        index = freedRequests_.back();
+        freedRequests_.pop_back();
+        outstandingRequests_[index] = request;
+    }
+    else
+    {
+        index = outstandingRequests_.size();
+        outstandingRequests_.push_back(request);
+    }
+
+    return index;
+}
+
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
diff --git a/src/Pstream/mpi/UIPstreamRead.C b/src/Pstream/mpi/UIPstreamRead.C
index d66285474f..40d67eabc9 100644
--- a/src/Pstream/mpi/UIPstreamRead.C
+++ b/src/Pstream/mpi/UIPstreamRead.C
@@ -218,7 +218,7 @@ Foam::label Foam::UIPstream::read
                 << Foam::endl;
         }
 
-        PstreamGlobals::outstandingRequests_.append(request);
+        PstreamGlobals::outstandingRequests_.push_back(request);
 
         // Assume the message is completely received.
         return bufSize;
diff --git a/src/Pstream/mpi/UOPstreamWrite.C b/src/Pstream/mpi/UOPstreamWrite.C
index 4ee1dc87ad..3c61afecbe 100644
--- a/src/Pstream/mpi/UOPstreamWrite.C
+++ b/src/Pstream/mpi/UOPstreamWrite.C
@@ -159,7 +159,7 @@ bool Foam::UOPstream::write
                 << Foam::endl;
         }
 
-        PstreamGlobals::outstandingRequests_.append(request);
+        PstreamGlobals::outstandingRequests_.push_back(request);
     }
     else
     {
diff --git a/src/Pstream/mpi/UPstream.C b/src/Pstream/mpi/UPstream.C
index 5ecd11b155..b9e2ab76a1 100644
--- a/src/Pstream/mpi/UPstream.C
+++ b/src/Pstream/mpi/UPstream.C
@@ -687,6 +687,9 @@ void Foam::UPstream::waitRequests(const label start)
             << " outstanding requests starting at " << start << endl;
     }
 
+    // TBD: check for
+    // (start < 0 || start > PstreamGlobals::outstandingRequests_.size())
+
     if (PstreamGlobals::outstandingRequests_.size())
     {
         SubList<MPI_Request> waitRequests
@@ -698,6 +701,7 @@ void Foam::UPstream::waitRequests(const label start)
 
         profilingPstream::beginTiming();
 
+        // On success: sets each request to MPI_REQUEST_NULL
         if
         (
             MPI_Waitall
@@ -714,7 +718,7 @@ void Foam::UPstream::waitRequests(const label start)
 
         profilingPstream::addWaitTime();
 
-        resetRequests(start);
+        PstreamGlobals::outstandingRequests_.resize(start);
     }
 
     if (debug)
@@ -749,6 +753,7 @@ void Foam::UPstream::waitRequest(const label i)
 
     profilingPstream::beginTiming();
 
+    // On success: sets request to MPI_REQUEST_NULL
     if
     (
         MPI_Wait
@@ -797,6 +802,7 @@ bool Foam::UPstream::finishedRequest(const label i)
             << Foam::abort(FatalError);
     }
 
+    // On success: sets request to MPI_REQUEST_NULL
     int flag;
     MPI_Test
     (
@@ -821,7 +827,7 @@ int Foam::UPstream::allocateTag(const char* const msg)
     if (PstreamGlobals::freedTags_.size())
     {
         tag = PstreamGlobals::freedTags_.back();
-        (void)PstreamGlobals::freedTags_.pop_back();
+        PstreamGlobals::freedTags_.pop_back();
     }
     else
     {
diff --git a/src/Pstream/mpi/UPstreamWrappingTemplates.C b/src/Pstream/mpi/UPstreamWrappingTemplates.C
index 8af13e7e16..65139268f4 100644
--- a/src/Pstream/mpi/UPstreamWrappingTemplates.C
+++ b/src/Pstream/mpi/UPstreamWrappingTemplates.C
@@ -179,16 +179,7 @@ void Foam::PstreamDetail::allReduce
                 << Foam::abort(FatalError);
         }
 
-        if (PstreamGlobals::freedRequests_.size())
-        {
-            *requestID = PstreamGlobals::freedRequests_.remove();
-            PstreamGlobals::outstandingRequests_[*requestID] = request;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::outstandingRequests_.size();
-            PstreamGlobals::outstandingRequests_.append(request);
-        }
+        *requestID = PstreamGlobals::push_request(request);
     }
 #endif
 
@@ -299,16 +290,7 @@ void Foam::PstreamDetail::allToAll
                 << Foam::abort(FatalError);
         }
 
-        if (PstreamGlobals::freedRequests_.size())
-        {
-            *requestID = PstreamGlobals::freedRequests_.remove();
-            PstreamGlobals::outstandingRequests_[*requestID] = request;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::outstandingRequests_.size();
-            PstreamGlobals::outstandingRequests_.append(request);
-        }
+        *requestID = PstreamGlobals::push_request(request);
     }
 #endif
 
@@ -449,16 +431,7 @@ void Foam::PstreamDetail::allToAllv
                 << Foam::abort(FatalError);
         }
 
-        if (PstreamGlobals::freedRequests_.size())
-        {
-            *requestID = PstreamGlobals::freedRequests_.remove();
-            PstreamGlobals::outstandingRequests_[*requestID] = request;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::outstandingRequests_.size();
-            PstreamGlobals::outstandingRequests_.append(request);
-        }
+        *requestID = PstreamGlobals::push_request(request);
     }
 #endif
 
@@ -568,16 +541,7 @@ void Foam::PstreamDetail::gather
                 << Foam::abort(FatalError);
         }
 
-        if (PstreamGlobals::freedRequests_.size())
-        {
-            *requestID = PstreamGlobals::freedRequests_.remove();
-            PstreamGlobals::outstandingRequests_[*requestID] = request;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::outstandingRequests_.size();
-            PstreamGlobals::outstandingRequests_.append(request);
-        }
+        *requestID = PstreamGlobals::push_request(request);
     }
 #endif
 
@@ -686,16 +650,7 @@ void Foam::PstreamDetail::scatter
                 << Foam::abort(FatalError);
         }
 
-        if (PstreamGlobals::freedRequests_.size())
-        {
-            *requestID = PstreamGlobals::freedRequests_.remove();
-            PstreamGlobals::outstandingRequests_[*requestID] = request;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::outstandingRequests_.size();
-            PstreamGlobals::outstandingRequests_.append(request);
-        }
+        *requestID = PstreamGlobals::push_request(request);
     }
 #endif
 
@@ -830,16 +785,7 @@ void Foam::PstreamDetail::gatherv
                 << Foam::abort(FatalError);
         }
 
-        if (PstreamGlobals::freedRequests_.size())
-        {
-            *requestID = PstreamGlobals::freedRequests_.remove();
-            PstreamGlobals::outstandingRequests_[*requestID] = request;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::outstandingRequests_.size();
-            PstreamGlobals::outstandingRequests_.append(request);
-        }
+        *requestID = PstreamGlobals::push_request(request);
     }
 #endif
 
@@ -968,16 +914,7 @@ void Foam::PstreamDetail::scatterv
                 << Foam::abort(FatalError);
         }
 
-        if (PstreamGlobals::freedRequests_.size())
-        {
-            *requestID = PstreamGlobals::freedRequests_.remove();
-            PstreamGlobals::outstandingRequests_[*requestID] = request;
-        }
-        else
-        {
-            *requestID = PstreamGlobals::outstandingRequests_.size();
-            PstreamGlobals::outstandingRequests_.append(request);
-        }
+        *requestID = PstreamGlobals::push_request(request);
     }
 #endif