diff --git a/applications/test/parallel-comm2/Test-parallel-comm2.C b/applications/test/parallel-comm2/Test-parallel-comm2.C
index ce77d5b38c..53b8fc3e5e 100644
--- a/applications/test/parallel-comm2/Test-parallel-comm2.C
+++ b/applications/test/parallel-comm2/Test-parallel-comm2.C
@@ -39,6 +39,7 @@ Description
 #include "Tuple2.H"
 #include "IOstreams.H"
 #include "PstreamReduceOps.H"
+#include "SHA1.H"
 #include <mpi.h>
 
 using namespace Foam;
@@ -99,51 +100,61 @@ int main(int argc, char *argv[])
     labelList subRanks;
     UPstream::communicator newComm;
 
-    #if 1
-    // With first ranks
-    subRanks = identity(UPstream::nProcs(UPstream::commWorld()) / 2);
-
-    newComm.reset(UPstream::commWorld(), 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::commWorld()));
-
-    for (label& val : subRanks)
+    if (!args.found("comm-split") && !args.found("host-comm"))
     {
-        if (val % 2) val = -1;
+        #if 1
+        // With first ranks
+        subRanks = identity(UPstream::nProcs(UPstream::commWorld()) / 2);
+
+        newComm.reset(UPstream::commWorld(), 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::commWorld()));
+
+        for (label& val : subRanks)
+        {
+            if (val % 2) val = -1;
+        }
+
+        newComm.reset(UPstream::commWorld(), 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
     }
 
-    newComm.reset(UPstream::commWorld(), 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
-
     if (Pstream::parRun() && args.found("comm-split"))
     {
+        int world_nprocs = 0;
+        int world_rank = -1;
+        MPI_Comm_size(MPI_COMM_WORLD, &world_nprocs);
+        MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
+
+        int host_nprocs = 0;
+        int host_rank = -1;
         MPI_Comm hostComm;
         MPI_Comm_split_type
         (
@@ -152,15 +163,98 @@ int main(int argc, char *argv[])
             0, MPI_INFO_NULL, &hostComm
         );
 
-        int host_nprocs = 0;
-        int host_rank = 0;
         MPI_Comm_size(hostComm, &host_nprocs);
         MPI_Comm_rank(hostComm, &host_rank);
 
-        Pout<< nl << "Host comm with "
-            << host_rank << " / " << host_nprocs
-            << " (using MPI_Comm_split_type)" << endl;
+        int leader_nprocs = 0;
+        int leader_rank = -1;
+        MPI_Comm hostMasterComm;
 
+        if (false)
+        {
+            // Easy enough to use MPI_Comm_split, but slightly annoying
+            // that it returns MPI_COMM_NULL for unused ranks...
+            MPI_Comm hostMasterComm;
+            MPI_Comm_split
+            (
+                MPI_COMM_WORLD,
+                (host_rank == 0) ? 0 : MPI_UNDEFINED,
+                0, &hostMasterComm
+            );
+
+            if (hostMasterComm != MPI_COMM_NULL)
+            {
+                MPI_Comm_size(hostMasterComm, &leader_nprocs);
+                MPI_Comm_rank(hostMasterComm, &leader_rank);
+            }
+        }
+        else
+        {
+            boolList isHostLeader(world_nprocs, false);
+            isHostLeader[world_rank] = (host_rank == 0);
+
+            MPI_Allgather
+            (
+                // recv is also send
+                MPI_IN_PLACE, 1, MPI_C_BOOL,
+                isHostLeader.data(), 1, MPI_C_BOOL,
+                MPI_COMM_WORLD
+            );
+
+            Pout<< "leaders: " << isHostLeader << endl;
+
+            DynamicList<int> subRanks(isHostLeader.size());
+            forAll(isHostLeader, proci)
+            {
+                if (isHostLeader[proci])
+                {
+                    subRanks.push_back(proci);
+                }
+            }
+            // Starting from parent
+            MPI_Group parent_group;
+            MPI_Comm_group(MPI_COMM_WORLD, &parent_group);
+
+            MPI_Group active_group;
+            MPI_Group_incl
+            (
+                parent_group,
+                subRanks.size(),
+                subRanks.cdata(),
+                &active_group
+            );
+
+            // Create new communicator for this group
+            MPI_Comm_create_group
+            (
+                MPI_COMM_WORLD,
+                active_group,
+                UPstream::msgType(),
+                &hostMasterComm
+            );
+
+            // Groups not needed after this...
+            MPI_Group_free(&parent_group);
+            MPI_Group_free(&active_group);
+
+            MPI_Comm_size(hostMasterComm, &leader_nprocs);
+            MPI_Comm_rank(hostMasterComm, &leader_rank);
+        }
+
+        Pout<< nl << "[MPI_Comm_split_type]" << nl
+            << "Host comm with " << host_rank << " / " << host_nprocs
+            << " on " << hostName()
+            << " master:" << (host_rank == 0)
+            << " leader rank: " << leader_rank
+            << " / " << leader_nprocs
+            << " host leader:" << (leader_rank == 0)
+            << " sub-rank:" << (leader_rank > 0)
+            << nl;
+
+        if (hostMasterComm != MPI_COMM_NULL)
+        {
+            MPI_Comm_free(&hostMasterComm);
+        }
         MPI_Comm_free(&hostComm);
     }
     if (Pstream::parRun() && args.found("host-comm"))
@@ -171,16 +265,56 @@ int main(int argc, char *argv[])
 
         label numprocs = UPstream::nProcs(UPstream::commGlobal());
 
+        // Option 1: using hostnames
+        // - pro: trivial coding
+        // - con: unequal lengths, more allocations and 'hops'
         stringList hosts(numprocs);
         hosts[Pstream::myProcNo(UPstream::commGlobal())] = hostName();
+        Pstream::gatherList(hosts, UPstream::msgType(), UPstream::commGlobal());
 
-        labelList hostIDs_;
+
+        // Option 2: using SHA1 of hostnames
+        // - con: uglier coding (but only needed locally!)
+        // - pro: fixed digest length enables direct MPI calls
+        //        can avoid Pstream::gatherList() during setup...
+
+        SHA1Digest myHostDigest(SHA1(hostName()).digest());
+
+        List<SHA1Digest> digests;
+        if (UPstream::master(UPstream::commGlobal()))
+        {
+            digests.resize(numprocs);
+        }
+
+        UPstream::mpiGather
+        (
+            reinterpret_cast<const char*>(myHostDigest.cdata_bytes()),
+            SHA1Digest::max_size(),     // Num send per proc
+            digests.data_bytes(),       // Recv
+            SHA1Digest::max_size(),     // Num recv per proc
+            UPstream::commGlobal()
+        );
+
+        // MPI_Gather
+        // (
+        //     myHostDigest.cdata_bytes(), // Send
+        //     SHA1Digest::max_size(),     // Num send per proc
+        //     MPI_BYTE,
+        //     digests.data_bytes(),       // Recv
+        //     SHA1Digest::max_size(),     // Num recv per proc
+        //     MPI_BYTE,
+        //     0,  //  root
+        //     MPI_COMM_WORLD
+        // );
+
+        Info<< "digests: " << digests << nl;
+
+        labelList hostIDs_(numprocs);
 
         // Compact
-        if (Pstream::master(UPstream::commGlobal()))
+        if (UPstream::master(UPstream::commGlobal()))
         {
             DynamicList<word> hostNames(numprocs);
-            hostIDs_.resize_nocopy(numprocs);
 
             forAll(hosts, proci)
             {
@@ -188,20 +322,72 @@ int main(int argc, char *argv[])
 
                 hostIDs_[proci] = hostNames.find(host);
 
-                if (hostIDs_[proci] == -1)
+                if (hostIDs_[proci] < 0)
                 {
-                    hostIDs_[proci] = hostNames.size();
+                    // First appearance of host (encode as leader)
+                    hostIDs_[proci] = -(hostNames.size() + 1);
                     hostNames.push_back(host);
                 }
             }
+
+            DynamicList<SHA1Digest> hostDigest(numprocs);
+
+            forAll(digests, proci)
+            {
+                const SHA1Digest& dig = digests[proci];
+
+                hostIDs_[proci] = hostDigest.find(dig);
+
+                if (hostIDs_[proci] < 0)
+                {
+                    // First appearance of host (encode as leader)
+                    hostIDs_[proci] = -(hostDigest.size() + 1);
+                    hostDigest.push_back(dig);
+                }
+            }
         }
 
-        Pstream::broadcasts(UPstream::commGlobal(), hostIDs_);
+        Info<< "hosts =  " << hosts << endl;
+        Info<< "hostIDs_ =  " << hostIDs_ << endl;
+
+        UPstream::broadcast
+        (
+            hostIDs_.data_bytes(),
+            hostIDs_.size_bytes(),
+            UPstream::commGlobal(),
+            UPstream::masterNo()
+        );
+
+        DynamicList<label> subRanks(numprocs);
+
+        // Ranks for world to hostMaster
+        forAll(hostIDs_, proci)
+        {
+            // Is host leader?
+            if (hostIDs_[proci] < 0)
+            {
+                subRanks.push_back(proci);
+
+                // Flip back to generic host id
+                hostIDs_[proci] = -(hostIDs_[proci] + 1);
+            }
+        }
+
+        // From world to hostMaster
+        const label hostMasterComm =
+            UPstream::allocateCommunicator
+            (
+                UPstream::commGlobal(),
+                subRanks,
+                true
+            );
+
 
         const label myHostId =
             hostIDs_[Pstream::myProcNo(UPstream::commGlobal())];
 
-        DynamicList<label> subRanks;
+        // Ranks for within a host
+        subRanks.clear();
         forAll(hostIDs_, proci)
         {
             if (hostIDs_[proci] == myHostId)
@@ -210,7 +396,7 @@ int main(int argc, char *argv[])
             }
         }
 
-        // Allocate new communicator with global communicator as its parent
+        // The intra-host ranks
         const label hostComm =
             UPstream::allocateCommunicator
             (
@@ -219,11 +405,19 @@ int main(int argc, char *argv[])
                 true
             );
 
-        Pout<< nl << "Host comm with "
+        Pout<< nl << "[manual split]" << nl
+            << nl << "Host comm with "
             << UPstream::myProcNo(hostComm)
             << " / " << UPstream::nProcs(hostComm)
+            << " on " << hostName()
+            << " master:" << UPstream::master(hostComm)
+            << " leader rank: " << UPstream::myProcNo(hostMasterComm)
+            << " / " << UPstream::nProcs(hostMasterComm)
+            << " host leader:" << UPstream::master(hostMasterComm)
+            << " sub-rank:" << UPstream::is_subrank(hostMasterComm)
             << nl;
 
+        UPstream::freeCommunicator(hostMasterComm, true);
         UPstream::freeCommunicator(hostComm, true);
     }
 
diff --git a/applications/test/parallel-external-init/Test-parallel-external-init.C b/applications/test/parallel-external-init/Test-parallel-external-init.C
index dbcead0d8b..362dc0b382 100644
--- a/applications/test/parallel-external-init/Test-parallel-external-init.C
+++ b/applications/test/parallel-external-init/Test-parallel-external-init.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2019-2022 OpenCFD Ltd.
+    Copyright (C) 2019-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -52,11 +52,6 @@ bool startMPI()
     int nprocs[3];
     int rank[3];
 
-    int group_nprocs[3];
-    int group_rank[3];
-
-    MPI_Group mpiGroup;
-
     MPI_Init(nullptr, nullptr);
 
     MPI_Comm_size(MPI_COMM_WORLD, &nprocs[worldComm]);
@@ -65,62 +60,27 @@ bool startMPI()
     const bool isMaster = (rank[worldComm] == 0);
     const string prefix = '[' + Foam::name(rank[worldComm]) + "] ";
 
-    MPI_Comm_group(MPI_COMM_WORLD, &mpiGroup);
-    MPI_Group_size(mpiGroup, &group_nprocs[worldComm]);
-    MPI_Group_rank(mpiGroup, &group_rank[worldComm]);
-
-
     if (isMaster && nprocs[worldComm])
     {
         std::cout
-            << nl << "Using MPI with " << nprocs[worldComm]
-            << " procs, group:"
-            << group_nprocs[worldComm] << nl
-            << "World group: " << Foam::name(mpiGroup) << nl
+            << nl << "Using MPI with "
+            << nprocs[worldComm] << " procs" << nl
             << nl;
     }
 
     MPI_Comm worldMpiComm;
-
     MPI_Comm_dup(MPI_COMM_WORLD, &worldMpiComm);
-
-    MPI_Comm_group(MPI_COMM_WORLD, &mpiGroup);
-
-    if (isMaster && nprocs[worldComm])
-    {
-        std::cout
-            << "dup comm group: " << Foam::name(mpiGroup) << nl;
-    }
-
     MPI_Comm_free(&worldMpiComm);
 
-    // May be a bad idea
-    MPI_Group_free(&mpiGroup);
-
     MPI_Comm_size(MPI_COMM_SELF, &nprocs[selfComm]);
     MPI_Comm_rank(MPI_COMM_SELF, &rank[selfComm]);
 
-    MPI_Comm_group(MPI_COMM_SELF, &mpiGroup);
-    MPI_Group_size(mpiGroup, &group_nprocs[selfComm]);
-    MPI_Group_rank(mpiGroup, &group_rank[selfComm]);
-
-    if (isMaster && nprocs[worldComm])
-    {
-        std::cout
-            << nl
-            << "Self group: " << Foam::name(mpiGroup) << nl;
-    }
-
-    // Should be a bad idea
-    MPI_Group_free(&mpiGroup);
-
     // if (nprocs && isMaster)
     {
         std::cout
             << prefix
-            << "Self: " << rank[selfComm] << " from " << nprocs[selfComm]
-            << " procs, group:"
-            << group_nprocs[selfComm] << nl;
+            << "Self: " << rank[selfComm]
+            << " from " << nprocs[selfComm] << " procs" << nl;
     }
 
     if (isMaster)
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
index e3cf730d71..6fc66f9021 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
@@ -155,6 +155,14 @@ Foam::label Foam::UPstream::allocateCommunicator
         // LIFO pop
         index = freeComms_.back();
         freeComms_.pop_back();
+
+        // Reset existing
+        myProcNo_[index] = -1;
+        parentComm_[index] = parentIndex;
+
+        procIDs_[index].clear();
+        linearCommunication_[index].clear();
+        treeCommunication_[index].clear();
     }
     else
     {
@@ -162,8 +170,9 @@ Foam::label Foam::UPstream::allocateCommunicator
         index = parentComm_.size();
 
         myProcNo_.push_back(-1);
+        parentComm_.push_back(parentIndex);
+
         procIDs_.emplace_back();
-        parentComm_.push_back(-1);
         linearCommunication_.emplace_back();
         treeCommunication_.emplace_back();
     }
@@ -213,8 +222,6 @@ Foam::label Foam::UPstream::allocateCommunicator
 
     procIds.resize(numSubRanks);
 
-    parentComm_[index] = parentIndex;
-
     // Size but do not fill structure - this is done on-the-fly
     linearCommunication_[index] = List<commsStruct>(numSubRanks);
     treeCommunication_[index] = List<commsStruct>(numSubRanks);
@@ -236,6 +243,14 @@ Foam::label Foam::UPstream::allocateCommunicator
         ///         myProcNo_[index] = -(myProcNo_[parentIndex]+1);
         ///     }
         /// }
+
+        // Did communicator allocation adjust procIDs_ as well?
+        if (numSubRanks != procIDs_[index].size())
+        {
+            numSubRanks = procIDs_[index].size();
+            linearCommunication_[index] = List<commsStruct>(numSubRanks);
+            treeCommunication_[index] = List<commsStruct>(numSubRanks);
+        }
     }
 
     return index;
@@ -268,8 +283,8 @@ void Foam::UPstream::freeCommunicator
     }
 
     myProcNo_[communicator] = -1;
-    //procIDs_[communicator].clear();
     parentComm_[communicator] = -1;
+    //procIDs_[communicator].clear();
     linearCommunication_[communicator].clear();
     treeCommunication_[communicator].clear();
 
@@ -449,26 +464,24 @@ bool Foam::UPstream::haveThreads_(false);
 int Foam::UPstream::msgType_(1);
 
 
-Foam::DynamicList<int> Foam::UPstream::myProcNo_(10);
-
-Foam::DynamicList<Foam::List<int>> Foam::UPstream::procIDs_(10);
-
-Foam::DynamicList<Foam::label> Foam::UPstream::parentComm_(10);
-
-Foam::DynamicList<Foam::label> Foam::UPstream::freeComms_;
-
 Foam::wordList Foam::UPstream::allWorlds_(Foam::one{}, "");
 Foam::labelList Foam::UPstream::worldIDs_(Foam::one{}, 0);
 
-Foam::DynamicList<Foam::List<Foam::UPstream::commsStruct>>
-Foam::UPstream::linearCommunication_(10);
+
+Foam::DynamicList<int> Foam::UPstream::myProcNo_(16);
+Foam::DynamicList<Foam::List<int>> Foam::UPstream::procIDs_(16);
+
+Foam::DynamicList<Foam::label> Foam::UPstream::parentComm_(16);
+Foam::DynamicList<Foam::label> Foam::UPstream::freeComms_;
 
 Foam::DynamicList<Foam::List<Foam::UPstream::commsStruct>>
-Foam::UPstream::treeCommunication_(10);
+Foam::UPstream::linearCommunication_(16);
+
+Foam::DynamicList<Foam::List<Foam::UPstream::commsStruct>>
+Foam::UPstream::treeCommunication_(16);
 
 
 Foam::label Foam::UPstream::worldComm(0);
-
 Foam::label Foam::UPstream::warnComm(-1);