ENH: use Improbe/Mrecv for NBX size exchange

- retains the queried message without secondary polling.
  Purported to be slightly faster. Better thread-safety.

applications/test/parallel-comm3a/Test-parallel-comm3a.C

@@ -131,14 +131,16 @@ int main(int argc, char *argv[])
     for (bool barrier_active = false, done = false; !done; /*nil*/)
     {
         int flag = 0;
+        MPI_Message message;
         MPI_Status status;
 
-        MPI_Iprobe
+        MPI_Improbe
         (
             MPI_ANY_SOURCE,
             tag,
             MPI_COMM_WORLD,
            &flag,
+           &message,
            &status
         );
 
@@ -153,14 +155,12 @@ int main(int argc, char *argv[])
             auto& buf = recvBufs(fromProci);
             buf.resize_nocopy(count);
 
-            MPI_Irecv
+            MPI_Imrecv
             (
                 buf.data_bytes(),
                 buf.size_bytes(),
                 MPI_BYTE,
-                fromProci,
+               &message,
-                tag,
-                MPI_COMM_WORLD,
                &recvRequests.emplace_back()
             );
         }

src/Pstream/mpi/UPstreamWrappingTemplates.C

@@ -157,15 +157,12 @@ void Foam::PstreamDetail::allReduce
         error::printStack(Pout);
     }
 
-    bool handled(false);
 
 #if defined(MPI_VERSION) && (MPI_VERSION >= 3)
-    // MPI-3 : eg, openmpi-1.7 (2013) and later
     if (immediate)
     {
+        // MPI-3 : eg, openmpi-1.7 (2013) and later
         profilingPstream::beginTiming();
-
-        handled = true;
         MPI_Request request;
 
         if
@@ -192,9 +189,8 @@ void Foam::PstreamDetail::allReduce
         PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
+    else
 #endif
-
-    if (!handled)
     {
         profilingPstream::beginTiming();
 
@@ -281,15 +277,12 @@ void Foam::PstreamDetail::allToAll
         return;
     }
 
-    bool handled(false);
 
 #if defined(MPI_VERSION) && (MPI_VERSION >= 3)
-    // MPI-3 : eg, openmpi-1.7 (2013) and later
     if (immediate)
     {
+        // MPI-3 : eg, openmpi-1.7 (2013) and later
         profilingPstream::beginTiming();
-
-        handled = true;
         MPI_Request request;
 
         if
@@ -318,9 +311,8 @@ void Foam::PstreamDetail::allToAll
         PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
+    else
 #endif
-
-    if (!handled)
     {
         profilingPstream::beginTiming();
 
@@ -432,15 +424,12 @@ void Foam::PstreamDetail::allToAllv
         return;
     }
 
-    bool handled(false);
 
 #if defined(MPI_VERSION) && (MPI_VERSION >= 3)
-    // MPI-3 : eg, openmpi-1.7 (2013) and later
     if (immediate)
     {
+        // MPI-3 : eg, openmpi-1.7 (2013) and later
         profilingPstream::beginTiming();
-
-        handled = true;
         MPI_Request request;
 
         if
@@ -470,9 +459,8 @@ void Foam::PstreamDetail::allToAllv
         PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
+    else
 #endif
-
-    if (!handled)
     {
         profilingPstream::beginTiming();
 
@@ -621,6 +609,19 @@ void Foam::PstreamDetail::allToAllConsensus
         int flag = 0;
         MPI_Status status;
 
+#if defined(MPI_VERSION) && (MPI_VERSION >= 3)
+        // MPI-3 : eg, openmpi-1.7 (2013) and later
+        MPI_Message message;
+        MPI_Improbe
+        (
+            MPI_ANY_SOURCE,
+            tag,
+            PstreamGlobals::MPICommunicators_[comm],
+           &flag,
+           &message,
+           &status
+        );
+#else
         MPI_Iprobe
         (
             MPI_ANY_SOURCE,
@@ -629,6 +630,7 @@ void Foam::PstreamDetail::allToAllConsensus
            &flag,
            &status
         );
+#endif
 
         if (flag)
         {
@@ -641,12 +643,24 @@ void Foam::PstreamDetail::allToAllConsensus
             if (count != 1)
             {
                 FatalErrorInFunction
-                    << "Incorrect message size. Expected 1 but had "
+                    << "Incorrect message size from proc=" << proci
-                    << count << nl
+                    << ". Expected 1 but had " << count << nl
                     << exit(FatalError);
             }
 
-            // Regular receive (the data are small)
+            // Regular blocking receive [the data are small]
+
+#if defined(MPI_VERSION) && (MPI_VERSION >= 3)
+            // MPI-3 : eg, openmpi-1.7 (2013) and later
+            MPI_Mrecv
+            (
+               &recvData[proci],
+                count,          // count=1 (see above)
+                datatype,
+               &message,
+                MPI_STATUS_IGNORE
+            );
+#else
             MPI_Recv
             (
                &recvData[proci],
@@ -657,6 +671,7 @@ void Foam::PstreamDetail::allToAllConsensus
                 PstreamGlobals::MPICommunicators_[comm],
                 MPI_STATUS_IGNORE
             );
+#endif
         }
 
         if (barrier_active)
@@ -800,6 +815,19 @@ void Foam::PstreamDetail::allToAllConsensus
         int flag = 0;
         MPI_Status status;
 
+#if defined(MPI_VERSION) && (MPI_VERSION >= 3)
+        // MPI-3 : eg, openmpi-1.7 (2013) and later
+        MPI_Message message;
+        MPI_Improbe
+        (
+            MPI_ANY_SOURCE,
+            tag,
+            PstreamGlobals::MPICommunicators_[comm],
+           &flag,
+           &message,
+           &status
+        );
+#else
         MPI_Iprobe
         (
             MPI_ANY_SOURCE,
@@ -808,6 +836,7 @@ void Foam::PstreamDetail::allToAllConsensus
            &flag,
            &status
         );
+#endif
 
         if (flag)
         {
@@ -821,14 +850,26 @@ void Foam::PstreamDetail::allToAllConsensus
             if (count != 1)
             {
                 FatalErrorInFunction
-                    << "Incorrect message size. Expected 1 but had "
+                    << "Incorrect message size from proc=" << proci
-                    << count << nl
+                    << ". Expected 1 but had " << count << nl
                     << exit(FatalError);
             }
 
             auto& recvData = recvBufs(proci);
 
-            // Regular receive [the data are small]
+            // Regular blocking receive [the data are small]
+
+#if defined(MPI_VERSION) && (MPI_VERSION >= 3)
+            // MPI-3 : eg, openmpi-1.7 (2013) and later
+            MPI_Mrecv
+            (
+               &recvData,
+                count,          // count=1 (see above)
+                datatype,
+               &message,
+                MPI_STATUS_IGNORE
+            );
+#else
             MPI_Recv
             (
                &recvData,
@@ -839,6 +880,7 @@ void Foam::PstreamDetail::allToAllConsensus
                 PstreamGlobals::MPICommunicators_[comm],
                 MPI_STATUS_IGNORE
             );
+#endif
         }
 
         if (barrier_active)
@@ -928,15 +970,12 @@ void Foam::PstreamDetail::gather
         error::printStack(Pout);
     }
 
-    bool handled(false);
 
 #if defined(MPI_VERSION) && (MPI_VERSION >= 3)
-    // MPI-3 : eg, openmpi-1.7 (2013) and later
     if (immediate)
     {
+        // MPI-3 : eg, openmpi-1.7 (2013) and later
         profilingPstream::beginTiming();
-
-        handled = true;
         MPI_Request request;
 
         if
@@ -965,9 +1004,8 @@ void Foam::PstreamDetail::gather
         PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
+    else
 #endif
-
-    if (!handled)
     {
         profilingPstream::beginTiming();
 
@@ -1048,15 +1086,12 @@ void Foam::PstreamDetail::scatter
         error::printStack(Pout);
     }
 
-    bool handled(false);
 
 #if defined(MPI_VERSION) && (MPI_VERSION >= 3)
-    // MPI-3 : eg, openmpi-1.7 (2013) and later
     if (immediate)
     {
+        // MPI-3 : eg, openmpi-1.7 (2013) and later
         profilingPstream::beginTiming();
-
-        handled = true;
         MPI_Request request;
 
         if
@@ -1085,9 +1120,8 @@ void Foam::PstreamDetail::scatter
         PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
+    else
 #endif
-
-    if (!handled)
     {
         profilingPstream::beginTiming();
 
@@ -1193,15 +1227,12 @@ void Foam::PstreamDetail::gatherv
         sendCount = 0;
     }
 
-    bool handled(false);
 
 #if defined(MPI_VERSION) && (MPI_VERSION >= 3)
-    // MPI-3 : eg, openmpi-1.7 (2013) and later
     if (immediate)
     {
+        // MPI-3 : eg, openmpi-1.7 (2013) and later
         profilingPstream::beginTiming();
-
-        handled = true;
         MPI_Request request;
 
         if
@@ -1231,9 +1262,8 @@ void Foam::PstreamDetail::gatherv
         PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
+    else
 #endif
-
-    if (!handled)
     {
         profilingPstream::beginTiming();
 
@@ -1333,15 +1363,12 @@ void Foam::PstreamDetail::scatterv
             << Foam::abort(FatalError);
     }
 
-    bool handled(false);
 
 #if defined(MPI_VERSION) && (MPI_VERSION >= 3)
-    // MPI-3 : eg, openmpi-1.7 (2013) and later
     if (immediate)
     {
+        // MPI-3 : eg, openmpi-1.7 (2013) and later
         profilingPstream::beginTiming();
-
-        handled = true;
         MPI_Request request;
 
         if
@@ -1371,9 +1398,8 @@ void Foam::PstreamDetail::scatterv
         PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
+    else
 #endif
-
-    if (!handled)
     {
         profilingPstream::beginTiming();
 
@@ -1445,15 +1471,12 @@ void Foam::PstreamDetail::allGather
         error::printStack(Pout);
     }
 
-    bool handled(false);
 
 #if defined(MPI_VERSION) && (MPI_VERSION >= 3)
-    // MPI-3 : eg, openmpi-1.7 (2013) and later
     if (immediate)
     {
+        // MPI-3 : eg, openmpi-1.7 (2013) and later
         profilingPstream::beginTiming();
-
-        handled = true;
         MPI_Request request;
 
         if
@@ -1475,9 +1498,8 @@ void Foam::PstreamDetail::allGather
         PstreamGlobals::push_request(request, req, requestID);
         profilingPstream::addRequestTime();
     }
+    else
 #endif
-
-    if (!handled)
     {
         profilingPstream::beginTiming();