From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Fri, 14 Jan 2022 11:58:12 +0000 (-0700)
Subject: In PEX, do not compute sources but just the number of sources.
X-Git-Tag: v9.4.0-rc1~592^2~2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ed163b97c9fc2bf19922890ab0f6695a56f54816;p=dealii.git

In PEX, do not compute sources but just the number of sources.

We don't actually need to know where we will get messages from, just how many messages
we will get. This is a question that at least in some contexts is cheaper to answer.
---

diff --git a/include/deal.II/base/mpi_consensus_algorithms.h b/include/deal.II/base/mpi_consensus_algorithms.h
index 3c34b591b4..b4570107dc 100644
--- a/include/deal.II/base/mpi_consensus_algorithms.h
+++ b/include/deal.II/base/mpi_consensus_algorithms.h
@@ -446,11 +446,6 @@ namespace Utilities
          */
         std::vector<unsigned int> targets;
 
-        /**
-         * List of ranks of processes wanting to send a request to this process.
-         */
-        std::vector<unsigned int> sources;
-
         // data structures to send and receive requests
 
         /**
diff --git a/include/deal.II/base/mpi_consensus_algorithms.templates.h b/include/deal.II/base/mpi_consensus_algorithms.templates.h
index b0c2993bed..f03687e7f5 100644
--- a/include/deal.II/base/mpi_consensus_algorithms.templates.h
+++ b/include/deal.II/base/mpi_consensus_algorithms.templates.h
@@ -479,13 +479,11 @@ namespace Utilities
                ExcMessage("The consensus algorithms expect that each process "
                           "only sends a single message to another process, "
                           "but the targets provided include duplicates."));
+        const unsigned int n_targets = targets.size();
 
         // 2) determine who wants to communicate with this process
-        sources =
-          compute_point_to_point_communication_pattern(this->comm, targets);
-
-        const unsigned int n_targets = targets.size();
-        const unsigned int n_sources = sources.size();
+        const unsigned int n_sources =
+          compute_n_point_to_point_communications(this->comm, targets);
 
         // 2) allocate memory
         recv_buffers.resize(n_targets);
@@ -529,7 +527,7 @@ namespace Utilities
             AssertThrowMPI(ierr);
           }
 
-        return sources.size();
+        return n_sources;
 #else
         return 0;
 #endif
@@ -548,17 +546,13 @@ namespace Utilities
           consensus_algorithm_pex_process_deliver;
 
         // Wait until we have a message ready for retrieval, though we don't
-        // care which process it is from. We know that the source must be
-        // listed in the 'sources' array, though.
+        // care which process it is from.
         MPI_Status status;
         int ierr = MPI_Probe(MPI_ANY_SOURCE, tag_request, this->comm, &status);
         AssertThrowMPI(ierr);
 
         // Get rank of incoming message and verify that it makes sense
         const unsigned int other_rank = status.MPI_SOURCE;
-        Assert(std::find(sources.begin(), sources.end(), other_rank) !=
-                 sources.end(),
-               ExcInternalError());
 
         Assert(requesting_processes.find(other_rank) ==
                  requesting_processes.end(),