From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Fri, 15 Dec 2023 08:03:37 +0000 (-0700)
Subject: Choose a more efficient algorithm.
X-Git-Tag: relicensing~241^2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=refs%2Fpull%2F16357%2Fhead;p=dealii.git

Choose a more efficient algorithm.
---

diff --git a/include/deal.II/lac/sparse_matrix_tools.h b/include/deal.II/lac/sparse_matrix_tools.h
index 455a633e26..4f49a41a18 100644
--- a/include/deal.II/lac/sparse_matrix_tools.h
+++ b/include/deal.II/lac/sparse_matrix_tools.h
@@ -157,6 +157,7 @@ namespace SparseMatrixTools
         {
           T prefix = {};
 
+          // First obtain every process's prefix sum:
           int ierr =
             MPI_Exscan(&value,
                        &prefix,
@@ -166,7 +167,12 @@ namespace SparseMatrixTools
                        comm);
           AssertThrowMPI(ierr);
 
-          T sum = Utilities::MPI::sum(value, comm);
+          // Then we also need the total sum. We could obtain it by
+          // calling Utilities::MPI::sum(), but it is cheaper if we
+          // broadcast it from the last process, which can compute it
+          // from its own prefix sum plus its own value.
+          T sum = Utilities::MPI::broadcast(
+            comm, prefix + value, Utilities::MPI::n_mpi_processes(comm) - 1);
 
           return {prefix, sum};
         }