From: Denis Davydov <davydden@gmail.com>
Date: Wed, 28 Mar 2018 09:00:23 +0000 (+0200)
Subject: extend description
X-Git-Tag: v9.0.0-rc1~273^2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=refs%2Fpull%2F6107%2Fhead;p=dealii.git

extend description
---

diff --git a/include/deal.II/lac/la_parallel_block_vector.h b/include/deal.II/lac/la_parallel_block_vector.h
index 4c6aa0ff13..f9f20a82fe 100644
--- a/include/deal.II/lac/la_parallel_block_vector.h
+++ b/include/deal.II/lac/la_parallel_block_vector.h
@@ -84,6 +84,13 @@ namespace LinearAlgebra
       /**
        * The chunks size to split communication in update_ghost_values()
        * and compress() calls.
+       *
+       * Most common MPI implementations will get slow when too many
+       * messages/requests are outstanding. Even when messages are small,
+       * say 1 kB only, we should collect enough data with @p communication_block_size
+       * to cover typical infiniband latencies which are around a few microseconds.
+       * Sending 20 kB at a throughput of 5 GB/s takes 4 microseconds,
+       * so we should arrive at the bandwidth dominated regime then which is good enough.
        */
       static constexpr unsigned int communication_block_size = 20;
     public: