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 4c6aa0ff13ec72df23a86a02e766d25449331814..f9f20a82fed88c6a5bad70823d48a34ecaa19c1a 100644 (file)
--- 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: