doc/doxygen/chapters/16mpi_support.doxy: add section describing debugging facilities for MPI

doc/doxygen/chapters/16mpi_support.doxy

@@ -1,7 +1,7 @@
 /*
  * This file is part of the StarPU Handbook.
  * Copyright (C) 2009--2011  Universit@'e de Bordeaux
- * Copyright (C) 2010, 2011, 2012, 2013, 2014, 2015  CNRS
+ * Copyright (C) 2010, 2011, 2012, 2013, 2014, 2015, 2016  CNRS
  * Copyright (C) 2011, 2012 INRIA
  * See the file version.doxy for copying conditions.
  */
@@ -577,6 +577,18 @@ starpu_mpi_gather_detached(data_handles, nblocks, 0, MPI_COMM_WORLD);
 
 */
 
+\section MPIDebug Debugging MPI
+
+Communication trace will be enabled when the environment variable \ref
+STARPU_MPI_COMM is set to 1, and StarPU has been configured with the
+option \ref enable-verbose "--enable-verbose".
+
+Statistics will be enabled for the communication cache when the
+environment variable \ref STARPU_MPI_CACHE_STATS is set to 1. It
+prints messages on the standard output when data are added or removed
+from the received communication cache.
+
+
 \section MPIExamples More MPI examples
 
 MPI examples are available in the StarPU source code in mpi/examples: