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First commit. VS2013, Codeblocks and Mac OSX configuration
author Geogaddi\David <d.m.ronan@qmul.ac.uk>
date Thu, 09 Jul 2015 01:12:16 +0100
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+<html lang="en">
+<head>
+<title>FFTW MPI Wisdom - FFTW 3.2alpha3</title>
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+<link rel="prev" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" title="FFTW MPI Transposes">
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+<!--
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+Copyright (C) 2003 Matteo Frigo.
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+<div class="node">
+<p>
+<a name="FFTW-MPI-Wisdom"></a>
+Next:&nbsp;<a rel="next" accesskey="n" href="Avoiding-MPI-Deadlocks.html#Avoiding-MPI-Deadlocks">Avoiding MPI Deadlocks</a>,
+Previous:&nbsp;<a rel="previous" accesskey="p" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes">FFTW MPI Transposes</a>,
+Up:&nbsp;<a rel="up" accesskey="u" href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI">Distributed-memory FFTW with MPI</a>
+<hr>
+</div>
+
+<h3 class="section">6.8 FFTW MPI Wisdom</h3>
+
+<p><a name="index-wisdom-380"></a><a name="index-saving-plans-to-disk-381"></a>
+FFTW's &ldquo;wisdom&rdquo; facility (see <a href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans">Words of Wisdom-Saving Plans</a>) can
+be used to save MPI plans as well as to save uniprocessor plans. 
+However, for MPI there are several unavoidable complications.
+
+   <p>First, the MPI standard does not guarantee that every process can
+perform file I/O (at least, not using C stdio routines)&mdash;only the
+process of rank <code>MPI_IO</code> can in general (unless <code>MPI_IO ==
+MPI_ANY_SOURCE</code>).  (In practice, <code>MPI_IO</code> is commonly assumed to
+be zero, i.e. at least process 0 can perform I/O, since otherwise a
+single-process run could not perform I/O.)  So, if we want to export
+the wisdom from a single process to a file, we must first export the
+wisdom to a string, then send it to process 0, then write it to a
+file. 
+<a name="index-MPI_005fIO-382"></a><a name="index-MPI_005fANY_005fSOURCE-383"></a>
+Second, in principle we may want to have separate wisdom for every
+process, since in general the processes may run on different hardware
+even for a single MPI program.  However, in practice FFTW's MPI code
+is designed for the case of homogeneous hardware (see <a href="Load-balancing.html#Load-balancing">Load balancing</a>), and in this case it is convenient to use the same wisdom
+for every process.  Thus, we need a mechanism to synchronize the wisdom.
+
+   <p>To address both of these problems, FFTW provides the following two
+functions:
+
+<pre class="example">     void fftw_mpi_broadcast_wisdom(MPI_Comm comm);
+     void fftw_mpi_gather_wisdom(MPI_Comm comm);
+</pre>
+   <p><a name="index-fftw_005fmpi_005fgather_005fwisdom-384"></a><a name="index-fftw_005fmpi_005fbroadcast_005fwisdom-385"></a>
+Given a communicator <code>comm</code>, <code>fftw_mpi_broadcast_wisdom</code>
+will broadcast the wisdom from process 0 to all other processes. 
+Conversely, <code>fftw_mpi_gather_wisdom</code> will collect wisdom from all
+processes onto process 0.  (If the plans created for the same problem
+by different processes are not the same, <code>fftw_mpi_gather_wisdom</code>
+will arbitrarily choose one of the plans.)  Both of these functions
+may result in suboptimal plans for different processes if the
+processes are running on non-identical hardware.  Both of these
+functions are <em>collective</em> calls, which means that they must be
+executed by all processes in the communicator. 
+<a name="index-collective-function-386"></a>
+So, for example, a typical code snippet to import wisdom from a file
+and use it on all processes would be:
+
+<pre class="example">     {
+         int rank;
+         FILE *f;
+     
+         fftw_mpi_init();
+         MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank);
+         if (rank == 0 &amp;&amp; (f = fopen("mywisdom", "r"))) {
+             fftw_import_wisdom_from_file(f);
+             fclose(f);
+         }
+         fftw_mpi_broadcast_wisdom(MPI_COMM_WORLD);
+     }
+</pre>
+   <p>(Note that we must call <code>fftw_mpi_init</code> before importing any
+wisdom that might contain MPI plans.)  Similarly, a typical code
+snippet to export wisdom from all processes to a file is:
+<a name="index-fftw_005fmpi_005finit-387"></a>
+<pre class="example">     {
+         int rank;
+         FILE *f;
+     
+         fftw_mpi_gather_wisdom(MPI_COMM_WORLD);
+         MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank);
+         if (rank == 0 &amp;&amp; (f = fopen("mywisdom", "w"))) {
+             fftw_export_wisdom_to_file(f);
+             fclose(f);
+         }
+     }
+</pre>
+   <!--  -->
+</body></html>
+