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<h3 class="section">6.8 FFTW MPI Wisdom</h3>

<p><a name="index-wisdom-410"></a><a name="index-saving-plans-to-disk-411"></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><a name="index-MPI-I_002fO-412"></a>First, the MPI standard does not guarantee that every process can
perform file I/O (at least, not using C stdio routines)&mdash;in general,
we may only assume that process 0 is capable of I/O.<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a> 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.

   <p>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-413"></a><a name="index-fftw_005fmpi_005fbroadcast_005fwisdom-414"></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-415"></a>

   <p>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;
     
         fftw_mpi_init();
         MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank);
         if (rank == 0) fftw_import_wisdom_from_filename("mywisdom");
         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-416"></a>
<pre class="example">     {
         int rank;
     
         fftw_mpi_gather_wisdom(MPI_COMM_WORLD);
         MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank);
         if (rank == 0) fftw_export_wisdom_to_filename("mywisdom");
     }
</pre>
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   <div class="footnote">
<hr>
<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> In fact,
even this assumption is not technically guaranteed by the standard,
although it seems to be universal in actual MPI implementations and is
widely assumed by MPI-using software.  Technically, you need to query
the <code>MPI_IO</code> attribute of <code>MPI_COMM_WORLD</code> with
<code>MPI_Attr_get</code>.  If this attribute is <code>MPI_PROC_NULL</code>, no
I/O is possible.  If it is <code>MPI_ANY_SOURCE</code>, any process can
perform I/O.  Otherwise, it is the rank of a process that can perform
I/O ... but since it is not guaranteed to yield the <em>same</em> rank
on all processes, you have to do an <code>MPI_Allreduce</code> of some kind
if you want all processes to agree about which is going to do I/O. 
And even then, the standard only guarantees that this process can
perform output, but not input. See e.g. <cite>Parallel Programming
with MPI</cite> by P. S. Pacheco, section 8.1.3.  Needless to say, in our
experience virtually no MPI programmers worry about this.</p>

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