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<title>FFTW 3.3.8: FFTW MPI Wisdom</title>

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<a name="FFTW-MPI-Wisdom"></a>
<div class="header">
<p>
Next: <a href="Avoiding-MPI-Deadlocks.html#Avoiding-MPI-Deadlocks" accesskey="n" rel="next">Avoiding MPI Deadlocks</a>, Previous: <a href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" accesskey="p" rel="prev">FFTW MPI Transposes</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="FFTW-MPI-Wisdom-1"></a>
<h3 class="section">6.8 FFTW MPI Wisdom</h3>
<a name="index-wisdom-2"></a>
<a name="index-saving-plans-to-disk-2"></a>

<p>FFTW&rsquo;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-1"></a>
<p>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 name="DOCF7" href="#FOOT7"><sup>7</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>
<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&rsquo;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>
<p>To address both of these problems, FFTW provides the following two
functions:
</p>
<div class="example">
<pre class="example">void fftw_mpi_broadcast_wisdom(MPI_Comm comm);
void fftw_mpi_gather_wisdom(MPI_Comm comm);
</pre></div>
<a name="index-fftw_005fmpi_005fgather_005fwisdom"></a>
<a name="index-fftw_005fmpi_005fbroadcast_005fwisdom"></a>

<p>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-1"></a>
</p>

<p>So, for example, a typical code snippet to import wisdom from a file
and use it on all processes would be:
</p>
<div class="example">
<pre class="example">{
    int rank;

    fftw_mpi_init();
    MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank);
    if (rank == 0) fftw_import_wisdom_from_filename(&quot;mywisdom&quot;);
    fftw_mpi_broadcast_wisdom(MPI_COMM_WORLD);
}
</pre></div>

<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-2"></a>
</p>
<div class="example">
<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(&quot;mywisdom&quot;);
}
</pre></div>

<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h3><a name="FOOT7" href="#DOCF7">(7)</a></h3>
<p>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>
</div>
<hr>
<div class="header">
<p>
Next: <a href="Avoiding-MPI-Deadlocks.html#Avoiding-MPI-Deadlocks" accesskey="n" rel="next">Avoiding MPI Deadlocks</a>, Previous: <a href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" accesskey="p" rel="prev">FFTW MPI Transposes</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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author	Chris Cannam <cannam@all-day-breakfast.com>
date	Tue, 19 Nov 2019 14:52:55 +0000
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