172 lines
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172 lines
7.8 KiB
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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<title>FFTW MPI Wisdom (FFTW 3.3.10)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Distributed_002dmemory-FFTW-with-MPI.html" rel="up" title="Distributed-memory FFTW with MPI">
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<link href="Avoiding-MPI-Deadlocks.html" rel="next" title="Avoiding MPI Deadlocks">
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<span id="FFTW-MPI-Wisdom"></span><div class="header">
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<p>
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Next: <a href="Avoiding-MPI-Deadlocks.html" accesskey="n" rel="next">Avoiding MPI Deadlocks</a>, Previous: <a href="FFTW-MPI-Transposes.html" accesskey="p" rel="prev">FFTW MPI Transposes</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="FFTW-MPI-Wisdom-1"></span><h3 class="section">6.8 FFTW MPI Wisdom</h3>
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<span id="index-wisdom-2"></span>
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<span id="index-saving-plans-to-disk-2"></span>
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<p>FFTW’s “wisdom” facility (see <a href="Words-of-Wisdom_002dSaving-Plans.html">Words of Wisdom-Saving Plans</a>) can
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be used to save MPI plans as well as to save uniprocessor plans.
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However, for MPI there are several unavoidable complications.
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</p>
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<span id="index-MPI-I_002fO-1"></span>
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<p>First, the MPI standard does not guarantee that every process can
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perform file I/O (at least, not using C stdio routines)—in general,
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we may only assume that process 0 is capable of I/O.<a id="DOCF7" href="#FOOT7"><sup>7</sup></a> So, if we
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want to export the wisdom from a single process to a file, we must
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first export the wisdom to a string, then send it to process 0, then
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write it to a file.
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</p>
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<p>Second, in principle we may want to have separate wisdom for every
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process, since in general the processes may run on different hardware
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even for a single MPI program. However, in practice FFTW’s MPI code
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is designed for the case of homogeneous hardware (see <a href="Load-balancing.html">Load balancing</a>), and in this case it is convenient to use the same wisdom
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for every process. Thus, we need a mechanism to synchronize the wisdom.
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</p>
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<p>To address both of these problems, FFTW provides the following two
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functions:
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</p>
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<div class="example">
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<pre class="example">void fftw_mpi_broadcast_wisdom(MPI_Comm comm);
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void fftw_mpi_gather_wisdom(MPI_Comm comm);
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</pre></div>
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<span id="index-fftw_005fmpi_005fgather_005fwisdom"></span>
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<span id="index-fftw_005fmpi_005fbroadcast_005fwisdom"></span>
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<p>Given a communicator <code>comm</code>, <code>fftw_mpi_broadcast_wisdom</code>
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will broadcast the wisdom from process 0 to all other processes.
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Conversely, <code>fftw_mpi_gather_wisdom</code> will collect wisdom from all
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processes onto process 0. (If the plans created for the same problem
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by different processes are not the same, <code>fftw_mpi_gather_wisdom</code>
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will arbitrarily choose one of the plans.) Both of these functions
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may result in suboptimal plans for different processes if the
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processes are running on non-identical hardware. Both of these
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functions are <em>collective</em> calls, which means that they must be
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executed by all processes in the communicator.
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<span id="index-collective-function-1"></span>
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</p>
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<p>So, for example, a typical code snippet to import wisdom from a file
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and use it on all processes would be:
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</p>
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<div class="example">
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<pre class="example">{
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int rank;
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fftw_mpi_init();
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MPI_Comm_rank(MPI_COMM_WORLD, &rank);
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if (rank == 0) fftw_import_wisdom_from_filename("mywisdom");
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fftw_mpi_broadcast_wisdom(MPI_COMM_WORLD);
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}
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</pre></div>
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<p>(Note that we must call <code>fftw_mpi_init</code> before importing any
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wisdom that might contain MPI plans.) Similarly, a typical code
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snippet to export wisdom from all processes to a file is:
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<span id="index-fftw_005fmpi_005finit-2"></span>
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</p>
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<div class="example">
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<pre class="example">{
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int rank;
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fftw_mpi_gather_wisdom(MPI_COMM_WORLD);
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MPI_Comm_rank(MPI_COMM_WORLD, &rank);
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if (rank == 0) fftw_export_wisdom_to_filename("mywisdom");
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}
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</pre></div>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h5><a id="FOOT7" href="#DOCF7">(7)</a></h3>
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<p>In fact,
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even this assumption is not technically guaranteed by the standard,
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although it seems to be universal in actual MPI implementations and is
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widely assumed by MPI-using software. Technically, you need to query
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the <code>MPI_IO</code> attribute of <code>MPI_COMM_WORLD</code> with
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<code>MPI_Attr_get</code>. If this attribute is <code>MPI_PROC_NULL</code>, no
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I/O is possible. If it is <code>MPI_ANY_SOURCE</code>, any process can
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perform I/O. Otherwise, it is the rank of a process that can perform
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I/O ... but since it is not guaranteed to yield the <em>same</em> rank
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on all processes, you have to do an <code>MPI_Allreduce</code> of some kind
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if you want all processes to agree about which is going to do I/O.
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And even then, the standard only guarantees that this process can
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perform output, but not input. See e.g. <cite>Parallel Programming
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with MPI</cite> by P. S. Pacheco, section 8.1.3. Needless to say, in our
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experience virtually no MPI programmers worry about this.</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Avoiding-MPI-Deadlocks.html" accesskey="n" rel="next">Avoiding MPI Deadlocks</a>, Previous: <a href="FFTW-MPI-Transposes.html" accesskey="p" rel="prev">FFTW MPI Transposes</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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