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Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents 37bf6b4a2645
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Chris@10 1 <html lang="en">
Chris@10 2 <head>
Chris@10 3 <title>FFTW MPI Wisdom - FFTW 3.3.3</title>
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Chris@10 13 This manual is for FFTW
Chris@10 14 (version 3.3.3, 25 November 2012).
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Chris@10 16 Copyright (C) 2003 Matteo Frigo.
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Chris@10 48 <div class="node">
Chris@10 49 <a name="FFTW-MPI-Wisdom"></a>
Chris@10 50 <p>
Chris@10 51 Next:&nbsp;<a rel="next" accesskey="n" href="Avoiding-MPI-Deadlocks.html#Avoiding-MPI-Deadlocks">Avoiding MPI Deadlocks</a>,
Chris@10 52 Previous:&nbsp;<a rel="previous" accesskey="p" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes">FFTW MPI Transposes</a>,
Chris@10 53 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>
Chris@10 54 <hr>
Chris@10 55 </div>
Chris@10 56
Chris@10 57 <h3 class="section">6.8 FFTW MPI Wisdom</h3>
Chris@10 58
Chris@10 59 <p><a name="index-wisdom-410"></a><a name="index-saving-plans-to-disk-411"></a>
Chris@10 60 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
Chris@10 61 be used to save MPI plans as well as to save uniprocessor plans.
Chris@10 62 However, for MPI there are several unavoidable complications.
Chris@10 63
Chris@10 64 <p><a name="index-MPI-I_002fO-412"></a>First, the MPI standard does not guarantee that every process can
Chris@10 65 perform file I/O (at least, not using C stdio routines)&mdash;in general,
Chris@10 66 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
Chris@10 67 want to export the wisdom from a single process to a file, we must
Chris@10 68 first export the wisdom to a string, then send it to process 0, then
Chris@10 69 write it to a file.
Chris@10 70
Chris@10 71 <p>Second, in principle we may want to have separate wisdom for every
Chris@10 72 process, since in general the processes may run on different hardware
Chris@10 73 even for a single MPI program. However, in practice FFTW's MPI code
Chris@10 74 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
Chris@10 75 for every process. Thus, we need a mechanism to synchronize the wisdom.
Chris@10 76
Chris@10 77 <p>To address both of these problems, FFTW provides the following two
Chris@10 78 functions:
Chris@10 79
Chris@10 80 <pre class="example"> void fftw_mpi_broadcast_wisdom(MPI_Comm comm);
Chris@10 81 void fftw_mpi_gather_wisdom(MPI_Comm comm);
Chris@10 82 </pre>
Chris@10 83 <p><a name="index-fftw_005fmpi_005fgather_005fwisdom-413"></a><a name="index-fftw_005fmpi_005fbroadcast_005fwisdom-414"></a>
Chris@10 84 Given a communicator <code>comm</code>, <code>fftw_mpi_broadcast_wisdom</code>
Chris@10 85 will broadcast the wisdom from process 0 to all other processes.
Chris@10 86 Conversely, <code>fftw_mpi_gather_wisdom</code> will collect wisdom from all
Chris@10 87 processes onto process 0. (If the plans created for the same problem
Chris@10 88 by different processes are not the same, <code>fftw_mpi_gather_wisdom</code>
Chris@10 89 will arbitrarily choose one of the plans.) Both of these functions
Chris@10 90 may result in suboptimal plans for different processes if the
Chris@10 91 processes are running on non-identical hardware. Both of these
Chris@10 92 functions are <em>collective</em> calls, which means that they must be
Chris@10 93 executed by all processes in the communicator.
Chris@10 94 <a name="index-collective-function-415"></a>
Chris@10 95
Chris@10 96 <p>So, for example, a typical code snippet to import wisdom from a file
Chris@10 97 and use it on all processes would be:
Chris@10 98
Chris@10 99 <pre class="example"> {
Chris@10 100 int rank;
Chris@10 101
Chris@10 102 fftw_mpi_init();
Chris@10 103 MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank);
Chris@10 104 if (rank == 0) fftw_import_wisdom_from_filename("mywisdom");
Chris@10 105 fftw_mpi_broadcast_wisdom(MPI_COMM_WORLD);
Chris@10 106 }
Chris@10 107 </pre>
Chris@10 108 <p>(Note that we must call <code>fftw_mpi_init</code> before importing any
Chris@10 109 wisdom that might contain MPI plans.) Similarly, a typical code
Chris@10 110 snippet to export wisdom from all processes to a file is:
Chris@10 111 <a name="index-fftw_005fmpi_005finit-416"></a>
Chris@10 112 <pre class="example"> {
Chris@10 113 int rank;
Chris@10 114
Chris@10 115 fftw_mpi_gather_wisdom(MPI_COMM_WORLD);
Chris@10 116 MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank);
Chris@10 117 if (rank == 0) fftw_export_wisdom_to_filename("mywisdom");
Chris@10 118 }
Chris@10 119 </pre>
Chris@10 120 <!-- -->
Chris@10 121 <div class="footnote">
Chris@10 122 <hr>
Chris@10 123 <h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> In fact,
Chris@10 124 even this assumption is not technically guaranteed by the standard,
Chris@10 125 although it seems to be universal in actual MPI implementations and is
Chris@10 126 widely assumed by MPI-using software. Technically, you need to query
Chris@10 127 the <code>MPI_IO</code> attribute of <code>MPI_COMM_WORLD</code> with
Chris@10 128 <code>MPI_Attr_get</code>. If this attribute is <code>MPI_PROC_NULL</code>, no
Chris@10 129 I/O is possible. If it is <code>MPI_ANY_SOURCE</code>, any process can
Chris@10 130 perform I/O. Otherwise, it is the rank of a process that can perform
Chris@10 131 I/O ... but since it is not guaranteed to yield the <em>same</em> rank
Chris@10 132 on all processes, you have to do an <code>MPI_Allreduce</code> of some kind
Chris@10 133 if you want all processes to agree about which is going to do I/O.
Chris@10 134 And even then, the standard only guarantees that this process can
Chris@10 135 perform output, but not input. See e.g. <cite>Parallel Programming
Chris@10 136 with MPI</cite> by P. S. Pacheco, section 8.1.3. Needless to say, in our
Chris@10 137 experience virtually no MPI programmers worry about this.</p>
Chris@10 138
Chris@10 139 <hr></div>
Chris@10 140
Chris@10 141 </body></html>
Chris@10 142