Mercurial > hg > sv-dependency-builds

annotate src/fftw-3.3.5/doc/html/FFTW-MPI-Wisdom.html @ 169:223a55898ab9 tip default

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
Add null config files
author Chris Cannam <cannam@all-day-breakfast.com>
date Mon, 02 Mar 2020 14:03:47 +0000
parents 7867fa7e1b6b
children
rev   line source
cannam@127 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
cannam@127 2 <html>
cannam@127 3 <!-- This manual is for FFTW
cannam@127 4 (version 3.3.5, 30 July 2016).
cannam@127 5
cannam@127 6 Copyright (C) 2003 Matteo Frigo.
cannam@127 7
cannam@127 8 Copyright (C) 2003 Massachusetts Institute of Technology.
cannam@127 9
cannam@127 10 Permission is granted to make and distribute verbatim copies of this
cannam@127 11 manual provided the copyright notice and this permission notice are
cannam@127 12 preserved on all copies.
cannam@127 13
cannam@127 14 Permission is granted to copy and distribute modified versions of this
cannam@127 15 manual under the conditions for verbatim copying, provided that the
cannam@127 16 entire resulting derived work is distributed under the terms of a
cannam@127 17 permission notice identical to this one.
cannam@127 18
cannam@127 19 Permission is granted to copy and distribute translations of this manual
cannam@127 20 into another language, under the above conditions for modified versions,
cannam@127 21 except that this permission notice may be stated in a translation
cannam@127 22 approved by the Free Software Foundation. -->
cannam@127 23 <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
cannam@127 24 <head>
cannam@127 25 <title>FFTW 3.3.5: FFTW MPI Wisdom</title>
cannam@127 26
cannam@127 27 <meta name="description" content="FFTW 3.3.5: FFTW MPI Wisdom">
cannam@127 28 <meta name="keywords" content="FFTW 3.3.5: FFTW MPI Wisdom">
cannam@127 29 <meta name="resource-type" content="document">
cannam@127 30 <meta name="distribution" content="global">
cannam@127 31 <meta name="Generator" content="makeinfo">
cannam@127 32 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
cannam@127 33 <link href="index.html#Top" rel="start" title="Top">
cannam@127 34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
cannam@127 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
cannam@127 36 <link href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="up" title="Distributed-memory FFTW with MPI">
cannam@127 37 <link href="Avoiding-MPI-Deadlocks.html#Avoiding-MPI-Deadlocks" rel="next" title="Avoiding MPI Deadlocks">
cannam@127 38 <link href="An-improved-replacement-for-MPI_005fAlltoall.html#An-improved-replacement-for-MPI_005fAlltoall" rel="prev" title="An improved replacement for MPI_Alltoall">
cannam@127 39 <style type="text/css">
cannam@127 40 <!--
cannam@127 41 a.summary-letter {text-decoration: none}
cannam@127 42 blockquote.smallquotation {font-size: smaller}
cannam@127 43 div.display {margin-left: 3.2em}
cannam@127 44 div.example {margin-left: 3.2em}
cannam@127 45 div.indentedblock {margin-left: 3.2em}
cannam@127 46 div.lisp {margin-left: 3.2em}
cannam@127 47 div.smalldisplay {margin-left: 3.2em}
cannam@127 48 div.smallexample {margin-left: 3.2em}
cannam@127 49 div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
cannam@127 50 div.smalllisp {margin-left: 3.2em}
cannam@127 51 kbd {font-style:oblique}
cannam@127 52 pre.display {font-family: inherit}
cannam@127 53 pre.format {font-family: inherit}
cannam@127 54 pre.menu-comment {font-family: serif}
cannam@127 55 pre.menu-preformatted {font-family: serif}
cannam@127 56 pre.smalldisplay {font-family: inherit; font-size: smaller}
cannam@127 57 pre.smallexample {font-size: smaller}
cannam@127 58 pre.smallformat {font-family: inherit; font-size: smaller}
cannam@127 59 pre.smalllisp {font-size: smaller}
cannam@127 60 span.nocodebreak {white-space:nowrap}
cannam@127 61 span.nolinebreak {white-space:nowrap}
cannam@127 62 span.roman {font-family:serif; font-weight:normal}
cannam@127 63 span.sansserif {font-family:sans-serif; font-weight:normal}
cannam@127 64 ul.no-bullet {list-style: none}
cannam@127 65 -->
cannam@127 66 </style>
cannam@127 67
cannam@127 68
cannam@127 69 </head>
cannam@127 70
cannam@127 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
cannam@127 72 <a name="FFTW-MPI-Wisdom"></a>
cannam@127 73 <div class="header">
cannam@127 74 <p>
cannam@127 75 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>
cannam@127 76 </div>
cannam@127 77 <hr>
cannam@127 78 <a name="FFTW-MPI-Wisdom-1"></a>
cannam@127 79 <h3 class="section">6.8 FFTW MPI Wisdom</h3>
cannam@127 80 <a name="index-wisdom-2"></a>
cannam@127 81 <a name="index-saving-plans-to-disk-2"></a>
cannam@127 82
cannam@127 83 <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
cannam@127 84 be used to save MPI plans as well as to save uniprocessor plans.
cannam@127 85 However, for MPI there are several unavoidable complications.
cannam@127 86 </p>
cannam@127 87 <a name="index-MPI-I_002fO-1"></a>
cannam@127 88 <p>First, the MPI standard does not guarantee that every process can
cannam@127 89 perform file I/O (at least, not using C stdio routines)&mdash;in general,
cannam@127 90 we may only assume that process 0 is capable of I/O.<a name="DOCF7" href="#FOOT7"><sup>7</sup></a> So, if we
cannam@127 91 want to export the wisdom from a single process to a file, we must
cannam@127 92 first export the wisdom to a string, then send it to process 0, then
cannam@127 93 write it to a file.
cannam@127 94 </p>
cannam@127 95 <p>Second, in principle we may want to have separate wisdom for every
cannam@127 96 process, since in general the processes may run on different hardware
cannam@127 97 even for a single MPI program. However, in practice FFTW&rsquo;s MPI code
cannam@127 98 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
cannam@127 99 for every process. Thus, we need a mechanism to synchronize the wisdom.
cannam@127 100 </p>
cannam@127 101 <p>To address both of these problems, FFTW provides the following two
cannam@127 102 functions:
cannam@127 103 </p>
cannam@127 104 <div class="example">
cannam@127 105 <pre class="example">void fftw_mpi_broadcast_wisdom(MPI_Comm comm);
cannam@127 106 void fftw_mpi_gather_wisdom(MPI_Comm comm);
cannam@127 107 </pre></div>
cannam@127 108 <a name="index-fftw_005fmpi_005fgather_005fwisdom"></a>
cannam@127 109 <a name="index-fftw_005fmpi_005fbroadcast_005fwisdom"></a>
cannam@127 110
cannam@127 111 <p>Given a communicator <code>comm</code>, <code>fftw_mpi_broadcast_wisdom</code>
cannam@127 112 will broadcast the wisdom from process 0 to all other processes.
cannam@127 113 Conversely, <code>fftw_mpi_gather_wisdom</code> will collect wisdom from all
cannam@127 114 processes onto process 0. (If the plans created for the same problem
cannam@127 115 by different processes are not the same, <code>fftw_mpi_gather_wisdom</code>
cannam@127 116 will arbitrarily choose one of the plans.) Both of these functions
cannam@127 117 may result in suboptimal plans for different processes if the
cannam@127 118 processes are running on non-identical hardware. Both of these
cannam@127 119 functions are <em>collective</em> calls, which means that they must be
cannam@127 120 executed by all processes in the communicator.
cannam@127 121 <a name="index-collective-function-1"></a>
cannam@127 122 </p>
cannam@127 123
cannam@127 124 <p>So, for example, a typical code snippet to import wisdom from a file
cannam@127 125 and use it on all processes would be:
cannam@127 126 </p>
cannam@127 127 <div class="example">
cannam@127 128 <pre class="example">{
cannam@127 129 int rank;
cannam@127 130
cannam@127 131 fftw_mpi_init();
cannam@127 132 MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank);
cannam@127 133 if (rank == 0) fftw_import_wisdom_from_filename(&quot;mywisdom&quot;);
cannam@127 134 fftw_mpi_broadcast_wisdom(MPI_COMM_WORLD);
cannam@127 135 }
cannam@127 136 </pre></div>
cannam@127 137
cannam@127 138 <p>(Note that we must call <code>fftw_mpi_init</code> before importing any
cannam@127 139 wisdom that might contain MPI plans.) Similarly, a typical code
cannam@127 140 snippet to export wisdom from all processes to a file is:
cannam@127 141 <a name="index-fftw_005fmpi_005finit-2"></a>
cannam@127 142 </p>
cannam@127 143 <div class="example">
cannam@127 144 <pre class="example">{
cannam@127 145 int rank;
cannam@127 146
cannam@127 147 fftw_mpi_gather_wisdom(MPI_COMM_WORLD);
cannam@127 148 MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank);
cannam@127 149 if (rank == 0) fftw_export_wisdom_to_filename(&quot;mywisdom&quot;);
cannam@127 150 }
cannam@127 151 </pre></div>
cannam@127 152
cannam@127 153 <div class="footnote">
cannam@127 154 <hr>
cannam@127 155 <h4 class="footnotes-heading">Footnotes</h4>
cannam@127 156
cannam@127 157 <h3><a name="FOOT7" href="#DOCF7">(7)</a></h3>
cannam@127 158 <p>In fact,
cannam@127 159 even this assumption is not technically guaranteed by the standard,
cannam@127 160 although it seems to be universal in actual MPI implementations and is
cannam@127 161 widely assumed by MPI-using software. Technically, you need to query
cannam@127 162 the <code>MPI_IO</code> attribute of <code>MPI_COMM_WORLD</code> with
cannam@127 163 <code>MPI_Attr_get</code>. If this attribute is <code>MPI_PROC_NULL</code>, no
cannam@127 164 I/O is possible. If it is <code>MPI_ANY_SOURCE</code>, any process can
cannam@127 165 perform I/O. Otherwise, it is the rank of a process that can perform
cannam@127 166 I/O ... but since it is not guaranteed to yield the <em>same</em> rank
cannam@127 167 on all processes, you have to do an <code>MPI_Allreduce</code> of some kind
cannam@127 168 if you want all processes to agree about which is going to do I/O.
cannam@127 169 And even then, the standard only guarantees that this process can
cannam@127 170 perform output, but not input. See e.g. <cite>Parallel Programming
cannam@127 171 with MPI</cite> by P. S. Pacheco, section 8.1.3. Needless to say, in our
cannam@127 172 experience virtually no MPI programmers worry about this.</p>
cannam@127 173 </div>
cannam@127 174 <hr>
cannam@127 175 <div class="header">
cannam@127 176 <p>
cannam@127 177 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>
cannam@127 178 </div>
cannam@127 179
cannam@127 180
cannam@127 181
cannam@127 182 </body>
cannam@127 183 </html>