Mercurial > hg > sv-dependency-builds
diff src/fftw-3.3.8/doc/html/FFTW-MPI-Wisdom.html @ 82:d0c2a83c1364
Add FFTW 3.3.8 source, and a Linux build
| author | Chris Cannam |
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| date | Tue, 19 Nov 2019 14:52:55 +0000 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.8/doc/html/FFTW-MPI-Wisdom.html Tue Nov 19 14:52:55 2019 +0000 @@ -0,0 +1,182 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This manual is for FFTW +(version 3.3.8, 24 May 2018). + +Copyright (C) 2003 Matteo Frigo. + +Copyright (C) 2003 Massachusetts Institute of Technology. + +Permission is granted to make and distribute verbatim copies of this +manual provided the copyright notice and this permission notice are +preserved on all copies. + +Permission is granted to copy and distribute modified versions of this +manual under the conditions for verbatim copying, provided that the +entire resulting derived work is distributed under the terms of a +permission notice identical to this one. + +Permission is granted to copy and distribute translations of this manual +into another language, under the above conditions for modified versions, +except that this permission notice may be stated in a translation +approved by the Free Software Foundation. --> +<!-- Created by GNU Texinfo 6.3, http://www.gnu.org/software/texinfo/ --> +<head> +<title>FFTW 3.3.8: FFTW MPI Wisdom</title> + +<meta name="description" content="FFTW 3.3.8: FFTW MPI Wisdom"> +<meta name="keywords" content="FFTW 3.3.8: FFTW MPI Wisdom"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="up" title="Distributed-memory FFTW with MPI"> +<link href="Avoiding-MPI-Deadlocks.html#Avoiding-MPI-Deadlocks" rel="next" title="Avoiding MPI Deadlocks"> +<link href="An-improved-replacement-for-MPI_005fAlltoall.html#An-improved-replacement-for-MPI_005fAlltoall" rel="prev" title="An improved replacement for MPI_Alltoall"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.indentedblock {margin-right: 0em} +blockquote.smallindentedblock {margin-right: 0em; font-size: smaller} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smalllisp {margin-left: 3.2em} +kbd {font-style: oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nolinebreak {white-space: nowrap} +span.roman {font-family: initial; font-weight: normal} +span.sansserif {font-family: sans-serif; font-weight: normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en"> +<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> [<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’s “wisdom” 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)—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’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, &rank); + if (rank == 0) fftw_import_wisdom_from_filename("mywisdom"); + 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, &rank); + if (rank == 0) fftw_export_wisdom_to_filename("mywisdom"); +} +</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> [<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> + + + +</body> +</html>