mirror of
https://github.com/tildearrow/furnace.git
synced 2024-11-30 08:23:01 +00:00
171 lines
7.8 KiB
HTML
171 lines
7.8 KiB
HTML
<!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 3.3.10, 10 December 2020).
|
|
|
|
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 6.7, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<title>FFTW MPI Wisdom (FFTW 3.3.10)</title>
|
|
|
|
<meta name="description" content="FFTW MPI Wisdom (FFTW 3.3.10)">
|
|
<meta name="keywords" content="FFTW MPI Wisdom (FFTW 3.3.10)">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content="makeinfo">
|
|
<link href="index.html" rel="start" title="Top">
|
|
<link href="Concept-Index.html" 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" rel="up" title="Distributed-memory FFTW with MPI">
|
|
<link href="Avoiding-MPI-Deadlocks.html" rel="next" title="Avoiding MPI Deadlocks">
|
|
<link href="An-improved-replacement-for-MPI_005fAlltoall.html" rel="prev" title="An improved replacement for MPI_Alltoall">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {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}
|
|
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">
|
|
<span id="FFTW-MPI-Wisdom"></span><div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
<hr>
|
|
<span id="FFTW-MPI-Wisdom-1"></span><h3 class="section">6.8 FFTW MPI Wisdom</h3>
|
|
<span id="index-wisdom-2"></span>
|
|
<span id="index-saving-plans-to-disk-2"></span>
|
|
|
|
<p>FFTW’s “wisdom” facility (see <a href="Words-of-Wisdom_002dSaving-Plans.html">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>
|
|
<span id="index-MPI-I_002fO-1"></span>
|
|
<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 id="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</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>
|
|
<span id="index-fftw_005fmpi_005fgather_005fwisdom"></span>
|
|
<span id="index-fftw_005fmpi_005fbroadcast_005fwisdom"></span>
|
|
|
|
<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.
|
|
<span id="index-collective-function-1"></span>
|
|
</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:
|
|
<span id="index-fftw_005fmpi_005finit-2"></span>
|
|
</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>
|
|
|
|
<h5><a id="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" 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>
|
|
</div>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|