mirror of
https://github.com/tildearrow/furnace.git
synced 2024-11-27 15:03:01 +00:00
127 lines
5.3 KiB
HTML
127 lines
5.3 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>An improved replacement for MPI_Alltoall (FFTW 3.3.10)</title>
|
|
|
|
<meta name="description" content="An improved replacement for MPI_Alltoall (FFTW 3.3.10)">
|
|
<meta name="keywords" content="An improved replacement for MPI_Alltoall (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="FFTW-MPI-Transposes.html" rel="up" title="FFTW MPI Transposes">
|
|
<link href="FFTW-MPI-Wisdom.html" rel="next" title="FFTW MPI Wisdom">
|
|
<link href="Advanced-distributed_002dtranspose-interface.html" rel="prev" title="Advanced distributed-transpose interface">
|
|
<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="An-improved-replacement-for-MPI_005fAlltoall"></span><div class="header">
|
|
<p>
|
|
Previous: <a href="Advanced-distributed_002dtranspose-interface.html" accesskey="p" rel="prev">Advanced distributed-transpose interface</a>, Up: <a href="FFTW-MPI-Transposes.html" accesskey="u" rel="up">FFTW MPI Transposes</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="An-improved-replacement-for-MPI_005fAlltoall-1"></span><h4 class="subsection">6.7.3 An improved replacement for MPI_Alltoall</h4>
|
|
|
|
<p>We close this section by noting that FFTW’s MPI transpose routines can
|
|
be thought of as a generalization for the <code>MPI_Alltoall</code> function
|
|
(albeit only for floating-point types), and in some circumstances can
|
|
function as an improved replacement.
|
|
<span id="index-MPI_005fAlltoall"></span>
|
|
</p>
|
|
|
|
<p><code>MPI_Alltoall</code> is defined by the MPI standard as:
|
|
</p>
|
|
<div class="example">
|
|
<pre class="example">int MPI_Alltoall(void *sendbuf, int sendcount, MPI_Datatype sendtype,
|
|
void *recvbuf, int recvcnt, MPI_Datatype recvtype,
|
|
MPI_Comm comm);
|
|
</pre></div>
|
|
|
|
<p>In particular, for <code>double*</code> arrays <code>in</code> and <code>out</code>,
|
|
consider the call:
|
|
</p>
|
|
<div class="example">
|
|
<pre class="example">MPI_Alltoall(in, howmany, MPI_DOUBLE, out, howmany MPI_DOUBLE, comm);
|
|
</pre></div>
|
|
|
|
<p>This is completely equivalent to:
|
|
</p>
|
|
<div class="example">
|
|
<pre class="example">MPI_Comm_size(comm, &P);
|
|
plan = fftw_mpi_plan_many_transpose(P, P, howmany, 1, 1, in, out, comm, FFTW_ESTIMATE);
|
|
fftw_execute(plan);
|
|
fftw_destroy_plan(plan);
|
|
</pre></div>
|
|
|
|
<p>That is, computing a P × P
|
|
transpose on <code>P</code> processes,
|
|
with a block size of 1, is just a standard all-to-all communication.
|
|
</p>
|
|
<p>However, using the FFTW routine instead of <code>MPI_Alltoall</code> may
|
|
have certain advantages. First of all, FFTW’s routine can operate
|
|
in-place (<code>in == out</code>) whereas <code>MPI_Alltoall</code> can only
|
|
operate out-of-place.
|
|
<span id="index-in_002dplace-8"></span>
|
|
</p>
|
|
|
|
<p>Second, even for out-of-place plans, FFTW’s routine may be faster,
|
|
especially if you need to perform the all-to-all communication many
|
|
times and can afford to use <code>FFTW_MEASURE</code> or
|
|
<code>FFTW_PATIENT</code>. It should certainly be no slower, not including
|
|
the time to create the plan, since one of the possible algorithms that
|
|
FFTW uses for an out-of-place transpose <em>is</em> simply to call
|
|
<code>MPI_Alltoall</code>. However, FFTW also considers several other
|
|
possible algorithms that, depending on your MPI implementation and
|
|
your hardware, may be faster.
|
|
<span id="index-FFTW_005fMEASURE-3"></span>
|
|
<span id="index-FFTW_005fPATIENT-4"></span>
|
|
</p>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|