Mercurial > hg > sv-dependency-builds

annotate src/fftw-3.3.5/doc/html/An-improved-replacement-for-MPI_005fAlltoall.html @ 148:b4bfdf10c4b3

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
Update Win64 capnp builds to v0.6
author Chris Cannam <cannam@all-day-breakfast.com>
date Mon, 22 May 2017 18:56:49 +0100
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: An improved replacement for MPI_Alltoall</title>
cannam@127 26
cannam@127 27 <meta name="description" content="FFTW 3.3.5: An improved replacement for MPI_Alltoall">
cannam@127 28 <meta name="keywords" content="FFTW 3.3.5: An improved replacement for MPI_Alltoall">
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="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" rel="up" title="FFTW MPI Transposes">
cannam@127 37 <link href="FFTW-MPI-Wisdom.html#FFTW-MPI-Wisdom" rel="next" title="FFTW MPI Wisdom">
cannam@127 38 <link href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface" rel="prev" title="Advanced distributed-transpose interface">
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="An-improved-replacement-for-MPI_005fAlltoall"></a>
cannam@127 73 <div class="header">
cannam@127 74 <p>
cannam@127 75 Previous: <a href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface" accesskey="p" rel="prev">Advanced distributed-transpose interface</a>, Up: <a href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" accesskey="u" rel="up">FFTW MPI Transposes</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="An-improved-replacement-for-MPI_005fAlltoall-1"></a>
cannam@127 79 <h4 class="subsection">6.7.3 An improved replacement for MPI_Alltoall</h4>
cannam@127 80
cannam@127 81 <p>We close this section by noting that FFTW&rsquo;s MPI transpose routines can
cannam@127 82 be thought of as a generalization for the <code>MPI_Alltoall</code> function
cannam@127 83 (albeit only for floating-point types), and in some circumstances can
cannam@127 84 function as an improved replacement.
cannam@127 85 <a name="index-MPI_005fAlltoall"></a>
cannam@127 86 </p>
cannam@127 87
cannam@127 88 <p><code>MPI_Alltoall</code> is defined by the MPI standard as:
cannam@127 89 </p>
cannam@127 90 <div class="example">
cannam@127 91 <pre class="example">int MPI_Alltoall(void *sendbuf, int sendcount, MPI_Datatype sendtype,
cannam@127 92 void *recvbuf, int recvcnt, MPI_Datatype recvtype,
cannam@127 93 MPI_Comm comm);
cannam@127 94 </pre></div>
cannam@127 95
cannam@127 96 <p>In particular, for <code>double*</code> arrays <code>in</code> and <code>out</code>,
cannam@127 97 consider the call:
cannam@127 98 </p>
cannam@127 99 <div class="example">
cannam@127 100 <pre class="example">MPI_Alltoall(in, howmany, MPI_DOUBLE, out, howmany MPI_DOUBLE, comm);
cannam@127 101 </pre></div>
cannam@127 102
cannam@127 103 <p>This is completely equivalent to:
cannam@127 104 </p>
cannam@127 105 <div class="example">
cannam@127 106 <pre class="example">MPI_Comm_size(comm, &amp;P);
cannam@127 107 plan = fftw_mpi_plan_many_transpose(P, P, howmany, 1, 1, in, out, comm, FFTW_ESTIMATE);
cannam@127 108 fftw_execute(plan);
cannam@127 109 fftw_destroy_plan(plan);
cannam@127 110 </pre></div>
cannam@127 111
cannam@127 112 <p>That is, computing a P&nbsp;&times;&nbsp;P transpose on <code>P</code> processes,
cannam@127 113 with a block size of 1, is just a standard all-to-all communication.
cannam@127 114 </p>
cannam@127 115 <p>However, using the FFTW routine instead of <code>MPI_Alltoall</code> may
cannam@127 116 have certain advantages. First of all, FFTW&rsquo;s routine can operate
cannam@127 117 in-place (<code>in == out</code>) whereas <code>MPI_Alltoall</code> can only
cannam@127 118 operate out-of-place.
cannam@127 119 <a name="index-in_002dplace-8"></a>
cannam@127 120 </p>
cannam@127 121
cannam@127 122 <p>Second, even for out-of-place plans, FFTW&rsquo;s routine may be faster,
cannam@127 123 especially if you need to perform the all-to-all communication many
cannam@127 124 times and can afford to use <code>FFTW_MEASURE</code> or
cannam@127 125 <code>FFTW_PATIENT</code>. It should certainly be no slower, not including
cannam@127 126 the time to create the plan, since one of the possible algorithms that
cannam@127 127 FFTW uses for an out-of-place transpose <em>is</em> simply to call
cannam@127 128 <code>MPI_Alltoall</code>. However, FFTW also considers several other
cannam@127 129 possible algorithms that, depending on your MPI implementation and
cannam@127 130 your hardware, may be faster.
cannam@127 131 <a name="index-FFTW_005fMEASURE-3"></a>
cannam@127 132 <a name="index-FFTW_005fPATIENT-4"></a>
cannam@127 133 </p>
cannam@127 134
cannam@127 135
cannam@127 136
cannam@127 137 </body>
cannam@127 138 </html>