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Add null config files
author Chris Cannam
date Mon, 02 Mar 2020 14:03:47 +0000
parents 2cd0e3b3e1fd
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Chris@42 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Chris@42 3 <!-- This manual is for FFTW
Chris@42 4 (version 3.3.5, 30 July 2016).
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Chris@42 6 Copyright (C) 2003 Matteo Frigo.
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: An improved replacement for MPI_Alltoall</title>
Chris@42 26
Chris@42 27 <meta name="description" content="FFTW 3.3.5: An improved replacement for MPI_Alltoall">
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Chris@42 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
Chris@42 36 <link href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" rel="up" title="FFTW MPI Transposes">
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Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="An-improved-replacement-for-MPI_005fAlltoall"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="An-improved-replacement-for-MPI_005fAlltoall-1"></a>
Chris@42 79 <h4 class="subsection">6.7.3 An improved replacement for MPI_Alltoall</h4>
Chris@42 80
Chris@42 81 <p>We close this section by noting that FFTW&rsquo;s MPI transpose routines can
Chris@42 82 be thought of as a generalization for the <code>MPI_Alltoall</code> function
Chris@42 83 (albeit only for floating-point types), and in some circumstances can
Chris@42 84 function as an improved replacement.
Chris@42 85 <a name="index-MPI_005fAlltoall"></a>
Chris@42 86 </p>
Chris@42 87
Chris@42 88 <p><code>MPI_Alltoall</code> is defined by the MPI standard as:
Chris@42 89 </p>
Chris@42 90 <div class="example">
Chris@42 91 <pre class="example">int MPI_Alltoall(void *sendbuf, int sendcount, MPI_Datatype sendtype,
Chris@42 92 void *recvbuf, int recvcnt, MPI_Datatype recvtype,
Chris@42 93 MPI_Comm comm);
Chris@42 94 </pre></div>
Chris@42 95
Chris@42 96 <p>In particular, for <code>double*</code> arrays <code>in</code> and <code>out</code>,
Chris@42 97 consider the call:
Chris@42 98 </p>
Chris@42 99 <div class="example">
Chris@42 100 <pre class="example">MPI_Alltoall(in, howmany, MPI_DOUBLE, out, howmany MPI_DOUBLE, comm);
Chris@42 101 </pre></div>
Chris@42 102
Chris@42 103 <p>This is completely equivalent to:
Chris@42 104 </p>
Chris@42 105 <div class="example">
Chris@42 106 <pre class="example">MPI_Comm_size(comm, &amp;P);
Chris@42 107 plan = fftw_mpi_plan_many_transpose(P, P, howmany, 1, 1, in, out, comm, FFTW_ESTIMATE);
Chris@42 108 fftw_execute(plan);
Chris@42 109 fftw_destroy_plan(plan);
Chris@42 110 </pre></div>
Chris@42 111
Chris@42 112 <p>That is, computing a P&nbsp;&times;&nbsp;P transpose on <code>P</code> processes,
Chris@42 113 with a block size of 1, is just a standard all-to-all communication.
Chris@42 114 </p>
Chris@42 115 <p>However, using the FFTW routine instead of <code>MPI_Alltoall</code> may
Chris@42 116 have certain advantages. First of all, FFTW&rsquo;s routine can operate
Chris@42 117 in-place (<code>in == out</code>) whereas <code>MPI_Alltoall</code> can only
Chris@42 118 operate out-of-place.
Chris@42 119 <a name="index-in_002dplace-8"></a>
Chris@42 120 </p>
Chris@42 121
Chris@42 122 <p>Second, even for out-of-place plans, FFTW&rsquo;s routine may be faster,
Chris@42 123 especially if you need to perform the all-to-all communication many
Chris@42 124 times and can afford to use <code>FFTW_MEASURE</code> or
Chris@42 125 <code>FFTW_PATIENT</code>. It should certainly be no slower, not including
Chris@42 126 the time to create the plan, since one of the possible algorithms that
Chris@42 127 FFTW uses for an out-of-place transpose <em>is</em> simply to call
Chris@42 128 <code>MPI_Alltoall</code>. However, FFTW also considers several other
Chris@42 129 possible algorithms that, depending on your MPI implementation and
Chris@42 130 your hardware, may be faster.
Chris@42 131 <a name="index-FFTW_005fMEASURE-3"></a>
Chris@42 132 <a name="index-FFTW_005fPATIENT-4"></a>
Chris@42 133 </p>
Chris@42 134
Chris@42 135
Chris@42 136
Chris@42 137 </body>
Chris@42 138 </html>