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Add FFTW 3.3.8 source, and a Linux build
author Chris Cannam
date Tue, 19 Nov 2019 14:52:55 +0000
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Chris@82 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Chris@82 3 <!-- This manual is for FFTW
Chris@82 4 (version 3.3.8, 24 May 2018).
Chris@82 5
Chris@82 6 Copyright (C) 2003 Matteo Frigo.
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Chris@82 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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Chris@82 24 <head>
Chris@82 25 <title>FFTW 3.3.8: An improved replacement for MPI_Alltoall</title>
Chris@82 26
Chris@82 27 <meta name="description" content="FFTW 3.3.8: An improved replacement for MPI_Alltoall">
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Chris@82 34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
Chris@82 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
Chris@82 36 <link href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" rel="up" title="FFTW MPI Transposes">
Chris@82 37 <link href="FFTW-MPI-Wisdom.html#FFTW-MPI-Wisdom" rel="next" title="FFTW MPI Wisdom">
Chris@82 38 <link href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface" rel="prev" title="Advanced distributed-transpose interface">
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Chris@82 67
Chris@82 68 </head>
Chris@82 69
Chris@82 70 <body lang="en">
Chris@82 71 <a name="An-improved-replacement-for-MPI_005fAlltoall"></a>
Chris@82 72 <div class="header">
Chris@82 73 <p>
Chris@82 74 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@82 75 </div>
Chris@82 76 <hr>
Chris@82 77 <a name="An-improved-replacement-for-MPI_005fAlltoall-1"></a>
Chris@82 78 <h4 class="subsection">6.7.3 An improved replacement for MPI_Alltoall</h4>
Chris@82 79
Chris@82 80 <p>We close this section by noting that FFTW&rsquo;s MPI transpose routines can
Chris@82 81 be thought of as a generalization for the <code>MPI_Alltoall</code> function
Chris@82 82 (albeit only for floating-point types), and in some circumstances can
Chris@82 83 function as an improved replacement.
Chris@82 84 <a name="index-MPI_005fAlltoall"></a>
Chris@82 85 </p>
Chris@82 86
Chris@82 87 <p><code>MPI_Alltoall</code> is defined by the MPI standard as:
Chris@82 88 </p>
Chris@82 89 <div class="example">
Chris@82 90 <pre class="example">int MPI_Alltoall(void *sendbuf, int sendcount, MPI_Datatype sendtype,
Chris@82 91 void *recvbuf, int recvcnt, MPI_Datatype recvtype,
Chris@82 92 MPI_Comm comm);
Chris@82 93 </pre></div>
Chris@82 94
Chris@82 95 <p>In particular, for <code>double*</code> arrays <code>in</code> and <code>out</code>,
Chris@82 96 consider the call:
Chris@82 97 </p>
Chris@82 98 <div class="example">
Chris@82 99 <pre class="example">MPI_Alltoall(in, howmany, MPI_DOUBLE, out, howmany MPI_DOUBLE, comm);
Chris@82 100 </pre></div>
Chris@82 101
Chris@82 102 <p>This is completely equivalent to:
Chris@82 103 </p>
Chris@82 104 <div class="example">
Chris@82 105 <pre class="example">MPI_Comm_size(comm, &amp;P);
Chris@82 106 plan = fftw_mpi_plan_many_transpose(P, P, howmany, 1, 1, in, out, comm, FFTW_ESTIMATE);
Chris@82 107 fftw_execute(plan);
Chris@82 108 fftw_destroy_plan(plan);
Chris@82 109 </pre></div>
Chris@82 110
Chris@82 111 <p>That is, computing a P&nbsp;&times;&nbsp;P
Chris@82 112 transpose on <code>P</code> processes,
Chris@82 113 with a block size of 1, is just a standard all-to-all communication.
Chris@82 114 </p>
Chris@82 115 <p>However, using the FFTW routine instead of <code>MPI_Alltoall</code> may
Chris@82 116 have certain advantages. First of all, FFTW&rsquo;s routine can operate
Chris@82 117 in-place (<code>in == out</code>) whereas <code>MPI_Alltoall</code> can only
Chris@82 118 operate out-of-place.
Chris@82 119 <a name="index-in_002dplace-8"></a>
Chris@82 120 </p>
Chris@82 121
Chris@82 122 <p>Second, even for out-of-place plans, FFTW&rsquo;s routine may be faster,
Chris@82 123 especially if you need to perform the all-to-all communication many
Chris@82 124 times and can afford to use <code>FFTW_MEASURE</code> or
Chris@82 125 <code>FFTW_PATIENT</code>. It should certainly be no slower, not including
Chris@82 126 the time to create the plan, since one of the possible algorithms that
Chris@82 127 FFTW uses for an out-of-place transpose <em>is</em> simply to call
Chris@82 128 <code>MPI_Alltoall</code>. However, FFTW also considers several other
Chris@82 129 possible algorithms that, depending on your MPI implementation and
Chris@82 130 your hardware, may be faster.
Chris@82 131 <a name="index-FFTW_005fMEASURE-3"></a>
Chris@82 132 <a name="index-FFTW_005fPATIENT-4"></a>
Chris@82 133 </p>
Chris@82 134
Chris@82 135
Chris@82 136
Chris@82 137 </body>
Chris@82 138 </html>