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