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Add FFTW3
author Chris Cannam
date Wed, 20 Mar 2013 15:35:50 +0000
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3 <title>An improved replacement for MPI_Alltoall - FFTW 3.3.3</title>
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48 <a name="An-improved-replacement-for-MPI_Alltoall"></a>
49 <a name="An-improved-replacement-for-MPI_005fAlltoall"></a>
50 <p>
51 Previous:&nbsp;<a rel="previous" accesskey="p" href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface">Advanced distributed-transpose interface</a>,
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54 </div>
55
56 <h4 class="subsection">6.7.3 An improved replacement for MPI_Alltoall</h4>
57
58 <p>We close this section by noting that FFTW's MPI transpose routines can
59 be thought of as a generalization for the <code>MPI_Alltoall</code> function
60 (albeit only for floating-point types), and in some circumstances can
61 function as an improved replacement.
62 <a name="index-MPI_005fAlltoall-406"></a>
63
64 <p><code>MPI_Alltoall</code> is defined by the MPI standard as:
65
66 <pre class="example"> int MPI_Alltoall(void *sendbuf, int sendcount, MPI_Datatype sendtype,
67 void *recvbuf, int recvcnt, MPI_Datatype recvtype,
68 MPI_Comm comm);
69 </pre>
70 <p>In particular, for <code>double*</code> arrays <code>in</code> and <code>out</code>,
71 consider the call:
72
73 <pre class="example"> MPI_Alltoall(in, howmany, MPI_DOUBLE, out, howmany MPI_DOUBLE, comm);
74 </pre>
75 <p>This is completely equivalent to:
76
77 <pre class="example"> MPI_Comm_size(comm, &amp;P);
78 plan = fftw_mpi_plan_many_transpose(P, P, howmany, 1, 1, in, out, comm, FFTW_ESTIMATE);
79 fftw_execute(plan);
80 fftw_destroy_plan(plan);
81 </pre>
82 <p>That is, computing a P&nbsp;&times;&nbsp;P transpose on <code>P</code> processes,
83 with a block size of 1, is just a standard all-to-all communication.
84
85 <p>However, using the FFTW routine instead of <code>MPI_Alltoall</code> may
86 have certain advantages. First of all, FFTW's routine can operate
87 in-place (<code>in == out</code>) whereas <code>MPI_Alltoall</code> can only
88 operate out-of-place.
89 <a name="index-in_002dplace-407"></a>
90
91 <p>Second, even for out-of-place plans, FFTW's routine may be faster,
92 especially if you need to perform the all-to-all communication many
93 times and can afford to use <code>FFTW_MEASURE</code> or
94 <code>FFTW_PATIENT</code>. It should certainly be no slower, not including
95 the time to create the plan, since one of the possible algorithms that
96 FFTW uses for an out-of-place transpose <em>is</em> simply to call
97 <code>MPI_Alltoall</code>. However, FFTW also considers several other
98 possible algorithms that, depending on your MPI implementation and
99 your hardware, may be faster.
100 <a name="index-FFTW_005fMEASURE-408"></a><a name="index-FFTW_005fPATIENT-409"></a>
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