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author Chris Cannam <cannam@all-day-breakfast.com>
date Mon, 25 Mar 2013 16:29:40 +0000
parents 89f5e221ed7b
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cannam@95 1 <html lang="en">
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cannam@95 3 <title>An improved replacement for MPI_Alltoall - FFTW 3.3.3</title>
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cannam@95 12 This manual is for FFTW
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cannam@95 15 Copyright (C) 2003 Matteo Frigo.
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cannam@95 48 <a name="An-improved-replacement-for-MPI_Alltoall"></a>
cannam@95 49 <a name="An-improved-replacement-for-MPI_005fAlltoall"></a>
cannam@95 50 <p>
cannam@95 51 Previous:&nbsp;<a rel="previous" accesskey="p" href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface">Advanced distributed-transpose interface</a>,
cannam@95 52 Up:&nbsp;<a rel="up" accesskey="u" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes">FFTW MPI Transposes</a>
cannam@95 53 <hr>
cannam@95 54 </div>
cannam@95 55
cannam@95 56 <h4 class="subsection">6.7.3 An improved replacement for MPI_Alltoall</h4>
cannam@95 57
cannam@95 58 <p>We close this section by noting that FFTW's MPI transpose routines can
cannam@95 59 be thought of as a generalization for the <code>MPI_Alltoall</code> function
cannam@95 60 (albeit only for floating-point types), and in some circumstances can
cannam@95 61 function as an improved replacement.
cannam@95 62 <a name="index-MPI_005fAlltoall-406"></a>
cannam@95 63
cannam@95 64 <p><code>MPI_Alltoall</code> is defined by the MPI standard as:
cannam@95 65
cannam@95 66 <pre class="example"> int MPI_Alltoall(void *sendbuf, int sendcount, MPI_Datatype sendtype,
cannam@95 67 void *recvbuf, int recvcnt, MPI_Datatype recvtype,
cannam@95 68 MPI_Comm comm);
cannam@95 69 </pre>
cannam@95 70 <p>In particular, for <code>double*</code> arrays <code>in</code> and <code>out</code>,
cannam@95 71 consider the call:
cannam@95 72
cannam@95 73 <pre class="example"> MPI_Alltoall(in, howmany, MPI_DOUBLE, out, howmany MPI_DOUBLE, comm);
cannam@95 74 </pre>
cannam@95 75 <p>This is completely equivalent to:
cannam@95 76
cannam@95 77 <pre class="example"> MPI_Comm_size(comm, &amp;P);
cannam@95 78 plan = fftw_mpi_plan_many_transpose(P, P, howmany, 1, 1, in, out, comm, FFTW_ESTIMATE);
cannam@95 79 fftw_execute(plan);
cannam@95 80 fftw_destroy_plan(plan);
cannam@95 81 </pre>
cannam@95 82 <p>That is, computing a P&nbsp;&times;&nbsp;P transpose on <code>P</code> processes,
cannam@95 83 with a block size of 1, is just a standard all-to-all communication.
cannam@95 84
cannam@95 85 <p>However, using the FFTW routine instead of <code>MPI_Alltoall</code> may
cannam@95 86 have certain advantages. First of all, FFTW's routine can operate
cannam@95 87 in-place (<code>in == out</code>) whereas <code>MPI_Alltoall</code> can only
cannam@95 88 operate out-of-place.
cannam@95 89 <a name="index-in_002dplace-407"></a>
cannam@95 90
cannam@95 91 <p>Second, even for out-of-place plans, FFTW's routine may be faster,
cannam@95 92 especially if you need to perform the all-to-all communication many
cannam@95 93 times and can afford to use <code>FFTW_MEASURE</code> or
cannam@95 94 <code>FFTW_PATIENT</code>. It should certainly be no slower, not including
cannam@95 95 the time to create the plan, since one of the possible algorithms that
cannam@95 96 FFTW uses for an out-of-place transpose <em>is</em> simply to call
cannam@95 97 <code>MPI_Alltoall</code>. However, FFTW also considers several other
cannam@95 98 possible algorithms that, depending on your MPI implementation and
cannam@95 99 your hardware, may be faster.
cannam@95 100 <a name="index-FFTW_005fMEASURE-408"></a><a name="index-FFTW_005fPATIENT-409"></a>
cannam@95 101 <!-- -->
cannam@95 102
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cannam@95 104