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Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents 37bf6b4a2645
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Chris@10 3 <title>An improved replacement for MPI_Alltoall - FFTW 3.3.3</title>
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Chris@10 48 <a name="An-improved-replacement-for-MPI_Alltoall"></a>
Chris@10 49 <a name="An-improved-replacement-for-MPI_005fAlltoall"></a>
Chris@10 50 <p>
Chris@10 51 Previous:&nbsp;<a rel="previous" accesskey="p" href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface">Advanced distributed-transpose interface</a>,
Chris@10 52 Up:&nbsp;<a rel="up" accesskey="u" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes">FFTW MPI Transposes</a>
Chris@10 53 <hr>
Chris@10 54 </div>
Chris@10 55
Chris@10 56 <h4 class="subsection">6.7.3 An improved replacement for MPI_Alltoall</h4>
Chris@10 57
Chris@10 58 <p>We close this section by noting that FFTW's MPI transpose routines can
Chris@10 59 be thought of as a generalization for the <code>MPI_Alltoall</code> function
Chris@10 60 (albeit only for floating-point types), and in some circumstances can
Chris@10 61 function as an improved replacement.
Chris@10 62 <a name="index-MPI_005fAlltoall-406"></a>
Chris@10 63
Chris@10 64 <p><code>MPI_Alltoall</code> is defined by the MPI standard as:
Chris@10 65
Chris@10 66 <pre class="example"> int MPI_Alltoall(void *sendbuf, int sendcount, MPI_Datatype sendtype,
Chris@10 67 void *recvbuf, int recvcnt, MPI_Datatype recvtype,
Chris@10 68 MPI_Comm comm);
Chris@10 69 </pre>
Chris@10 70 <p>In particular, for <code>double*</code> arrays <code>in</code> and <code>out</code>,
Chris@10 71 consider the call:
Chris@10 72
Chris@10 73 <pre class="example"> MPI_Alltoall(in, howmany, MPI_DOUBLE, out, howmany MPI_DOUBLE, comm);
Chris@10 74 </pre>
Chris@10 75 <p>This is completely equivalent to:
Chris@10 76
Chris@10 77 <pre class="example"> MPI_Comm_size(comm, &amp;P);
Chris@10 78 plan = fftw_mpi_plan_many_transpose(P, P, howmany, 1, 1, in, out, comm, FFTW_ESTIMATE);
Chris@10 79 fftw_execute(plan);
Chris@10 80 fftw_destroy_plan(plan);
Chris@10 81 </pre>
Chris@10 82 <p>That is, computing a P&nbsp;&times;&nbsp;P transpose on <code>P</code> processes,
Chris@10 83 with a block size of 1, is just a standard all-to-all communication.
Chris@10 84
Chris@10 85 <p>However, using the FFTW routine instead of <code>MPI_Alltoall</code> may
Chris@10 86 have certain advantages. First of all, FFTW's routine can operate
Chris@10 87 in-place (<code>in == out</code>) whereas <code>MPI_Alltoall</code> can only
Chris@10 88 operate out-of-place.
Chris@10 89 <a name="index-in_002dplace-407"></a>
Chris@10 90
Chris@10 91 <p>Second, even for out-of-place plans, FFTW's routine may be faster,
Chris@10 92 especially if you need to perform the all-to-all communication many
Chris@10 93 times and can afford to use <code>FFTW_MEASURE</code> or
Chris@10 94 <code>FFTW_PATIENT</code>. It should certainly be no slower, not including
Chris@10 95 the time to create the plan, since one of the possible algorithms that
Chris@10 96 FFTW uses for an out-of-place transpose <em>is</em> simply to call
Chris@10 97 <code>MPI_Alltoall</code>. However, FFTW also considers several other
Chris@10 98 possible algorithms that, depending on your MPI implementation and
Chris@10 99 your hardware, may be faster.
Chris@10 100 <a name="index-FFTW_005fMEASURE-408"></a><a name="index-FFTW_005fPATIENT-409"></a>
Chris@10 101 <!-- -->
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Chris@10 104