128 lines
5.3 KiB
HTML
128 lines
5.3 KiB
HTML
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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<title>An improved replacement for MPI_Alltoall (FFTW 3.3.10)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="FFTW-MPI-Transposes.html" rel="up" title="FFTW MPI Transposes">
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<span id="An-improved-replacement-for-MPI_005fAlltoall"></span><div class="header">
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<p>
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Previous: <a href="Advanced-distributed_002dtranspose-interface.html" accesskey="p" rel="prev">Advanced distributed-transpose interface</a>, Up: <a href="FFTW-MPI-Transposes.html" accesskey="u" rel="up">FFTW MPI Transposes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="An-improved-replacement-for-MPI_005fAlltoall-1"></span><h4 class="subsection">6.7.3 An improved replacement for MPI_Alltoall</h4>
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<p>We close this section by noting that FFTW’s MPI transpose routines can
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be thought of as a generalization for the <code>MPI_Alltoall</code> function
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(albeit only for floating-point types), and in some circumstances can
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function as an improved replacement.
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<span id="index-MPI_005fAlltoall"></span>
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</p>
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<p><code>MPI_Alltoall</code> is defined by the MPI standard as:
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</p>
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<div class="example">
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<pre class="example">int MPI_Alltoall(void *sendbuf, int sendcount, MPI_Datatype sendtype,
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void *recvbuf, int recvcnt, MPI_Datatype recvtype,
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MPI_Comm comm);
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</pre></div>
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<p>In particular, for <code>double*</code> arrays <code>in</code> and <code>out</code>,
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consider the call:
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</p>
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<div class="example">
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<pre class="example">MPI_Alltoall(in, howmany, MPI_DOUBLE, out, howmany MPI_DOUBLE, comm);
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</pre></div>
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<p>This is completely equivalent to:
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</p>
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<div class="example">
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<pre class="example">MPI_Comm_size(comm, &P);
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plan = fftw_mpi_plan_many_transpose(P, P, howmany, 1, 1, in, out, comm, FFTW_ESTIMATE);
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fftw_execute(plan);
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fftw_destroy_plan(plan);
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</pre></div>
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<p>That is, computing a P × P
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transpose on <code>P</code> processes,
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with a block size of 1, is just a standard all-to-all communication.
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</p>
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<p>However, using the FFTW routine instead of <code>MPI_Alltoall</code> may
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have certain advantages. First of all, FFTW’s routine can operate
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in-place (<code>in == out</code>) whereas <code>MPI_Alltoall</code> can only
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operate out-of-place.
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<span id="index-in_002dplace-8"></span>
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</p>
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<p>Second, even for out-of-place plans, FFTW’s routine may be faster,
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especially if you need to perform the all-to-all communication many
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times and can afford to use <code>FFTW_MEASURE</code> or
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<code>FFTW_PATIENT</code>. It should certainly be no slower, not including
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the time to create the plan, since one of the possible algorithms that
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FFTW uses for an out-of-place transpose <em>is</em> simply to call
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<code>MPI_Alltoall</code>. However, FFTW also considers several other
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possible algorithms that, depending on your MPI implementation and
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your hardware, may be faster.
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<span id="index-FFTW_005fMEASURE-3"></span>
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<span id="index-FFTW_005fPATIENT-4"></span>
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</p>
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