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| author | Geogaddi\David <d.m.ronan@qmul.ac.uk> |
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| date | Thu, 09 Jul 2015 01:12:16 +0100 |
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| 1 <html lang="en"> | |
| 2 <head> | |
| 3 <title>An improved replacement for MPI_Alltoall - FFTW 3.2alpha3</title> | |
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| 5 <meta name="description" content="FFTW 3.2alpha3"> | |
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| 8 <link rel="up" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" title="FFTW MPI Transposes"> | |
| 9 <link rel="prev" href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface" title="Advanced distributed-transpose interface"> | |
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| 11 <!-- | |
| 12 This manual is for FFTW | |
| 13 (version 3.2alpha3, 14 August 2007). | |
| 14 | |
| 15 Copyright (C) 2003 Matteo Frigo. | |
| 16 | |
| 17 Copyright (C) 2003 Massachusetts Institute of Technology. | |
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| 25 that the entire resulting derived work is distributed under the | |
| 26 terms of a permission notice identical to this one. | |
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| 28 Permission is granted to copy and distribute translations of this | |
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| 45 </head> | |
| 46 <body> | |
| 47 <div class="node"> | |
| 48 <p> | |
| 49 <a name="An-improved-replacement-for-MPI_Alltoall"></a> | |
| 50 <a name="An-improved-replacement-for-MPI_005fAlltoall"></a> | |
| 51 Previous: <a rel="previous" accesskey="p" href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface">Advanced distributed-transpose interface</a>, | |
| 52 Up: <a rel="up" accesskey="u" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes">FFTW MPI Transposes</a> | |
| 53 <hr> | |
| 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-376"></a> | |
| 63 <code>MPI_Alltoall</code> is defined by the MPI standard as: | |
| 64 | |
| 65 <pre class="example"> int MPI_Alltoall(void *sendbuf, int sendcount, MPI_Datatype sendtype, | |
| 66 void *recvbuf, int recvcnt, MPI_Datatype recvtype, | |
| 67 MPI_Comm comm); | |
| 68 </pre> | |
| 69 <p>In particular, for <code>double*</code> arrays <code>in</code> and <code>out</code>, | |
| 70 consider the call: | |
| 71 | |
| 72 <pre class="example"> MPI_Alltoall(in, howmany, MPI_DOUBLE, out, howmany MPI_DOUBLE, comm); | |
| 73 </pre> | |
| 74 <p>This is completely equivalent to: | |
| 75 | |
| 76 <pre class="example"> MPI_Comm_size(comm, &P); | |
| 77 plan = fftw_mpi_plan_many_transpose(P, P, howmany, 1, 1, in, out, comm, FFTW_ESTIMATE); | |
| 78 fftw_execute(plan); | |
| 79 fftw_destroy_plan(plan); | |
| 80 </pre> | |
| 81 <p>That is, computing a P × P transpose on <code>P</code> processes, | |
| 82 with a block size of 1, is just a standard all-to-all communication. | |
| 83 | |
| 84 <p>However, using the FFTW routine instead of <code>MPI_Alltoall</code> may | |
| 85 have certain advantages. First of all, FFTW's routine can operate | |
| 86 in-place <code>in == out</code>) whereas <code>MPI_Alltoall</code> can only | |
| 87 operate out-of-place. | |
| 88 <a name="index-in_002dplace-377"></a> | |
| 89 Second, even for out-of-place plans, FFTW's routine may be faster, | |
| 90 especially if you need to perform the all-to-all communication many | |
| 91 times and can afford to use <code>FFTW_MEASURE</code> or | |
| 92 <code>FFTW_PATIENT</code>. It should certainly be no slower, not including | |
| 93 the time to create the plan, since one of the possible algorithms that | |
| 94 FFTW uses for an out-of-place transpose <em>is</em> simply to call | |
| 95 <code>MPI_Alltoall</code>. However, FFTW also considers several other | |
| 96 possible algorithms that, depending on your MPI implementation and | |
| 97 your hardware, may be faster. | |
| 98 <a name="index-FFTW_005fMEASURE-378"></a><a name="index-FFTW_005fPATIENT-379"></a> | |
| 99 <!-- --> | |
| 100 | |
| 101 </body></html> | |
| 102 |