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| author | Chris Cannam |
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| date | Tue, 06 Oct 2015 13:08:39 +0100 |
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| 18:8db794ca3e0b | 19:26056e866c29 |
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| 1 <html lang="en"> | |
| 2 <head> | |
| 3 <title>An improved replacement for MPI_Alltoall - FFTW 3.3.4</title> | |
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| 5 <meta name="description" content="FFTW 3.3.4"> | |
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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"> | |
| 10 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> | |
| 11 <!-- | |
| 12 This manual is for FFTW | |
| 13 (version 3.3.4, 20 September 2013). | |
| 14 | |
| 15 Copyright (C) 2003 Matteo Frigo. | |
| 16 | |
| 17 Copyright (C) 2003 Massachusetts Institute of Technology. | |
| 18 | |
| 19 Permission is granted to make and distribute verbatim copies of | |
| 20 this manual provided the copyright notice and this permission | |
| 21 notice are preserved on all copies. | |
| 22 | |
| 23 Permission is granted to copy and distribute modified versions of | |
| 24 this manual under the conditions for verbatim copying, provided | |
| 25 that the entire resulting derived work is distributed under the | |
| 26 terms of a permission notice identical to this one. | |
| 27 | |
| 28 Permission is granted to copy and distribute translations of this | |
| 29 manual into another language, under the above conditions for | |
| 30 modified versions, except that this permission notice may be | |
| 31 stated in a translation approved by the Free Software Foundation. | |
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| 45 </head> | |
| 46 <body> | |
| 47 <div class="node"> | |
| 48 <a name="An-improved-replacement-for-MPI_Alltoall"></a> | |
| 49 <a name="An-improved-replacement-for-MPI_005fAlltoall"></a> | |
| 50 <p> | |
| 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-409"></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, &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 × 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-410"></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-411"></a><a name="index-FFTW_005fPATIENT-412"></a> | |
| 101 <!-- --> | |
| 102 | |
| 103 </body></html> | |
| 104 |