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1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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2 <html>
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3 <!-- This manual is for FFTW
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4 (version 3.3.5, 30 July 2016).
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5
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6 Copyright (C) 2003 Matteo Frigo.
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7
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8 Copyright (C) 2003 Massachusetts Institute of Technology.
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9
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10 Permission is granted to make and distribute verbatim copies of this
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11 manual provided the copyright notice and this permission notice are
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12 preserved on all copies.
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13
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14 Permission is granted to copy and distribute modified versions of this
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15 manual under the conditions for verbatim copying, provided that the
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16 entire resulting derived work is distributed under the terms of a
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17 permission notice identical to this one.
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18
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19 Permission is granted to copy and distribute translations of this manual
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20 into another language, under the above conditions for modified versions,
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21 except that this permission notice may be stated in a translation
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22 approved by the Free Software Foundation. -->
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23 <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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24 <head>
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25 <title>FFTW 3.3.5: An improved replacement for MPI_Alltoall</title>
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26
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27 <meta name="description" content="FFTW 3.3.5: An improved replacement for MPI_Alltoall">
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28 <meta name="keywords" content="FFTW 3.3.5: An improved replacement for MPI_Alltoall">
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33 <link href="index.html#Top" rel="start" title="Top">
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34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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36 <link href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" rel="up" title="FFTW MPI Transposes">
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37 <link href="FFTW-MPI-Wisdom.html#FFTW-MPI-Wisdom" rel="next" title="FFTW MPI Wisdom">
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38 <link href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface" rel="prev" title="Advanced distributed-transpose interface">
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64 ul.no-bullet {list-style: none}
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65 -->
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66 </style>
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67
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68
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69 </head>
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70
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71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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72 <a name="An-improved-replacement-for-MPI_005fAlltoall"></a>
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73 <div class="header">
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74 <p>
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75 Previous: <a href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface" accesskey="p" rel="prev">Advanced distributed-transpose interface</a>, Up: <a href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" 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#Concept-Index" title="Index" rel="index">Index</a>]</p>
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76 </div>
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77 <hr>
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78 <a name="An-improved-replacement-for-MPI_005fAlltoall-1"></a>
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79 <h4 class="subsection">6.7.3 An improved replacement for MPI_Alltoall</h4>
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80
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81 <p>We close this section by noting that FFTW’s MPI transpose routines can
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82 be thought of as a generalization for the <code>MPI_Alltoall</code> function
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83 (albeit only for floating-point types), and in some circumstances can
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84 function as an improved replacement.
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85 <a name="index-MPI_005fAlltoall"></a>
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86 </p>
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87
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88 <p><code>MPI_Alltoall</code> is defined by the MPI standard as:
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89 </p>
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90 <div class="example">
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91 <pre class="example">int MPI_Alltoall(void *sendbuf, int sendcount, MPI_Datatype sendtype,
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92 void *recvbuf, int recvcnt, MPI_Datatype recvtype,
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93 MPI_Comm comm);
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94 </pre></div>
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95
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96 <p>In particular, for <code>double*</code> arrays <code>in</code> and <code>out</code>,
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97 consider the call:
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98 </p>
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99 <div class="example">
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100 <pre class="example">MPI_Alltoall(in, howmany, MPI_DOUBLE, out, howmany MPI_DOUBLE, comm);
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101 </pre></div>
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102
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103 <p>This is completely equivalent to:
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104 </p>
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105 <div class="example">
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106 <pre class="example">MPI_Comm_size(comm, &P);
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107 plan = fftw_mpi_plan_many_transpose(P, P, howmany, 1, 1, in, out, comm, FFTW_ESTIMATE);
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108 fftw_execute(plan);
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109 fftw_destroy_plan(plan);
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110 </pre></div>
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111
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112 <p>That is, computing a P × P transpose on <code>P</code> processes,
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113 with a block size of 1, is just a standard all-to-all communication.
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114 </p>
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115 <p>However, using the FFTW routine instead of <code>MPI_Alltoall</code> may
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116 have certain advantages. First of all, FFTW’s routine can operate
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117 in-place (<code>in == out</code>) whereas <code>MPI_Alltoall</code> can only
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118 operate out-of-place.
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119 <a name="index-in_002dplace-8"></a>
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120 </p>
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121
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122 <p>Second, even for out-of-place plans, FFTW’s routine may be faster,
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123 especially if you need to perform the all-to-all communication many
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124 times and can afford to use <code>FFTW_MEASURE</code> or
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125 <code>FFTW_PATIENT</code>. It should certainly be no slower, not including
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126 the time to create the plan, since one of the possible algorithms that
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127 FFTW uses for an out-of-place transpose <em>is</em> simply to call
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128 <code>MPI_Alltoall</code>. However, FFTW also considers several other
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129 possible algorithms that, depending on your MPI implementation and
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130 your hardware, may be faster.
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131 <a name="index-FFTW_005fMEASURE-3"></a>
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132 <a name="index-FFTW_005fPATIENT-4"></a>
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133 </p>
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134
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135
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136
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137 </body>
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138 </html>
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