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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 2cd0e3b3e1fd
children
rev   line source
Chris@42 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
Chris@42 2 <html>
Chris@42 3 <!-- This manual is for FFTW
Chris@42 4 (version 3.3.5, 30 July 2016).
Chris@42 5
Chris@42 6 Copyright (C) 2003 Matteo Frigo.
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Chris@42 8 Copyright (C) 2003 Massachusetts Institute of Technology.
Chris@42 9
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Chris@42 22 approved by the Free Software Foundation. -->
Chris@42 23 <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: 2d MPI example</title>
Chris@42 26
Chris@42 27 <meta name="description" content="FFTW 3.3.5: 2d MPI example">
Chris@42 28 <meta name="keywords" content="FFTW 3.3.5: 2d MPI example">
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Chris@42 33 <link href="index.html#Top" rel="start" title="Top">
Chris@42 34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
Chris@42 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
Chris@42 36 <link href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="up" title="Distributed-memory FFTW with MPI">
Chris@42 37 <link href="MPI-Data-Distribution.html#MPI-Data-Distribution" rel="next" title="MPI Data Distribution">
Chris@42 38 <link href="Linking-and-Initializing-MPI-FFTW.html#Linking-and-Initializing-MPI-FFTW" rel="prev" title="Linking and Initializing MPI FFTW">
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Chris@42 69 </head>
Chris@42 70
Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="g_t2d-MPI-example"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 Next: <a href="MPI-Data-Distribution.html#MPI-Data-Distribution" accesskey="n" rel="next">MPI Data Distribution</a>, Previous: <a href="Linking-and-Initializing-MPI-FFTW.html#Linking-and-Initializing-MPI-FFTW" accesskey="p" rel="prev">Linking and Initializing MPI FFTW</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> &nbsp; [<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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="g_t2d-MPI-example-1"></a>
Chris@42 79 <h3 class="section">6.3 2d MPI example</h3>
Chris@42 80
Chris@42 81 <p>Before we document the FFTW MPI interface in detail, we begin with a
Chris@42 82 simple example outlining how one would perform a two-dimensional
Chris@42 83 <code>N0</code> by <code>N1</code> complex DFT.
Chris@42 84 </p>
Chris@42 85 <div class="example">
Chris@42 86 <pre class="example">#include &lt;fftw3-mpi.h&gt;
Chris@42 87
Chris@42 88 int main(int argc, char **argv)
Chris@42 89 {
Chris@42 90 const ptrdiff_t N0 = ..., N1 = ...;
Chris@42 91 fftw_plan plan;
Chris@42 92 fftw_complex *data;
Chris@42 93 ptrdiff_t alloc_local, local_n0, local_0_start, i, j;
Chris@42 94
Chris@42 95 MPI_Init(&amp;argc, &amp;argv);
Chris@42 96 fftw_mpi_init();
Chris@42 97
Chris@42 98 /* <span class="roman">get local data size and allocate</span> */
Chris@42 99 alloc_local = fftw_mpi_local_size_2d(N0, N1, MPI_COMM_WORLD,
Chris@42 100 &amp;local_n0, &amp;local_0_start);
Chris@42 101 data = fftw_alloc_complex(alloc_local);
Chris@42 102
Chris@42 103 /* <span class="roman">create plan for in-place forward DFT</span> */
Chris@42 104 plan = fftw_mpi_plan_dft_2d(N0, N1, data, data, MPI_COMM_WORLD,
Chris@42 105 FFTW_FORWARD, FFTW_ESTIMATE);
Chris@42 106
Chris@42 107 /* <span class="roman">initialize data to some function</span> my_function(x,y) */
Chris@42 108 for (i = 0; i &lt; local_n0; ++i) for (j = 0; j &lt; N1; ++j)
Chris@42 109 data[i*N1 + j] = my_function(local_0_start + i, j);
Chris@42 110
Chris@42 111 /* <span class="roman">compute transforms, in-place, as many times as desired</span> */
Chris@42 112 fftw_execute(plan);
Chris@42 113
Chris@42 114 fftw_destroy_plan(plan);
Chris@42 115
Chris@42 116 MPI_Finalize();
Chris@42 117 }
Chris@42 118 </pre></div>
Chris@42 119
Chris@42 120 <p>As can be seen above, the MPI interface follows the same basic style
Chris@42 121 of allocate/plan/execute/destroy as the serial FFTW routines. All of
Chris@42 122 the MPI-specific routines are prefixed with &lsquo;<samp>fftw_mpi_</samp>&rsquo; instead
Chris@42 123 of &lsquo;<samp>fftw_</samp>&rsquo;. There are a few important differences, however:
Chris@42 124 </p>
Chris@42 125 <p>First, we must call <code>fftw_mpi_init()</code> after calling
Chris@42 126 <code>MPI_Init</code> (required in all MPI programs) and before calling any
Chris@42 127 other &lsquo;<samp>fftw_mpi_</samp>&rsquo; routine.
Chris@42 128 <a name="index-MPI_005fInit"></a>
Chris@42 129 <a name="index-fftw_005fmpi_005finit-1"></a>
Chris@42 130 </p>
Chris@42 131
Chris@42 132 <p>Second, when we create the plan with <code>fftw_mpi_plan_dft_2d</code>,
Chris@42 133 analogous to <code>fftw_plan_dft_2d</code>, we pass an additional argument:
Chris@42 134 the communicator, indicating which processes will participate in the
Chris@42 135 transform (here <code>MPI_COMM_WORLD</code>, indicating all processes).
Chris@42 136 Whenever you create, execute, or destroy a plan for an MPI transform,
Chris@42 137 you must call the corresponding FFTW routine on <em>all</em> processes
Chris@42 138 in the communicator for that transform. (That is, these are
Chris@42 139 <em>collective</em> calls.) Note that the plan for the MPI transform
Chris@42 140 uses the standard <code>fftw_execute</code> and <code>fftw_destroy</code> routines
Chris@42 141 (on the other hand, there are MPI-specific new-array execute functions
Chris@42 142 documented below).
Chris@42 143 <a name="index-collective-function"></a>
Chris@42 144 <a name="index-fftw_005fmpi_005fplan_005fdft_005f2d"></a>
Chris@42 145 <a name="index-MPI_005fCOMM_005fWORLD-1"></a>
Chris@42 146 </p>
Chris@42 147
Chris@42 148 <p>Third, all of the FFTW MPI routines take <code>ptrdiff_t</code> arguments
Chris@42 149 instead of <code>int</code> as for the serial FFTW. <code>ptrdiff_t</code> is a
Chris@42 150 standard C integer type which is (at least) 32 bits wide on a 32-bit
Chris@42 151 machine and 64 bits wide on a 64-bit machine. This is to make it easy
Chris@42 152 to specify very large parallel transforms on a 64-bit machine. (You
Chris@42 153 can specify 64-bit transform sizes in the serial FFTW, too, but only
Chris@42 154 by using the &lsquo;<samp>guru64</samp>&rsquo; planner interface. See <a href="64_002dbit-Guru-Interface.html#g_t64_002dbit-Guru-Interface">64-bit Guru Interface</a>.)
Chris@42 155 <a name="index-ptrdiff_005ft-1"></a>
Chris@42 156 <a name="index-64_002dbit-architecture-1"></a>
Chris@42 157 </p>
Chris@42 158
Chris@42 159 <p>Fourth, and most importantly, you don&rsquo;t allocate the entire
Chris@42 160 two-dimensional array on each process. Instead, you call
Chris@42 161 <code>fftw_mpi_local_size_2d</code> to find out what <em>portion</em> of the
Chris@42 162 array resides on each processor, and how much space to allocate.
Chris@42 163 Here, the portion of the array on each process is a <code>local_n0</code> by
Chris@42 164 <code>N1</code> slice of the total array, starting at index
Chris@42 165 <code>local_0_start</code>. The total number of <code>fftw_complex</code> numbers
Chris@42 166 to allocate is given by the <code>alloc_local</code> return value, which
Chris@42 167 <em>may</em> be greater than <code>local_n0 * N1</code> (in case some
Chris@42 168 intermediate calculations require additional storage). The data
Chris@42 169 distribution in FFTW&rsquo;s MPI interface is described in more detail by
Chris@42 170 the next section.
Chris@42 171 <a name="index-fftw_005fmpi_005flocal_005fsize_005f2d"></a>
Chris@42 172 <a name="index-data-distribution-1"></a>
Chris@42 173 </p>
Chris@42 174
Chris@42 175 <p>Given the portion of the array that resides on the local process, it
Chris@42 176 is straightforward to initialize the data (here to a function
Chris@42 177 <code>myfunction</code>) and otherwise manipulate it. Of course, at the end
Chris@42 178 of the program you may want to output the data somehow, but
Chris@42 179 synchronizing this output is up to you and is beyond the scope of this
Chris@42 180 manual. (One good way to output a large multi-dimensional distributed
Chris@42 181 array in MPI to a portable binary file is to use the free HDF5
Chris@42 182 library; see the <a href="http://www.hdfgroup.org/">HDF home page</a>.)
Chris@42 183 <a name="index-HDF5"></a>
Chris@42 184 <a name="index-MPI-I_002fO"></a>
Chris@42 185 </p>
Chris@42 186 <hr>
Chris@42 187 <div class="header">
Chris@42 188 <p>
Chris@42 189 Next: <a href="MPI-Data-Distribution.html#MPI-Data-Distribution" accesskey="n" rel="next">MPI Data Distribution</a>, Previous: <a href="Linking-and-Initializing-MPI-FFTW.html#Linking-and-Initializing-MPI-FFTW" accesskey="p" rel="prev">Linking and Initializing MPI FFTW</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> &nbsp; [<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>
Chris@42 190 </div>
Chris@42 191
Chris@42 192
Chris@42 193
Chris@42 194 </body>
Chris@42 195 </html>