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author Chris Cannam <cannam@all-day-breakfast.com>
date Mon, 02 Mar 2020 14:03:47 +0000
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cannam@127 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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cannam@127 3 <!-- This manual is for FFTW
cannam@127 4 (version 3.3.5, 30 July 2016).
cannam@127 5
cannam@127 6 Copyright (C) 2003 Matteo Frigo.
cannam@127 7
cannam@127 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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cannam@127 24 <head>
cannam@127 25 <title>FFTW 3.3.5: 2d MPI example</title>
cannam@127 26
cannam@127 27 <meta name="description" content="FFTW 3.3.5: 2d MPI example">
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cannam@127 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
cannam@127 36 <link href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="up" title="Distributed-memory FFTW with MPI">
cannam@127 37 <link href="MPI-Data-Distribution.html#MPI-Data-Distribution" rel="next" title="MPI Data Distribution">
cannam@127 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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cannam@127 69 </head>
cannam@127 70
cannam@127 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
cannam@127 72 <a name="g_t2d-MPI-example"></a>
cannam@127 73 <div class="header">
cannam@127 74 <p>
cannam@127 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>
cannam@127 76 </div>
cannam@127 77 <hr>
cannam@127 78 <a name="g_t2d-MPI-example-1"></a>
cannam@127 79 <h3 class="section">6.3 2d MPI example</h3>
cannam@127 80
cannam@127 81 <p>Before we document the FFTW MPI interface in detail, we begin with a
cannam@127 82 simple example outlining how one would perform a two-dimensional
cannam@127 83 <code>N0</code> by <code>N1</code> complex DFT.
cannam@127 84 </p>
cannam@127 85 <div class="example">
cannam@127 86 <pre class="example">#include &lt;fftw3-mpi.h&gt;
cannam@127 87
cannam@127 88 int main(int argc, char **argv)
cannam@127 89 {
cannam@127 90 const ptrdiff_t N0 = ..., N1 = ...;
cannam@127 91 fftw_plan plan;
cannam@127 92 fftw_complex *data;
cannam@127 93 ptrdiff_t alloc_local, local_n0, local_0_start, i, j;
cannam@127 94
cannam@127 95 MPI_Init(&amp;argc, &amp;argv);
cannam@127 96 fftw_mpi_init();
cannam@127 97
cannam@127 98 /* <span class="roman">get local data size and allocate</span> */
cannam@127 99 alloc_local = fftw_mpi_local_size_2d(N0, N1, MPI_COMM_WORLD,
cannam@127 100 &amp;local_n0, &amp;local_0_start);
cannam@127 101 data = fftw_alloc_complex(alloc_local);
cannam@127 102
cannam@127 103 /* <span class="roman">create plan for in-place forward DFT</span> */
cannam@127 104 plan = fftw_mpi_plan_dft_2d(N0, N1, data, data, MPI_COMM_WORLD,
cannam@127 105 FFTW_FORWARD, FFTW_ESTIMATE);
cannam@127 106
cannam@127 107 /* <span class="roman">initialize data to some function</span> my_function(x,y) */
cannam@127 108 for (i = 0; i &lt; local_n0; ++i) for (j = 0; j &lt; N1; ++j)
cannam@127 109 data[i*N1 + j] = my_function(local_0_start + i, j);
cannam@127 110
cannam@127 111 /* <span class="roman">compute transforms, in-place, as many times as desired</span> */
cannam@127 112 fftw_execute(plan);
cannam@127 113
cannam@127 114 fftw_destroy_plan(plan);
cannam@127 115
cannam@127 116 MPI_Finalize();
cannam@127 117 }
cannam@127 118 </pre></div>
cannam@127 119
cannam@127 120 <p>As can be seen above, the MPI interface follows the same basic style
cannam@127 121 of allocate/plan/execute/destroy as the serial FFTW routines. All of
cannam@127 122 the MPI-specific routines are prefixed with &lsquo;<samp>fftw_mpi_</samp>&rsquo; instead
cannam@127 123 of &lsquo;<samp>fftw_</samp>&rsquo;. There are a few important differences, however:
cannam@127 124 </p>
cannam@127 125 <p>First, we must call <code>fftw_mpi_init()</code> after calling
cannam@127 126 <code>MPI_Init</code> (required in all MPI programs) and before calling any
cannam@127 127 other &lsquo;<samp>fftw_mpi_</samp>&rsquo; routine.
cannam@127 128 <a name="index-MPI_005fInit"></a>
cannam@127 129 <a name="index-fftw_005fmpi_005finit-1"></a>
cannam@127 130 </p>
cannam@127 131
cannam@127 132 <p>Second, when we create the plan with <code>fftw_mpi_plan_dft_2d</code>,
cannam@127 133 analogous to <code>fftw_plan_dft_2d</code>, we pass an additional argument:
cannam@127 134 the communicator, indicating which processes will participate in the
cannam@127 135 transform (here <code>MPI_COMM_WORLD</code>, indicating all processes).
cannam@127 136 Whenever you create, execute, or destroy a plan for an MPI transform,
cannam@127 137 you must call the corresponding FFTW routine on <em>all</em> processes
cannam@127 138 in the communicator for that transform. (That is, these are
cannam@127 139 <em>collective</em> calls.) Note that the plan for the MPI transform
cannam@127 140 uses the standard <code>fftw_execute</code> and <code>fftw_destroy</code> routines
cannam@127 141 (on the other hand, there are MPI-specific new-array execute functions
cannam@127 142 documented below).
cannam@127 143 <a name="index-collective-function"></a>
cannam@127 144 <a name="index-fftw_005fmpi_005fplan_005fdft_005f2d"></a>
cannam@127 145 <a name="index-MPI_005fCOMM_005fWORLD-1"></a>
cannam@127 146 </p>
cannam@127 147
cannam@127 148 <p>Third, all of the FFTW MPI routines take <code>ptrdiff_t</code> arguments
cannam@127 149 instead of <code>int</code> as for the serial FFTW. <code>ptrdiff_t</code> is a
cannam@127 150 standard C integer type which is (at least) 32 bits wide on a 32-bit
cannam@127 151 machine and 64 bits wide on a 64-bit machine. This is to make it easy
cannam@127 152 to specify very large parallel transforms on a 64-bit machine. (You
cannam@127 153 can specify 64-bit transform sizes in the serial FFTW, too, but only
cannam@127 154 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>.)
cannam@127 155 <a name="index-ptrdiff_005ft-1"></a>
cannam@127 156 <a name="index-64_002dbit-architecture-1"></a>
cannam@127 157 </p>
cannam@127 158
cannam@127 159 <p>Fourth, and most importantly, you don&rsquo;t allocate the entire
cannam@127 160 two-dimensional array on each process. Instead, you call
cannam@127 161 <code>fftw_mpi_local_size_2d</code> to find out what <em>portion</em> of the
cannam@127 162 array resides on each processor, and how much space to allocate.
cannam@127 163 Here, the portion of the array on each process is a <code>local_n0</code> by
cannam@127 164 <code>N1</code> slice of the total array, starting at index
cannam@127 165 <code>local_0_start</code>. The total number of <code>fftw_complex</code> numbers
cannam@127 166 to allocate is given by the <code>alloc_local</code> return value, which
cannam@127 167 <em>may</em> be greater than <code>local_n0 * N1</code> (in case some
cannam@127 168 intermediate calculations require additional storage). The data
cannam@127 169 distribution in FFTW&rsquo;s MPI interface is described in more detail by
cannam@127 170 the next section.
cannam@127 171 <a name="index-fftw_005fmpi_005flocal_005fsize_005f2d"></a>
cannam@127 172 <a name="index-data-distribution-1"></a>
cannam@127 173 </p>
cannam@127 174
cannam@127 175 <p>Given the portion of the array that resides on the local process, it
cannam@127 176 is straightforward to initialize the data (here to a function
cannam@127 177 <code>myfunction</code>) and otherwise manipulate it. Of course, at the end
cannam@127 178 of the program you may want to output the data somehow, but
cannam@127 179 synchronizing this output is up to you and is beyond the scope of this
cannam@127 180 manual. (One good way to output a large multi-dimensional distributed
cannam@127 181 array in MPI to a portable binary file is to use the free HDF5
cannam@127 182 library; see the <a href="http://www.hdfgroup.org/">HDF home page</a>.)
cannam@127 183 <a name="index-HDF5"></a>
cannam@127 184 <a name="index-MPI-I_002fO"></a>
cannam@127 185 </p>
cannam@127 186 <hr>
cannam@127 187 <div class="header">
cannam@127 188 <p>
cannam@127 189 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>
cannam@127 190 </div>
cannam@127 191
cannam@127 192
cannam@127 193
cannam@127 194 </body>
cannam@127 195 </html>