Mercurial > hg > sv-dependency-builds

annotate src/fftw-3.3.5/doc/html/2d-MPI-example.html @ 42:2cd0e3b3e1fd

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
parents
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.
Chris@42 7
Chris@42 8 Copyright (C) 2003 Massachusetts Institute of Technology.
Chris@42 9
Chris@42 10 Permission is granted to make and distribute verbatim copies of this
Chris@42 11 manual provided the copyright notice and this permission notice are
Chris@42 12 preserved on all copies.
Chris@42 13
Chris@42 14 Permission is granted to copy and distribute modified versions of this
Chris@42 15 manual under the conditions for verbatim copying, provided that the
Chris@42 16 entire resulting derived work is distributed under the terms of a
Chris@42 17 permission notice identical to this one.
Chris@42 18
Chris@42 19 Permission is granted to copy and distribute translations of this manual
Chris@42 20 into another language, under the above conditions for modified versions,
Chris@42 21 except that this permission notice may be stated in a translation
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">
Chris@42 29 <meta name="resource-type" content="document">
Chris@42 30 <meta name="distribution" content="global">
Chris@42 31 <meta name="Generator" content="makeinfo">
Chris@42 32 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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">
Chris@42 39 <style type="text/css">
Chris@42 40 <!--
Chris@42 41 a.summary-letter {text-decoration: none}
Chris@42 42 blockquote.smallquotation {font-size: smaller}
Chris@42 43 div.display {margin-left: 3.2em}
Chris@42 44 div.example {margin-left: 3.2em}
Chris@42 45 div.indentedblock {margin-left: 3.2em}
Chris@42 46 div.lisp {margin-left: 3.2em}
Chris@42 47 div.smalldisplay {margin-left: 3.2em}
Chris@42 48 div.smallexample {margin-left: 3.2em}
Chris@42 49 div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
Chris@42 50 div.smalllisp {margin-left: 3.2em}
Chris@42 51 kbd {font-style:oblique}
Chris@42 52 pre.display {font-family: inherit}
Chris@42 53 pre.format {font-family: inherit}
Chris@42 54 pre.menu-comment {font-family: serif}
Chris@42 55 pre.menu-preformatted {font-family: serif}
Chris@42 56 pre.smalldisplay {font-family: inherit; font-size: smaller}
Chris@42 57 pre.smallexample {font-size: smaller}
Chris@42 58 pre.smallformat {font-family: inherit; font-size: smaller}
Chris@42 59 pre.smalllisp {font-size: smaller}
Chris@42 60 span.nocodebreak {white-space:nowrap}
Chris@42 61 span.nolinebreak {white-space:nowrap}
Chris@42 62 span.roman {font-family:serif; font-weight:normal}
Chris@42 63 span.sansserif {font-family:sans-serif; font-weight:normal}
Chris@42 64 ul.no-bullet {list-style: none}
Chris@42 65 -->
Chris@42 66 </style>
Chris@42 67
Chris@42 68
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>