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author Chris Cannam <cannam@all-day-breakfast.com>
date Wed, 20 Mar 2013 15:35:50 +0000
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1 <html lang="en">
2 <head>
3 <title>2d MPI example - FFTW 3.3.3</title>
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13 This manual is for FFTW
14 (version 3.3.3, 25 November 2012).
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16 Copyright (C) 2003 Matteo Frigo.
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48 <div class="node">
49 <a name="g_t2d-MPI-example"></a>
50 <p>
51 Next:&nbsp;<a rel="next" accesskey="n" href="MPI-Data-Distribution.html#MPI-Data-Distribution">MPI Data Distribution</a>,
52 Previous:&nbsp;<a rel="previous" accesskey="p" href="Linking-and-Initializing-MPI-FFTW.html#Linking-and-Initializing-MPI-FFTW">Linking and Initializing MPI FFTW</a>,
53 Up:&nbsp;<a rel="up" accesskey="u" href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI">Distributed-memory FFTW with MPI</a>
54 <hr>
55 </div>
56
57 <h3 class="section">6.3 2d MPI example</h3>
58
59 <p>Before we document the FFTW MPI interface in detail, we begin with a
60 simple example outlining how one would perform a two-dimensional
61 <code>N0</code> by <code>N1</code> complex DFT.
62
63 <pre class="example"> #include &lt;fftw3-mpi.h&gt;
64
65 int main(int argc, char **argv)
66 {
67 const ptrdiff_t N0 = ..., N1 = ...;
68 fftw_plan plan;
69 fftw_complex *data;
70 ptrdiff_t alloc_local, local_n0, local_0_start, i, j;
71
72 MPI_Init(&amp;argc, &amp;argv);
73 fftw_mpi_init();
74
75 /* <span class="roman">get local data size and allocate</span> */
76 alloc_local = fftw_mpi_local_size_2d(N0, N1, MPI_COMM_WORLD,
77 &amp;local_n0, &amp;local_0_start);
78 data = fftw_alloc_complex(alloc_local);
79
80 /* <span class="roman">create plan for in-place forward DFT</span> */
81 plan = fftw_mpi_plan_dft_2d(N0, N1, data, data, MPI_COMM_WORLD,
82 FFTW_FORWARD, FFTW_ESTIMATE);
83
84 /* <span class="roman">initialize data to some function</span> my_function(x,y) */
85 for (i = 0; i &lt; local_n0; ++i) for (j = 0; j &lt; N1; ++j)
86 data[i*N1 + j] = my_function(local_0_start + i, j);
87
88 /* <span class="roman">compute transforms, in-place, as many times as desired</span> */
89 fftw_execute(plan);
90
91 fftw_destroy_plan(plan);
92
93 MPI_Finalize();
94 }
95 </pre>
96 <p>As can be seen above, the MPI interface follows the same basic style
97 of allocate/plan/execute/destroy as the serial FFTW routines. All of
98 the MPI-specific routines are prefixed with &lsquo;<samp><span class="samp">fftw_mpi_</span></samp>&rsquo; instead
99 of &lsquo;<samp><span class="samp">fftw_</span></samp>&rsquo;. There are a few important differences, however:
100
101 <p>First, we must call <code>fftw_mpi_init()</code> after calling
102 <code>MPI_Init</code> (required in all MPI programs) and before calling any
103 other &lsquo;<samp><span class="samp">fftw_mpi_</span></samp>&rsquo; routine.
104 <a name="index-MPI_005fInit-357"></a><a name="index-fftw_005fmpi_005finit-358"></a>
105
106 <p>Second, when we create the plan with <code>fftw_mpi_plan_dft_2d</code>,
107 analogous to <code>fftw_plan_dft_2d</code>, we pass an additional argument:
108 the communicator, indicating which processes will participate in the
109 transform (here <code>MPI_COMM_WORLD</code>, indicating all processes).
110 Whenever you create, execute, or destroy a plan for an MPI transform,
111 you must call the corresponding FFTW routine on <em>all</em> processes
112 in the communicator for that transform. (That is, these are
113 <em>collective</em> calls.) Note that the plan for the MPI transform
114 uses the standard <code>fftw_execute</code> and <code>fftw_destroy</code> routines
115 (on the other hand, there are MPI-specific new-array execute functions
116 documented below).
117 <a name="index-collective-function-359"></a><a name="index-fftw_005fmpi_005fplan_005fdft_005f2d-360"></a><a name="index-MPI_005fCOMM_005fWORLD-361"></a>
118
119 <p>Third, all of the FFTW MPI routines take <code>ptrdiff_t</code> arguments
120 instead of <code>int</code> as for the serial FFTW. <code>ptrdiff_t</code> is a
121 standard C integer type which is (at least) 32 bits wide on a 32-bit
122 machine and 64 bits wide on a 64-bit machine. This is to make it easy
123 to specify very large parallel transforms on a 64-bit machine. (You
124 can specify 64-bit transform sizes in the serial FFTW, too, but only
125 by using the &lsquo;<samp><span class="samp">guru64</span></samp>&rsquo; planner interface. See <a href="64_002dbit-Guru-Interface.html#g_t64_002dbit-Guru-Interface">64-bit Guru Interface</a>.)
126 <a name="index-ptrdiff_005ft-362"></a><a name="index-g_t64_002dbit-architecture-363"></a>
127
128 <p>Fourth, and most importantly, you don't allocate the entire
129 two-dimensional array on each process. Instead, you call
130 <code>fftw_mpi_local_size_2d</code> to find out what <em>portion</em> of the
131 array resides on each processor, and how much space to allocate.
132 Here, the portion of the array on each process is a <code>local_n0</code> by
133 <code>N1</code> slice of the total array, starting at index
134 <code>local_0_start</code>. The total number of <code>fftw_complex</code> numbers
135 to allocate is given by the <code>alloc_local</code> return value, which
136 <em>may</em> be greater than <code>local_n0 * N1</code> (in case some
137 intermediate calculations require additional storage). The data
138 distribution in FFTW's MPI interface is described in more detail by
139 the next section.
140 <a name="index-fftw_005fmpi_005flocal_005fsize_005f2d-364"></a><a name="index-data-distribution-365"></a>
141
142 <p>Given the portion of the array that resides on the local process, it
143 is straightforward to initialize the data (here to a function
144 <code>myfunction</code>) and otherwise manipulate it. Of course, at the end
145 of the program you may want to output the data somehow, but
146 synchronizing this output is up to you and is beyond the scope of this
147 manual. (One good way to output a large multi-dimensional distributed
148 array in MPI to a portable binary file is to use the free HDF5
149 library; see the <a href="http://www.hdfgroup.org/">HDF home page</a>.)
150 <a name="index-HDF5-366"></a><a name="index-MPI-I_002fO-367"></a>
151 <!-- -->
152
153 </body></html>
154