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183 lines
8.6 KiB
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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into another language, under the above conditions for modified versions,
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except that this permission notice may be stated in a translation
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approved by the Free Software Foundation. -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>2d MPI example (FFTW 3.3.10)</title>
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<meta name="description" content="2d MPI example (FFTW 3.3.10)">
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<meta name="keywords" content="2d MPI example (FFTW 3.3.10)">
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<meta name="distribution" content="global">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Distributed_002dmemory-FFTW-with-MPI.html" rel="up" title="Distributed-memory FFTW with MPI">
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<link href="MPI-Data-Distribution.html" rel="next" title="MPI Data Distribution">
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<link href="Linking-and-Initializing-MPI-FFTW.html" rel="prev" title="Linking and Initializing MPI FFTW">
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</head>
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<body lang="en">
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<span id="g_t2d-MPI-example"></span><div class="header">
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<p>
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Next: <a href="MPI-Data-Distribution.html" accesskey="n" rel="next">MPI Data Distribution</a>, Previous: <a href="Linking-and-Initializing-MPI-FFTW.html" accesskey="p" rel="prev">Linking and Initializing MPI FFTW</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="g_t2d-MPI-example-1"></span><h3 class="section">6.3 2d MPI example</h3>
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<p>Before we document the FFTW MPI interface in detail, we begin with a
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simple example outlining how one would perform a two-dimensional
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<code>N0</code> by <code>N1</code> complex DFT.
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</p>
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<div class="example">
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<pre class="example">#include <fftw3-mpi.h>
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int main(int argc, char **argv)
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{
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const ptrdiff_t N0 = ..., N1 = ...;
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fftw_plan plan;
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fftw_complex *data;
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ptrdiff_t alloc_local, local_n0, local_0_start, i, j;
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MPI_Init(&argc, &argv);
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fftw_mpi_init();
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/* <span class="roman">get local data size and allocate</span> */
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alloc_local = fftw_mpi_local_size_2d(N0, N1, MPI_COMM_WORLD,
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&local_n0, &local_0_start);
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data = fftw_alloc_complex(alloc_local);
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/* <span class="roman">create plan for in-place forward DFT</span> */
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plan = fftw_mpi_plan_dft_2d(N0, N1, data, data, MPI_COMM_WORLD,
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FFTW_FORWARD, FFTW_ESTIMATE);
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/* <span class="roman">initialize data to some function</span> my_function(x,y) */
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for (i = 0; i < local_n0; ++i) for (j = 0; j < N1; ++j)
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data[i*N1 + j] = my_function(local_0_start + i, j);
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/* <span class="roman">compute transforms, in-place, as many times as desired</span> */
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fftw_execute(plan);
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fftw_destroy_plan(plan);
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MPI_Finalize();
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}
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</pre></div>
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<p>As can be seen above, the MPI interface follows the same basic style
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of allocate/plan/execute/destroy as the serial FFTW routines. All of
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the MPI-specific routines are prefixed with ‘<samp>fftw_mpi_</samp>’ instead
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of ‘<samp>fftw_</samp>’. There are a few important differences, however:
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</p>
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<p>First, we must call <code>fftw_mpi_init()</code> after calling
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<code>MPI_Init</code> (required in all MPI programs) and before calling any
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other ‘<samp>fftw_mpi_</samp>’ routine.
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<span id="index-MPI_005fInit"></span>
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<span id="index-fftw_005fmpi_005finit-1"></span>
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</p>
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<p>Second, when we create the plan with <code>fftw_mpi_plan_dft_2d</code>,
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analogous to <code>fftw_plan_dft_2d</code>, we pass an additional argument:
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the communicator, indicating which processes will participate in the
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transform (here <code>MPI_COMM_WORLD</code>, indicating all processes).
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Whenever you create, execute, or destroy a plan for an MPI transform,
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you must call the corresponding FFTW routine on <em>all</em> processes
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in the communicator for that transform. (That is, these are
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<em>collective</em> calls.) Note that the plan for the MPI transform
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uses the standard <code>fftw_execute</code> and <code>fftw_destroy</code> routines
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(on the other hand, there are MPI-specific new-array execute functions
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documented below).
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<span id="index-collective-function"></span>
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<span id="index-fftw_005fmpi_005fplan_005fdft_005f2d"></span>
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<span id="index-MPI_005fCOMM_005fWORLD-1"></span>
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</p>
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<p>Third, all of the FFTW MPI routines take <code>ptrdiff_t</code> arguments
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instead of <code>int</code> as for the serial FFTW. <code>ptrdiff_t</code> is a
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standard C integer type which is (at least) 32 bits wide on a 32-bit
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machine and 64 bits wide on a 64-bit machine. This is to make it easy
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to specify very large parallel transforms on a 64-bit machine. (You
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can specify 64-bit transform sizes in the serial FFTW, too, but only
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by using the ‘<samp>guru64</samp>’ planner interface. See <a href="64_002dbit-Guru-Interface.html">64-bit Guru Interface</a>.)
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<span id="index-ptrdiff_005ft-1"></span>
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<span id="index-64_002dbit-architecture-1"></span>
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</p>
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<p>Fourth, and most importantly, you don’t allocate the entire
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two-dimensional array on each process. Instead, you call
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<code>fftw_mpi_local_size_2d</code> to find out what <em>portion</em> of the
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array resides on each processor, and how much space to allocate.
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Here, the portion of the array on each process is a <code>local_n0</code> by
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<code>N1</code> slice of the total array, starting at index
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<code>local_0_start</code>. The total number of <code>fftw_complex</code> numbers
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to allocate is given by the <code>alloc_local</code> return value, which
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<em>may</em> be greater than <code>local_n0 * N1</code> (in case some
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intermediate calculations require additional storage). The data
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distribution in FFTW’s MPI interface is described in more detail by
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the next section.
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<span id="index-fftw_005fmpi_005flocal_005fsize_005f2d"></span>
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<span id="index-data-distribution-1"></span>
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</p>
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<p>Given the portion of the array that resides on the local process, it
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is straightforward to initialize the data (here to a function
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<code>myfunction</code>) and otherwise manipulate it. Of course, at the end
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of the program you may want to output the data somehow, but
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synchronizing this output is up to you and is beyond the scope of this
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manual. (One good way to output a large multi-dimensional distributed
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array in MPI to a portable binary file is to use the free HDF5
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library; see the <a href="http://www.hdfgroup.org/">HDF home page</a>.)
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<span id="index-HDF5"></span>
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<span id="index-MPI-I_002fO"></span>
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="MPI-Data-Distribution.html" accesskey="n" rel="next">MPI Data Distribution</a>, Previous: <a href="Linking-and-Initializing-MPI-FFTW.html" accesskey="p" rel="prev">Linking and Initializing MPI FFTW</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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