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| author | Chris Cannam <cannam@all-day-breakfast.com> |
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| date | Wed, 20 Mar 2013 15:35:50 +0000 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.3/doc/html/2d-MPI-example.html Wed Mar 20 15:35:50 2013 +0000 @@ -0,0 +1,154 @@ +<html lang="en"> +<head> +<title>2d MPI example - FFTW 3.3.3</title> +<meta http-equiv="Content-Type" content="text/html"> +<meta name="description" content="FFTW 3.3.3"> +<meta name="generator" content="makeinfo 4.13"> +<link title="Top" rel="start" href="index.html#Top"> +<link rel="up" href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" title="Distributed-memory FFTW with MPI"> +<link rel="prev" href="Linking-and-Initializing-MPI-FFTW.html#Linking-and-Initializing-MPI-FFTW" title="Linking and Initializing MPI FFTW"> +<link rel="next" href="MPI-Data-Distribution.html#MPI-Data-Distribution" title="MPI Data Distribution"> +<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> +<!-- +This manual is for FFTW +(version 3.3.3, 25 November 2012). + +Copyright (C) 2003 Matteo Frigo. + +Copyright (C) 2003 Massachusetts Institute of Technology. + + Permission is granted to make and distribute verbatim copies of + this manual provided the copyright notice and this permission + notice are preserved on all copies. + + Permission is granted to copy and distribute modified versions of + this manual under the conditions for verbatim copying, provided + that the entire resulting derived work is distributed under the + terms of a permission notice identical to this one. + + Permission is granted to copy and distribute translations of this + manual into another language, under the above conditions for + modified versions, except that this permission notice may be + stated in a translation approved by the Free Software Foundation. + --> +<meta http-equiv="Content-Style-Type" content="text/css"> +<style type="text/css"><!-- + pre.display { font-family:inherit } + pre.format { font-family:inherit } + pre.smalldisplay { font-family:inherit; font-size:smaller } + pre.smallformat { font-family:inherit; font-size:smaller } + pre.smallexample { font-size:smaller } + pre.smalllisp { font-size:smaller } + span.sc { font-variant:small-caps } + span.roman { font-family:serif; font-weight:normal; } + span.sansserif { font-family:sans-serif; font-weight:normal; } +--></style> +</head> +<body> +<div class="node"> +<a name="g_t2d-MPI-example"></a> +<p> +Next: <a rel="next" accesskey="n" href="MPI-Data-Distribution.html#MPI-Data-Distribution">MPI Data Distribution</a>, +Previous: <a rel="previous" accesskey="p" href="Linking-and-Initializing-MPI-FFTW.html#Linking-and-Initializing-MPI-FFTW">Linking and Initializing MPI FFTW</a>, +Up: <a rel="up" accesskey="u" href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI">Distributed-memory FFTW with MPI</a> +<hr> +</div> + +<h3 class="section">6.3 2d MPI example</h3> + +<p>Before we document the FFTW MPI interface in detail, we begin with a +simple example outlining how one would perform a two-dimensional +<code>N0</code> by <code>N1</code> complex DFT. + +<pre class="example"> #include <fftw3-mpi.h> + + int main(int argc, char **argv) + { + const ptrdiff_t N0 = ..., N1 = ...; + fftw_plan plan; + fftw_complex *data; + ptrdiff_t alloc_local, local_n0, local_0_start, i, j; + + MPI_Init(&argc, &argv); + fftw_mpi_init(); + + /* <span class="roman">get local data size and allocate</span> */ + alloc_local = fftw_mpi_local_size_2d(N0, N1, MPI_COMM_WORLD, + &local_n0, &local_0_start); + data = fftw_alloc_complex(alloc_local); + + /* <span class="roman">create plan for in-place forward DFT</span> */ + plan = fftw_mpi_plan_dft_2d(N0, N1, data, data, MPI_COMM_WORLD, + FFTW_FORWARD, FFTW_ESTIMATE); + + /* <span class="roman">initialize data to some function</span> my_function(x,y) */ + for (i = 0; i < local_n0; ++i) for (j = 0; j < N1; ++j) + data[i*N1 + j] = my_function(local_0_start + i, j); + + /* <span class="roman">compute transforms, in-place, as many times as desired</span> */ + fftw_execute(plan); + + fftw_destroy_plan(plan); + + MPI_Finalize(); + } +</pre> + <p>As can be seen above, the MPI interface follows the same basic style +of allocate/plan/execute/destroy as the serial FFTW routines. All of +the MPI-specific routines are prefixed with ‘<samp><span class="samp">fftw_mpi_</span></samp>’ instead +of ‘<samp><span class="samp">fftw_</span></samp>’. There are a few important differences, however: + + <p>First, we must call <code>fftw_mpi_init()</code> after calling +<code>MPI_Init</code> (required in all MPI programs) and before calling any +other ‘<samp><span class="samp">fftw_mpi_</span></samp>’ routine. +<a name="index-MPI_005fInit-357"></a><a name="index-fftw_005fmpi_005finit-358"></a> + + <p>Second, when we create the plan with <code>fftw_mpi_plan_dft_2d</code>, +analogous to <code>fftw_plan_dft_2d</code>, we pass an additional argument: +the communicator, indicating which processes will participate in the +transform (here <code>MPI_COMM_WORLD</code>, indicating all processes). +Whenever you create, execute, or destroy a plan for an MPI transform, +you must call the corresponding FFTW routine on <em>all</em> processes +in the communicator for that transform. (That is, these are +<em>collective</em> calls.) Note that the plan for the MPI transform +uses the standard <code>fftw_execute</code> and <code>fftw_destroy</code> routines +(on the other hand, there are MPI-specific new-array execute functions +documented below). +<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> + + <p>Third, all of the FFTW MPI routines take <code>ptrdiff_t</code> arguments +instead of <code>int</code> as for the serial FFTW. <code>ptrdiff_t</code> is a +standard C integer type which is (at least) 32 bits wide on a 32-bit +machine and 64 bits wide on a 64-bit machine. This is to make it easy +to specify very large parallel transforms on a 64-bit machine. (You +can specify 64-bit transform sizes in the serial FFTW, too, but only +by using the ‘<samp><span class="samp">guru64</span></samp>’ planner interface. See <a href="64_002dbit-Guru-Interface.html#g_t64_002dbit-Guru-Interface">64-bit Guru Interface</a>.) +<a name="index-ptrdiff_005ft-362"></a><a name="index-g_t64_002dbit-architecture-363"></a> + + <p>Fourth, and most importantly, you don't allocate the entire +two-dimensional array on each process. Instead, you call +<code>fftw_mpi_local_size_2d</code> to find out what <em>portion</em> of the +array resides on each processor, and how much space to allocate. +Here, the portion of the array on each process is a <code>local_n0</code> by +<code>N1</code> slice of the total array, starting at index +<code>local_0_start</code>. The total number of <code>fftw_complex</code> numbers +to allocate is given by the <code>alloc_local</code> return value, which +<em>may</em> be greater than <code>local_n0 * N1</code> (in case some +intermediate calculations require additional storage). The data +distribution in FFTW's MPI interface is described in more detail by +the next section. +<a name="index-fftw_005fmpi_005flocal_005fsize_005f2d-364"></a><a name="index-data-distribution-365"></a> + + <p>Given the portion of the array that resides on the local process, it +is straightforward to initialize the data (here to a function +<code>myfunction</code>) and otherwise manipulate it. Of course, at the end +of the program you may want to output the data somehow, but +synchronizing this output is up to you and is beyond the scope of this +manual. (One good way to output a large multi-dimensional distributed +array in MPI to a portable binary file is to use the free HDF5 +library; see the <a href="http://www.hdfgroup.org/">HDF home page</a>.) +<a name="index-HDF5-366"></a><a name="index-MPI-I_002fO-367"></a> +<!-- --> + + </body></html> +