184 lines
8.6 KiB
HTML
184 lines
8.6 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- This manual is for FFTW
|
|
(version 3.3.10, 10 December 2020).
|
|
|
|
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. -->
|
|
<!-- Created by GNU Texinfo 6.7, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<title>2d MPI example (FFTW 3.3.10)</title>
|
|
|
|
<meta name="description" content="2d MPI example (FFTW 3.3.10)">
|
|
<meta name="keywords" content="2d MPI example (FFTW 3.3.10)">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content="makeinfo">
|
|
<link href="index.html" rel="start" title="Top">
|
|
<link href="Concept-Index.html" rel="index" title="Concept Index">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="Distributed_002dmemory-FFTW-with-MPI.html" rel="up" title="Distributed-memory FFTW with MPI">
|
|
<link href="MPI-Data-Distribution.html" rel="next" title="MPI Data Distribution">
|
|
<link href="Linking-and-Initializing-MPI-FFTW.html" rel="prev" title="Linking and Initializing MPI FFTW">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
kbd {font-style: oblique}
|
|
pre.display {font-family: inherit}
|
|
pre.format {font-family: inherit}
|
|
pre.menu-comment {font-family: serif}
|
|
pre.menu-preformatted {font-family: serif}
|
|
span.nolinebreak {white-space: nowrap}
|
|
span.roman {font-family: initial; font-weight: normal}
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<span id="g_t2d-MPI-example"></span><div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
<hr>
|
|
<span id="g_t2d-MPI-example-1"></span><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.
|
|
</p>
|
|
<div class="example">
|
|
<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></div>
|
|
|
|
<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>fftw_mpi_</samp>’ instead
|
|
of ‘<samp>fftw_</samp>’. There are a few important differences, however:
|
|
</p>
|
|
<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>fftw_mpi_</samp>’ routine.
|
|
<span id="index-MPI_005fInit"></span>
|
|
<span id="index-fftw_005fmpi_005finit-1"></span>
|
|
</p>
|
|
|
|
<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).
|
|
<span id="index-collective-function"></span>
|
|
<span id="index-fftw_005fmpi_005fplan_005fdft_005f2d"></span>
|
|
<span id="index-MPI_005fCOMM_005fWORLD-1"></span>
|
|
</p>
|
|
|
|
<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>guru64</samp>’ planner interface. See <a href="64_002dbit-Guru-Interface.html">64-bit Guru Interface</a>.)
|
|
<span id="index-ptrdiff_005ft-1"></span>
|
|
<span id="index-64_002dbit-architecture-1"></span>
|
|
</p>
|
|
|
|
<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.
|
|
<span id="index-fftw_005fmpi_005flocal_005fsize_005f2d"></span>
|
|
<span id="index-data-distribution-1"></span>
|
|
</p>
|
|
|
|
<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>.)
|
|
<span id="index-HDF5"></span>
|
|
<span id="index-MPI-I_002fO"></span>
|
|
</p>
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|