140 lines
7.3 KiB
HTML
140 lines
7.3 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>MPI Data Distribution (FFTW 3.3.10)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="MPI Data Distribution (FFTW 3.3.10)">
|
||
|
|
<meta name="keywords" content="MPI Data Distribution (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="Basic-and-advanced-distribution-interfaces.html" rel="next" title="Basic and advanced distribution interfaces">
|
||
|
|
<link href="2d-MPI-example.html" rel="prev" title="2d MPI example">
|
||
|
|
<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="MPI-Data-Distribution"></span><div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Multi_002ddimensional-MPI-DFTs-of-Real-Data.html" accesskey="n" rel="next">Multi-dimensional MPI DFTs of Real Data</a>, Previous: <a href="2d-MPI-example.html" accesskey="p" rel="prev">2d MPI example</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="MPI-Data-Distribution-1"></span><h3 class="section">6.4 MPI Data Distribution</h3>
|
||
|
|
<span id="index-data-distribution-2"></span>
|
||
|
|
|
||
|
|
<p>The most important concept to understand in using FFTW’s MPI interface
|
||
|
|
is the data distribution. With a serial or multithreaded FFT, all of
|
||
|
|
the inputs and outputs are stored as a single contiguous chunk of
|
||
|
|
memory. With a distributed-memory FFT, the inputs and outputs are
|
||
|
|
broken into disjoint blocks, one per process.
|
||
|
|
</p>
|
||
|
|
<p>In particular, FFTW uses a <em>1d block distribution</em> of the data,
|
||
|
|
distributed along the <em>first dimension</em>. For example, if you
|
||
|
|
want to perform a 100 × 200
|
||
|
|
complex DFT, distributed over 4
|
||
|
|
processes, each process will get a 25 × 200
|
||
|
|
slice of the data.
|
||
|
|
That is, process 0 will get rows 0 through 24, process 1 will get rows
|
||
|
|
25 through 49, process 2 will get rows 50 through 74, and process 3
|
||
|
|
will get rows 75 through 99. If you take the same array but
|
||
|
|
distribute it over 3 processes, then it is not evenly divisible so the
|
||
|
|
different processes will have unequal chunks. FFTW’s default choice
|
||
|
|
in this case is to assign 34 rows to processes 0 and 1, and 32 rows to
|
||
|
|
process 2.
|
||
|
|
<span id="index-block-distribution"></span>
|
||
|
|
</p>
|
||
|
|
|
||
|
|
<p>FFTW provides several ‘<samp>fftw_mpi_local_size</samp>’ routines that you can
|
||
|
|
call to find out what portion of an array is stored on the current
|
||
|
|
process. In most cases, you should use the default block sizes picked
|
||
|
|
by FFTW, but it is also possible to specify your own block size. For
|
||
|
|
example, with a 100 × 200
|
||
|
|
array on three processes, you can
|
||
|
|
tell FFTW to use a block size of 40, which would assign 40 rows to
|
||
|
|
processes 0 and 1, and 20 rows to process 2. FFTW’s default is to
|
||
|
|
divide the data equally among the processes if possible, and as best
|
||
|
|
it can otherwise. The rows are always assigned in “rank order,”
|
||
|
|
i.e. process 0 gets the first block of rows, then process 1, and so
|
||
|
|
on. (You can change this by using <code>MPI_Comm_split</code> to create a
|
||
|
|
new communicator with re-ordered processes.) However, you should
|
||
|
|
always call the ‘<samp>fftw_mpi_local_size</samp>’ routines, if possible,
|
||
|
|
rather than trying to predict FFTW’s distribution choices.
|
||
|
|
</p>
|
||
|
|
<p>In particular, it is critical that you allocate the storage size that
|
||
|
|
is returned by ‘<samp>fftw_mpi_local_size</samp>’, which is <em>not</em>
|
||
|
|
necessarily the size of the local slice of the array. The reason is
|
||
|
|
that intermediate steps of FFTW’s algorithms involve transposing the
|
||
|
|
array and redistributing the data, so at these intermediate steps FFTW
|
||
|
|
may require more local storage space (albeit always proportional to
|
||
|
|
the total size divided by the number of processes). The
|
||
|
|
‘<samp>fftw_mpi_local_size</samp>’ functions know how much storage is required
|
||
|
|
for these intermediate steps and tell you the correct amount to
|
||
|
|
allocate.
|
||
|
|
</p>
|
||
|
|
<table class="menu" border="0" cellspacing="0">
|
||
|
|
<tr><td align="left" valign="top">• <a href="Basic-and-advanced-distribution-interfaces.html" accesskey="1">Basic and advanced distribution interfaces</a></td><td> </td><td align="left" valign="top">
|
||
|
|
</td></tr>
|
||
|
|
<tr><td align="left" valign="top">• <a href="Load-balancing.html" accesskey="2">Load balancing</a></td><td> </td><td align="left" valign="top">
|
||
|
|
</td></tr>
|
||
|
|
<tr><td align="left" valign="top">• <a href="Transposed-distributions.html" accesskey="3">Transposed distributions</a></td><td> </td><td align="left" valign="top">
|
||
|
|
</td></tr>
|
||
|
|
<tr><td align="left" valign="top">• <a href="One_002ddimensional-distributions.html" accesskey="4">One-dimensional distributions</a></td><td> </td><td align="left" valign="top">
|
||
|
|
</td></tr>
|
||
|
|
</table>
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Multi_002ddimensional-MPI-DFTs-of-Real-Data.html" accesskey="n" rel="next">Multi-dimensional MPI DFTs of Real Data</a>, Previous: <a href="2d-MPI-example.html" accesskey="p" rel="prev">2d MPI example</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>
|