103 lines
4.7 KiB
HTML
103 lines
4.7 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>Load balancing (FFTW 3.3.10)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Load balancing (FFTW 3.3.10)">
|
||
|
|
<meta name="keywords" content="Load balancing (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="MPI-Data-Distribution.html" rel="up" title="MPI Data Distribution">
|
||
|
|
<link href="Transposed-distributions.html" rel="next" title="Transposed distributions">
|
||
|
|
<link href="Basic-and-advanced-distribution-interfaces.html" rel="prev" title="Basic and advanced distribution interfaces">
|
||
|
|
<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="Load-balancing"></span><div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Transposed-distributions.html" accesskey="n" rel="next">Transposed distributions</a>, Previous: <a href="Basic-and-advanced-distribution-interfaces.html" accesskey="p" rel="prev">Basic and advanced distribution interfaces</a>, Up: <a href="MPI-Data-Distribution.html" accesskey="u" rel="up">MPI Data Distribution</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="Load-balancing-1"></span><h4 class="subsection">6.4.2 Load balancing</h4>
|
||
|
|
<span id="index-load-balancing"></span>
|
||
|
|
|
||
|
|
<p>Ideally, when you parallelize a transform over some <em>P</em>
|
||
|
|
processes, each process should end up with work that takes equal time.
|
||
|
|
Otherwise, all of the processes end up waiting on whichever process is
|
||
|
|
slowest. This goal is known as “load balancing.” In this section,
|
||
|
|
we describe the circumstances under which FFTW is able to load-balance
|
||
|
|
well, and in particular how you should choose your transform size in
|
||
|
|
order to load balance.
|
||
|
|
</p>
|
||
|
|
<p>Load balancing is especially difficult when you are parallelizing over
|
||
|
|
heterogeneous machines; for example, if one of your processors is a
|
||
|
|
old 486 and another is a Pentium IV, obviously you should give the
|
||
|
|
Pentium more work to do than the 486 since the latter is much slower.
|
||
|
|
FFTW does not deal with this problem, however—it assumes that your
|
||
|
|
processes run on hardware of comparable speed, and that the goal is
|
||
|
|
therefore to divide the problem as equally as possible.
|
||
|
|
</p>
|
||
|
|
<p>For a multi-dimensional complex DFT, FFTW can divide the problem
|
||
|
|
equally among the processes if: (i) the <em>first</em> dimension
|
||
|
|
<code>n0</code> is divisible by <em>P</em>; and (ii), the <em>product</em> of
|
||
|
|
the subsequent dimensions is divisible by <em>P</em>. (For the advanced
|
||
|
|
interface, where you can specify multiple simultaneous transforms via
|
||
|
|
some “vector” length <code>howmany</code>, a factor of <code>howmany</code> is
|
||
|
|
included in the product of the subsequent dimensions.)
|
||
|
|
</p>
|
||
|
|
<p>For a one-dimensional complex DFT, the length <code>N</code> of the data
|
||
|
|
should be divisible by <em>P</em> <em>squared</em> to be able to divide
|
||
|
|
the problem equally among the processes.
|
||
|
|
</p>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|