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diff src/fftw-3.3.5/doc/html/Load-balancing.html @ 42:2cd0e3b3e1fd
Current fftw source
| author | Chris Cannam |
|---|---|
| date | Tue, 18 Oct 2016 13:40:26 +0100 |
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| children |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.5/doc/html/Load-balancing.html Tue Oct 18 13:40:26 2016 +0100 @@ -0,0 +1,114 @@ +<!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 3.3.5, 30 July 2016). + +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 5.2, http://www.gnu.org/software/texinfo/ --> +<head> +<title>FFTW 3.3.5: Load balancing</title> + +<meta name="description" content="FFTW 3.3.5: Load balancing"> +<meta name="keywords" content="FFTW 3.3.5: Load balancing"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="MPI-Data-Distribution.html#MPI-Data-Distribution" rel="up" title="MPI Data Distribution"> +<link href="Transposed-distributions.html#Transposed-distributions" rel="next" title="Transposed distributions"> +<link href="Basic-and-advanced-distribution-interfaces.html#Basic-and-advanced-distribution-interfaces" rel="prev" title="Basic and advanced distribution interfaces"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {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} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Load-balancing"></a> +<div class="header"> +<p> +Next: <a href="Transposed-distributions.html#Transposed-distributions" accesskey="n" rel="next">Transposed distributions</a>, Previous: <a href="Basic-and-advanced-distribution-interfaces.html#Basic-and-advanced-distribution-interfaces" accesskey="p" rel="prev">Basic and advanced distribution interfaces</a>, Up: <a href="MPI-Data-Distribution.html#MPI-Data-Distribution" 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#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Load-balancing-1"></a> +<h4 class="subsection">6.4.2 Load balancing</h4> +<a name="index-load-balancing"></a> + +<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>