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comparison src/fftw-3.3.5/doc/html/Load-balancing.html @ 127:7867fa7e1b6b
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author | Chris Cannam <cannam@all-day-breakfast.com> |
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date | Tue, 18 Oct 2016 13:40:26 +0100 |
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1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> | |
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3 <!-- This manual is for FFTW | |
4 (version 3.3.5, 30 July 2016). | |
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25 <title>FFTW 3.3.5: Load balancing</title> | |
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72 <a name="Load-balancing"></a> | |
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74 <p> | |
75 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> | |
76 </div> | |
77 <hr> | |
78 <a name="Load-balancing-1"></a> | |
79 <h4 class="subsection">6.4.2 Load balancing</h4> | |
80 <a name="index-load-balancing"></a> | |
81 | |
82 <p>Ideally, when you parallelize a transform over some <em>P</em> | |
83 processes, each process should end up with work that takes equal time. | |
84 Otherwise, all of the processes end up waiting on whichever process is | |
85 slowest. This goal is known as “load balancing.” In this section, | |
86 we describe the circumstances under which FFTW is able to load-balance | |
87 well, and in particular how you should choose your transform size in | |
88 order to load balance. | |
89 </p> | |
90 <p>Load balancing is especially difficult when you are parallelizing over | |
91 heterogeneous machines; for example, if one of your processors is a | |
92 old 486 and another is a Pentium IV, obviously you should give the | |
93 Pentium more work to do than the 486 since the latter is much slower. | |
94 FFTW does not deal with this problem, however—it assumes that your | |
95 processes run on hardware of comparable speed, and that the goal is | |
96 therefore to divide the problem as equally as possible. | |
97 </p> | |
98 <p>For a multi-dimensional complex DFT, FFTW can divide the problem | |
99 equally among the processes if: (i) the <em>first</em> dimension | |
100 <code>n0</code> is divisible by <em>P</em>; and (ii), the <em>product</em> of | |
101 the subsequent dimensions is divisible by <em>P</em>. (For the advanced | |
102 interface, where you can specify multiple simultaneous transforms via | |
103 some “vector” length <code>howmany</code>, a factor of <code>howmany</code> is | |
104 included in the product of the subsequent dimensions.) | |
105 </p> | |
106 <p>For a one-dimensional complex DFT, the length <code>N</code> of the data | |
107 should be divisible by <em>P</em> <em>squared</em> to be able to divide | |
108 the problem equally among the processes. | |
109 </p> | |
110 | |
111 | |
112 | |
113 </body> | |
114 </html> |