103 lines
4.7 KiB
HTML
103 lines
4.7 KiB
HTML
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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<title>Load balancing (FFTW 3.3.10)</title>
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<span id="Load-balancing"></span><div class="header">
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<p>
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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>
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<hr>
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<span id="Load-balancing-1"></span><h4 class="subsection">6.4.2 Load balancing</h4>
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<span id="index-load-balancing"></span>
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<p>Ideally, when you parallelize a transform over some <em>P</em>
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processes, each process should end up with work that takes equal time.
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Otherwise, all of the processes end up waiting on whichever process is
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slowest. This goal is known as “load balancing.” In this section,
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we describe the circumstances under which FFTW is able to load-balance
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well, and in particular how you should choose your transform size in
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order to load balance.
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</p>
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<p>Load balancing is especially difficult when you are parallelizing over
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heterogeneous machines; for example, if one of your processors is a
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old 486 and another is a Pentium IV, obviously you should give the
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Pentium more work to do than the 486 since the latter is much slower.
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FFTW does not deal with this problem, however—it assumes that your
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processes run on hardware of comparable speed, and that the goal is
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therefore to divide the problem as equally as possible.
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</p>
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<p>For a multi-dimensional complex DFT, FFTW can divide the problem
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equally among the processes if: (i) the <em>first</em> dimension
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<code>n0</code> is divisible by <em>P</em>; and (ii), the <em>product</em> of
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the subsequent dimensions is divisible by <em>P</em>. (For the advanced
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interface, where you can specify multiple simultaneous transforms via
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some “vector” length <code>howmany</code>, a factor of <code>howmany</code> is
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included in the product of the subsequent dimensions.)
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</p>
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<p>For a one-dimensional complex DFT, the length <code>N</code> of the data
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should be divisible by <em>P</em> <em>squared</em> to be able to divide
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the problem equally among the processes.
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</p>
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