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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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Permission is granted to copy and distribute modified versions of this
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<title>How Many Threads to Use? (FFTW 3.3.10)</title>
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Multi_002dthreaded-FFTW.html" rel="up" title="Multi-threaded FFTW">
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<link href="Thread-safety.html" rel="next" title="Thread safety">
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<span id="How-Many-Threads-to-Use_003f"></span><div class="header">
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<p>
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Next: <a href="Thread-safety.html" accesskey="n" rel="next">Thread safety</a>, Previous: <a href="Usage-of-Multi_002dthreaded-FFTW.html" accesskey="p" rel="prev">Usage of Multi-threaded FFTW</a>, Up: <a href="Multi_002dthreaded-FFTW.html" accesskey="u" rel="up">Multi-threaded FFTW</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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</div>
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<hr>
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<span id="How-Many-Threads-to-Use_003f-1"></span><h3 class="section">5.3 How Many Threads to Use?</h3>
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<span id="index-number-of-threads"></span>
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<p>There is a fair amount of overhead involved in synchronizing threads,
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so the optimal number of threads to use depends upon the size of the
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transform as well as on the number of processors you have.
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</p>
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<p>As a general rule, you don’t want to use more threads than you have
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processors. (Using more threads will work, but there will be extra
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overhead with no benefit.) In fact, if the problem size is too small,
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you may want to use fewer threads than you have processors.
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</p>
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<p>You will have to experiment with your system to see what level of
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parallelization is best for your problem size. Typically, the problem
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will have to involve at least a few thousand data points before threads
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become beneficial. If you plan with <code>FFTW_PATIENT</code>, it will
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automatically disable threads for sizes that don’t benefit from
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parallelization.
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<span id="index-FFTW_005fPATIENT-3"></span>
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</p>
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