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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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+<!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.
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+manual provided the copyright notice and this permission notice are
+preserved on all copies.
+
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+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: How Many Threads to Use?</title>
+
+<meta name="description" content="FFTW 3.3.5: How Many Threads to Use?">
+<meta name="keywords" content="FFTW 3.3.5: How Many Threads to Use?">
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+<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="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" rel="up" title="Multi-threaded FFTW">
+<link href="Thread-safety.html#Thread-safety" rel="next" title="Thread safety">
+<link href="Usage-of-Multi_002dthreaded-FFTW.html#Usage-of-Multi_002dthreaded-FFTW" rel="prev" title="Usage of Multi-threaded FFTW">
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+<a name="How-Many-Threads-to-Use_003f"></a>
+<div class="header">
+<p>
+Next: <a href="Thread-safety.html#Thread-safety" accesskey="n" rel="next">Thread safety</a>, Previous: <a href="Usage-of-Multi_002dthreaded-FFTW.html#Usage-of-Multi_002dthreaded-FFTW" accesskey="p" rel="prev">Usage of Multi-threaded FFTW</a>, Up: <a href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" accesskey="u" rel="up">Multi-threaded FFTW</a> &nbsp; [<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="How-Many-Threads-to-Use_003f-1"></a>
+<h3 class="section">5.3 How Many Threads to Use?</h3>
+
+<a name="index-number-of-threads"></a>
+<p>There is a fair amount of overhead involved in synchronizing threads,
+so the optimal number of threads to use depends upon the size of the
+transform as well as on the number of processors you have.
+</p>
+<p>As a general rule, you don&rsquo;t want to use more threads than you have
+processors.  (Using more threads will work, but there will be extra
+overhead with no benefit.)  In fact, if the problem size is too small,
+you may want to use fewer threads than you have processors.
+</p>
+<p>You will have to experiment with your system to see what level of
+parallelization is best for your problem size.  Typically, the problem
+will have to involve at least a few thousand data points before threads
+become beneficial.  If you plan with <code>FFTW_PATIENT</code>, it will
+automatically disable threads for sizes that don&rsquo;t benefit from
+parallelization.
+<a name="index-FFTW_005fPATIENT-3"></a>
+</p>
+
+
+
+</body>
+</html>