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Add FFTW 3.3.8 source, and a Linux build
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 19 Nov 2019 14:52:55 +0000
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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 3.3.8, 24 May 2018).
5
6 Copyright (C) 2003 Matteo Frigo.
7
8 Copyright (C) 2003 Massachusetts Institute of Technology.
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11 manual provided the copyright notice and this permission notice are
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25 <title>FFTW 3.3.8: Thread safety</title>
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70 <body lang="en">
71 <a name="Thread-safety"></a>
72 <div class="header">
73 <p>
74 Previous: <a href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" accesskey="p" rel="prev">How Many Threads to Use?</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>
75 </div>
76 <hr>
77 <a name="Thread-safety-1"></a>
78 <h3 class="section">5.4 Thread safety</h3>
79
80 <a name="index-threads-1"></a>
81 <a name="index-OpenMP-3"></a>
82 <a name="index-thread-safety-1"></a>
83 <p>Users writing multi-threaded programs (including OpenMP) must concern
84 themselves with the <em>thread safety</em> of the libraries they
85 use&mdash;that is, whether it is safe to call routines in parallel from
86 multiple threads. FFTW can be used in such an environment, but some
87 care must be taken because the planner routines share data
88 (e.g. wisdom and trigonometric tables) between calls and plans.
89 </p>
90 <p>The upshot is that the only thread-safe routine in FFTW is
91 <code>fftw_execute</code> (and the new-array variants thereof). All other routines
92 (e.g. the planner) should only be called from one thread at a time. So,
93 for example, you can wrap a semaphore lock around any calls to the
94 planner; even more simply, you can just create all of your plans from
95 one thread. We do not think this should be an important restriction
96 (FFTW is designed for the situation where the only performance-sensitive
97 code is the actual execution of the transform), and the benefits of
98 shared data between plans are great.
99 </p>
100 <p>Note also that, since the plan is not modified by <code>fftw_execute</code>,
101 it is safe to execute the <em>same plan</em> in parallel by multiple
102 threads. However, since a given plan operates by default on a fixed
103 array, you need to use one of the new-array execute functions (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>) so that different threads compute the transform of different data.
104 </p>
105 <p>(Users should note that these comments only apply to programs using
106 shared-memory threads or OpenMP. Parallelism using MPI or forked processes
107 involves a separate address-space and global variables for each process,
108 and is not susceptible to problems of this sort.)
109 </p>
110 <p>The FFTW planner is intended to be called from a single thread. If you
111 really must call it from multiple threads, you are expected to grab
112 whatever lock makes sense for your application, with the understanding
113 that you may be holding that lock for a long time, which is undesirable.
114 </p>
115 <p>Neither strategy works, however, in the following situation. The
116 &ldquo;application&rdquo; is structured as a set of &ldquo;plugins&rdquo; which are unaware
117 of each other, and for whatever reason the &ldquo;plugins&rdquo; cannot coordinate
118 on grabbing the lock. (This is not a technical problem, but an
119 organizational one. The &ldquo;plugins&rdquo; are written by independent agents,
120 and from the perspective of each plugin&rsquo;s author, each plugin is using
121 FFTW correctly from a single thread.) To cope with this situation,
122 starting from FFTW-3.3.5, FFTW supports an API to make the planner
123 thread-safe:
124 </p>
125 <div class="example">
126 <pre class="example">void fftw_make_planner_thread_safe(void);
127 </pre></div>
128 <a name="index-fftw_005fmake_005fplanner_005fthread_005fsafe"></a>
129
130 <p>This call operates by brute force: It just installs a hook that wraps a
131 lock (chosen by us) around all planner calls. So there is no magic and
132 you get the worst of all worlds. The planner is still single-threaded,
133 but you cannot choose which lock to use. The planner still holds the
134 lock for a long time, but you cannot impose a timeout on lock
135 acquisition. As of FFTW-3.3.5 and FFTW-3.3.6, this call does not work
136 when using OpenMP as threading substrate. (Suggestions on what to do
137 about this bug are welcome.) <em>Do not use
138 <code>fftw_make_planner_thread_safe</code> unless there is no other choice,</em>
139 such as in the application/plugin situation.
140 </p><hr>
141 <div class="header">
142 <p>
143 Previous: <a href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" accesskey="p" rel="prev">How Many Threads to Use?</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>
144 </div>
145
146
147
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149 </html>