mirror of
https://github.com/tildearrow/furnace.git
synced 2024-11-05 20:35:06 +00:00
139 lines
6.8 KiB
HTML
139 lines
6.8 KiB
HTML
|
<!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 3.3.10, 10 December 2020).
|
||
|
|
|
||
|
|
Copyright (C) 2003 Matteo Frigo.
|
||
|
|
|
||
|
|
Copyright (C) 2003 Massachusetts Institute of Technology.
|
||
|
|
|
||
|
|
Permission is granted to make and distribute verbatim copies of this
|
||
|
|
manual provided the copyright notice and this permission notice are
|
||
|
|
preserved on all copies.
|
||
|
|
|
||
|
|
Permission is granted to copy and distribute modified versions of this
|
||
|
|
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 6.7, http://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<title>Thread safety (FFTW 3.3.10)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Thread safety (FFTW 3.3.10)">
|
||
|
|
<meta name="keywords" content="Thread safety (FFTW 3.3.10)">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content="makeinfo">
|
||
|
|
<link href="index.html" rel="start" title="Top">
|
||
|
|
<link href="Concept-Index.html" rel="index" title="Concept Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="Multi_002dthreaded-FFTW.html" rel="up" title="Multi-threaded FFTW">
|
||
|
|
<link href="Distributed_002dmemory-FFTW-with-MPI.html" rel="next" title="Distributed-memory FFTW with MPI">
|
||
|
|
<link href="How-Many-Threads-to-Use_003f.html" rel="prev" title="How Many Threads to Use?">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.summary-letter {text-decoration: none}
|
||
|
|
blockquote.indentedblock {margin-right: 0em}
|
||
|
|
div.display {margin-left: 3.2em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
div.lisp {margin-left: 3.2em}
|
||
|
|
kbd {font-style: oblique}
|
||
|
|
pre.display {font-family: inherit}
|
||
|
|
pre.format {font-family: inherit}
|
||
|
|
pre.menu-comment {font-family: serif}
|
||
|
|
pre.menu-preformatted {font-family: serif}
|
||
|
|
span.nolinebreak {white-space: nowrap}
|
||
|
|
span.roman {font-family: initial; font-weight: normal}
|
||
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
||
|
|
ul.no-bullet {list-style: none}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<span id="Thread-safety"></span><div class="header">
|
||
|
|
<p>
|
||
|
|
Previous: <a href="How-Many-Threads-to-Use_003f.html" accesskey="p" rel="prev">How Many Threads to Use?</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>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<span id="Thread-safety-1"></span><h3 class="section">5.4 Thread safety</h3>
|
||
|
|
|
||
|
|
<span id="index-threads-1"></span>
|
||
|
|
<span id="index-OpenMP-3"></span>
|
||
|
|
<span id="index-thread-safety-1"></span>
|
||
|
|
<p>Users writing multi-threaded programs (including OpenMP) must concern
|
||
|
|
themselves with the <em>thread safety</em> of the libraries they
|
||
|
|
use—that is, whether it is safe to call routines in parallel from
|
||
|
|
multiple threads. FFTW can be used in such an environment, but some
|
||
|
|
care must be taken because the planner routines share data
|
||
|
|
(e.g. wisdom and trigonometric tables) between calls and plans.
|
||
|
|
</p>
|
||
|
|
<p>The upshot is that the only thread-safe routine in FFTW is
|
||
|
|
<code>fftw_execute</code> (and the new-array variants thereof). All other routines
|
||
|
|
(e.g. the planner) should only be called from one thread at a time. So,
|
||
|
|
for example, you can wrap a semaphore lock around any calls to the
|
||
|
|
planner; even more simply, you can just create all of your plans from
|
||
|
|
one thread. We do not think this should be an important restriction
|
||
|
|
(FFTW is designed for the situation where the only performance-sensitive
|
||
|
|
code is the actual execution of the transform), and the benefits of
|
||
|
|
shared data between plans are great.
|
||
|
|
</p>
|
||
|
|
<p>Note also that, since the plan is not modified by <code>fftw_execute</code>,
|
||
|
|
it is safe to execute the <em>same plan</em> in parallel by multiple
|
||
|
|
threads. However, since a given plan operates by default on a fixed
|
||
|
|
array, you need to use one of the new-array execute functions (see <a href="New_002darray-Execute-Functions.html">New-array Execute Functions</a>) so that different threads compute the transform of different data.
|
||
|
|
</p>
|
||
|
|
<p>(Users should note that these comments only apply to programs using
|
||
|
|
shared-memory threads or OpenMP. Parallelism using MPI or forked processes
|
||
|
|
involves a separate address-space and global variables for each process,
|
||
|
|
and is not susceptible to problems of this sort.)
|
||
|
|
</p>
|
||
|
|
<p>The FFTW planner is intended to be called from a single thread. If you
|
||
|
|
really must call it from multiple threads, you are expected to grab
|
||
|
|
whatever lock makes sense for your application, with the understanding
|
||
|
|
that you may be holding that lock for a long time, which is undesirable.
|
||
|
|
</p>
|
||
|
|
<p>Neither strategy works, however, in the following situation. The
|
||
|
|
“application” is structured as a set of “plugins” which are unaware
|
||
|
|
of each other, and for whatever reason the “plugins” cannot coordinate
|
||
|
|
on grabbing the lock. (This is not a technical problem, but an
|
||
|
|
organizational one. The “plugins” are written by independent agents,
|
||
|
|
and from the perspective of each plugin’s author, each plugin is using
|
||
|
|
FFTW correctly from a single thread.) To cope with this situation,
|
||
|
|
starting from FFTW-3.3.5, FFTW supports an API to make the planner
|
||
|
|
thread-safe:
|
||
|
|
</p>
|
||
|
|
<div class="example">
|
||
|
|
<pre class="example">void fftw_make_planner_thread_safe(void);
|
||
|
|
</pre></div>
|
||
|
|
<span id="index-fftw_005fmake_005fplanner_005fthread_005fsafe"></span>
|
||
|
|
|
||
|
|
<p>This call operates by brute force: It just installs a hook that wraps a
|
||
|
|
lock (chosen by us) around all planner calls. So there is no magic and
|
||
|
|
you get the worst of all worlds. The planner is still single-threaded,
|
||
|
|
but you cannot choose which lock to use. The planner still holds the
|
||
|
|
lock for a long time, but you cannot impose a timeout on lock
|
||
|
|
acquisition. As of FFTW-3.3.5 and FFTW-3.3.6, this call does not work
|
||
|
|
when using OpenMP as threading substrate. (Suggestions on what to do
|
||
|
|
about this bug are welcome.) <em>Do not use
|
||
|
|
<code>fftw_make_planner_thread_safe</code> unless there is no other choice,</em>
|
||
|
|
such as in the application/plugin situation.
|
||
|
|
</p><hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Previous: <a href="How-Many-Threads-to-Use_003f.html" accesskey="p" rel="prev">How Many Threads to Use?</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>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|