Mercurial > hg > sv-dependency-builds
diff src/fftw-3.3.8/doc/html/Thread-safety.html @ 82:d0c2a83c1364
Add FFTW 3.3.8 source, and a Linux build
| author | Chris Cannam |
|---|---|
| date | Tue, 19 Nov 2019 14:52:55 +0000 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.8/doc/html/Thread-safety.html Tue Nov 19 14:52:55 2019 +0000 @@ -0,0 +1,149 @@ +<!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 3.3.8, 24 May 2018). + +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 6.3, http://www.gnu.org/software/texinfo/ --> +<head> +<title>FFTW 3.3.8: Thread safety</title> + +<meta name="description" content="FFTW 3.3.8: Thread safety"> +<meta name="keywords" content="FFTW 3.3.8: Thread safety"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<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="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="next" title="Distributed-memory FFTW with MPI"> +<link href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" rel="prev" title="How Many Threads to Use?"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.indentedblock {margin-right: 0em} +blockquote.smallindentedblock {margin-right: 0em; font-size: smaller} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smalllisp {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} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +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"> +<a name="Thread-safety"></a> +<div class="header"> +<p> +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> [<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="Thread-safety-1"></a> +<h3 class="section">5.4 Thread safety</h3> + +<a name="index-threads-1"></a> +<a name="index-OpenMP-3"></a> +<a name="index-thread-safety-1"></a> +<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_002darray-Execute-Functions">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> +<a name="index-fftw_005fmake_005fplanner_005fthread_005fsafe"></a> + +<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#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> [<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> + + + +</body> +</html>