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| author | Chris Cannam |
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| date | Wed, 20 Mar 2013 15:35:50 +0000 |
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| 9:c0fb53affa76 | 10:37bf6b4a2645 |
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| 1 <html lang="en"> | |
| 2 <head> | |
| 3 <title>Thread safety - FFTW 3.3.3</title> | |
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| 5 <meta name="description" content="FFTW 3.3.3"> | |
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| 8 <link rel="up" href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" title="Multi-threaded FFTW"> | |
| 9 <link rel="prev" href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" title="How Many Threads to Use?"> | |
| 10 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> | |
| 11 <!-- | |
| 12 This manual is for FFTW | |
| 13 (version 3.3.3, 25 November 2012). | |
| 14 | |
| 15 Copyright (C) 2003 Matteo Frigo. | |
| 16 | |
| 17 Copyright (C) 2003 Massachusetts Institute of Technology. | |
| 18 | |
| 19 Permission is granted to make and distribute verbatim copies of | |
| 20 this manual provided the copyright notice and this permission | |
| 21 notice are preserved on all copies. | |
| 22 | |
| 23 Permission is granted to copy and distribute modified versions of | |
| 24 this manual under the conditions for verbatim copying, provided | |
| 25 that the entire resulting derived work is distributed under the | |
| 26 terms of a permission notice identical to this one. | |
| 27 | |
| 28 Permission is granted to copy and distribute translations of this | |
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| 46 <body> | |
| 47 <div class="node"> | |
| 48 <a name="Thread-safety"></a> | |
| 49 <p> | |
| 50 Previous: <a rel="previous" accesskey="p" href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f">How Many Threads to Use?</a>, | |
| 51 Up: <a rel="up" accesskey="u" href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW">Multi-threaded FFTW</a> | |
| 52 <hr> | |
| 53 </div> | |
| 54 | |
| 55 <h3 class="section">5.4 Thread safety</h3> | |
| 56 | |
| 57 <p><a name="index-threads-341"></a><a name="index-OpenMP-342"></a><a name="index-thread-safety-343"></a>Users writing multi-threaded programs (including OpenMP) must concern | |
| 58 themselves with the <dfn>thread safety</dfn> of the libraries they | |
| 59 use—that is, whether it is safe to call routines in parallel from | |
| 60 multiple threads. FFTW can be used in such an environment, but some | |
| 61 care must be taken because the planner routines share data | |
| 62 (e.g. wisdom and trigonometric tables) between calls and plans. | |
| 63 | |
| 64 <p>The upshot is that the only thread-safe (re-entrant) routine in FFTW is | |
| 65 <code>fftw_execute</code> (and the new-array variants thereof). All other routines | |
| 66 (e.g. the planner) should only be called from one thread at a time. So, | |
| 67 for example, you can wrap a semaphore lock around any calls to the | |
| 68 planner; even more simply, you can just create all of your plans from | |
| 69 one thread. We do not think this should be an important restriction | |
| 70 (FFTW is designed for the situation where the only performance-sensitive | |
| 71 code is the actual execution of the transform), and the benefits of | |
| 72 shared data between plans are great. | |
| 73 | |
| 74 <p>Note also that, since the plan is not modified by <code>fftw_execute</code>, | |
| 75 it is safe to execute the <em>same plan</em> in parallel by multiple | |
| 76 threads. However, since a given plan operates by default on a fixed | |
| 77 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. | |
| 78 | |
| 79 <p>(Users should note that these comments only apply to programs using | |
| 80 shared-memory threads or OpenMP. Parallelism using MPI or forked processes | |
| 81 involves a separate address-space and global variables for each process, | |
| 82 and is not susceptible to problems of this sort.) | |
| 83 | |
| 84 <p>If you are configured FFTW with the <code>--enable-debug</code> or | |
| 85 <code>--enable-debug-malloc</code> flags (see <a href="Installation-on-Unix.html#Installation-on-Unix">Installation on Unix</a>), | |
| 86 then <code>fftw_execute</code> is not thread-safe. These flags are not | |
| 87 documented because they are intended only for developing | |
| 88 and debugging FFTW, but if you must use <code>--enable-debug</code> then you | |
| 89 should also specifically pass <code>--disable-debug-malloc</code> for | |
| 90 <code>fftw_execute</code> to be thread-safe. | |
| 91 | |
| 92 </body></html> | |
| 93 |