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2 <head>
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3 <title>How Many Threads to Use? - FFTW 3.3.3</title>
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8 <link rel="up" href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" title="Multi-threaded FFTW">
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9 <link rel="prev" href="Usage-of-Multi_002dthreaded-FFTW.html#Usage-of-Multi_002dthreaded-FFTW" title="Usage of Multi-threaded FFTW">
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12 <!--
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13 This manual is for FFTW
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14 (version 3.3.3, 25 November 2012).
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15
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16 Copyright (C) 2003 Matteo Frigo.
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17
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18 Copyright (C) 2003 Massachusetts Institute of Technology.
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20 Permission is granted to make and distribute verbatim copies of
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24 Permission is granted to copy and distribute modified versions of
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25 this manual under the conditions for verbatim copying, provided
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26 that the entire resulting derived work is distributed under the
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27 terms of a permission notice identical to this one.
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29 Permission is granted to copy and distribute translations of this
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31 modified versions, except that this permission notice may be
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32 stated in a translation approved by the Free Software Foundation.
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47 <body>
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48 <div class="node">
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49 <a name="How-Many-Threads-to-Use%3f"></a>
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50 <a name="How-Many-Threads-to-Use_003f"></a>
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51 <p>
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52 Next: <a rel="next" accesskey="n" href="Thread-safety.html#Thread-safety">Thread safety</a>,
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53 Previous: <a rel="previous" accesskey="p" href="Usage-of-Multi_002dthreaded-FFTW.html#Usage-of-Multi_002dthreaded-FFTW">Usage of Multi-threaded FFTW</a>,
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54 Up: <a rel="up" accesskey="u" href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW">Multi-threaded FFTW</a>
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55 <hr>
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56 </div>
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57
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58 <h3 class="section">5.3 How Many Threads to Use?</h3>
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59
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60 <p><a name="index-number-of-threads-339"></a>There is a fair amount of overhead involved in synchronizing threads,
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61 so the optimal number of threads to use depends upon the size of the
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62 transform as well as on the number of processors you have.
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63
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64 <p>As a general rule, you don't want to use more threads than you have
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65 processors. (Using more threads will work, but there will be extra
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66 overhead with no benefit.) In fact, if the problem size is too small,
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67 you may want to use fewer threads than you have processors.
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68
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69 <p>You will have to experiment with your system to see what level of
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70 parallelization is best for your problem size. Typically, the problem
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71 will have to involve at least a few thousand data points before threads
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72 become beneficial. If you plan with <code>FFTW_PATIENT</code>, it will
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73 automatically disable threads for sizes that don't benefit from
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74 parallelization.
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75 <a name="index-FFTW_005fPATIENT-340"></a>
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76 <!-- -->
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77
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78 </body></html>
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79
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