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3 <title>Caveats in Using Wisdom - FFTW 3.3.3</title>
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9 <link rel="prev" href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans" title="Words of Wisdom-Saving Plans">
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11 <!--
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12 This manual is for FFTW
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13 (version 3.3.3, 25 November 2012).
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14
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15 Copyright (C) 2003 Matteo Frigo.
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16
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17 Copyright (C) 2003 Massachusetts Institute of Technology.
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18
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19 Permission is granted to make and distribute verbatim copies of
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23 Permission is granted to copy and distribute modified versions of
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24 this manual under the conditions for verbatim copying, provided
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25 that the entire resulting derived work is distributed under the
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26 terms of a permission notice identical to this one.
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27
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28 Permission is granted to copy and distribute translations of this
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29 manual into another language, under the above conditions for
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30 modified versions, except that this permission notice may be
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31 stated in a translation approved by the Free Software Foundation.
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44 --></style>
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45 </head>
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46 <body>
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47 <div class="node">
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48 <a name="Caveats-in-Using-Wisdom"></a>
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49 <p>
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50 Previous: <a rel="previous" accesskey="p" href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans">Words of Wisdom-Saving Plans</a>,
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51 Up: <a rel="up" accesskey="u" href="Other-Important-Topics.html#Other-Important-Topics">Other Important Topics</a>
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52 <hr>
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53 </div>
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54
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55 <h3 class="section">3.4 Caveats in Using Wisdom</h3>
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56
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57 <p><a name="index-wisdom_002c-problems-with-133"></a>
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58 <blockquote>
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59 <i>For in much wisdom is much grief, and he that increaseth knowledge
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60 increaseth sorrow.
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61 </i>[Ecclesiastes 1:18]
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62 <a name="index-Ecclesiastes-134"></a></blockquote>
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63
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64 <p><a name="index-portability-135"></a>There are pitfalls to using wisdom, in that it can negate FFTW's
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65 ability to adapt to changing hardware and other conditions. For
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66 example, it would be perfectly possible to export wisdom from a
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67 program running on one processor and import it into a program running
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68 on another processor. Doing so, however, would mean that the second
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69 program would use plans optimized for the first processor, instead of
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70 the one it is running on.
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71
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72 <p>It should be safe to reuse wisdom as long as the hardware and program
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73 binaries remain unchanged. (Actually, the optimal plan may change even
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74 between runs of the same binary on identical hardware, due to
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75 differences in the virtual memory environment, etcetera. Users
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76 seriously interested in performance should worry about this problem,
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77 too.) It is likely that, if the same wisdom is used for two
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78 different program binaries, even running on the same machine, the
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79 plans may be sub-optimal because of differing code alignments. It is
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80 therefore wise to recreate wisdom every time an application is
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81 recompiled. The more the underlying hardware and software changes
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82 between the creation of wisdom and its use, the greater grows
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83 the risk of sub-optimal plans.
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84
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85 <p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
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86 using possibly-suboptimal wisdom (created on the same machine, but for a
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87 different binary), the wisdom is likely to be better. For this reason,
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88 we provide a function to import wisdom from a standard system-wide
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89 location (<code>/etc/fftw/wisdom</code> on Unix):
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90 <a name="index-wisdom_002c-system_002dwide-136"></a>
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91 <pre class="example"> int fftw_import_system_wisdom(void);
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92 </pre>
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93 <p><a name="index-fftw_005fimport_005fsystem_005fwisdom-137"></a>
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94 FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
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95 by its own <code>man</code> page on Unix) with which users can create wisdom,
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96 e.g. for a canonical set of sizes to store in the system wisdom file.
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97 See <a href="Wisdom-Utilities.html#Wisdom-Utilities">Wisdom Utilities</a>.
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98 <a name="index-fftw_002dwisdom-utility-138"></a>
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99
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100 </body></html>
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101
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