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author Chris Cannam <cannam@all-day-breakfast.com>
date Wed, 20 Mar 2013 15:35:50 +0000
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3 <title>Caveats in Using Wisdom - FFTW 3.3.3</title>
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48 <a name="Caveats-in-Using-Wisdom"></a>
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54
55 <h3 class="section">3.4 Caveats in Using Wisdom</h3>
56
57 <p><a name="index-wisdom_002c-problems-with-133"></a>
58 <blockquote>
59 <i>For in much wisdom is much grief, and he that increaseth knowledge
60 increaseth sorrow.
61 </i>[Ecclesiastes 1:18]
62 <a name="index-Ecclesiastes-134"></a></blockquote>
63
64 <p><a name="index-portability-135"></a>There are pitfalls to using wisdom, in that it can negate FFTW's
65 ability to adapt to changing hardware and other conditions. For
66 example, it would be perfectly possible to export wisdom from a
67 program running on one processor and import it into a program running
68 on another processor. Doing so, however, would mean that the second
69 program would use plans optimized for the first processor, instead of
70 the one it is running on.
71
72 <p>It should be safe to reuse wisdom as long as the hardware and program
73 binaries remain unchanged. (Actually, the optimal plan may change even
74 between runs of the same binary on identical hardware, due to
75 differences in the virtual memory environment, etcetera. Users
76 seriously interested in performance should worry about this problem,
77 too.) It is likely that, if the same wisdom is used for two
78 different program binaries, even running on the same machine, the
79 plans may be sub-optimal because of differing code alignments. It is
80 therefore wise to recreate wisdom every time an application is
81 recompiled. The more the underlying hardware and software changes
82 between the creation of wisdom and its use, the greater grows
83 the risk of sub-optimal plans.
84
85 <p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
86 using possibly-suboptimal wisdom (created on the same machine, but for a
87 different binary), the wisdom is likely to be better. For this reason,
88 we provide a function to import wisdom from a standard system-wide
89 location (<code>/etc/fftw/wisdom</code> on Unix):
90 <a name="index-wisdom_002c-system_002dwide-136"></a>
91 <pre class="example"> int fftw_import_system_wisdom(void);
92 </pre>
93 <p><a name="index-fftw_005fimport_005fsystem_005fwisdom-137"></a>
94 FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
95 by its own <code>man</code> page on Unix) with which users can create wisdom,
96 e.g. for a canonical set of sizes to store in the system wisdom file.
97 See <a href="Wisdom-Utilities.html#Wisdom-Utilities">Wisdom Utilities</a>.
98 <a name="index-fftw_002dwisdom-utility-138"></a>
99
100 </body></html>
101