129 lines
5.7 KiB
HTML
129 lines
5.7 KiB
HTML
<!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 3.3.10, 10 December 2020).
|
|
|
|
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 6.7, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<title>Caveats in Using Wisdom (FFTW 3.3.10)</title>
|
|
|
|
<meta name="description" content="Caveats in Using Wisdom (FFTW 3.3.10)">
|
|
<meta name="keywords" content="Caveats in Using Wisdom (FFTW 3.3.10)">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content="makeinfo">
|
|
<link href="index.html" rel="start" title="Top">
|
|
<link href="Concept-Index.html" rel="index" title="Concept Index">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="Other-Important-Topics.html" rel="up" title="Other Important Topics">
|
|
<link href="FFTW-Reference.html" rel="next" title="FFTW Reference">
|
|
<link href="Words-of-Wisdom_002dSaving-Plans.html" rel="prev" title="Words of Wisdom-Saving Plans">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {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}
|
|
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">
|
|
<span id="Caveats-in-Using-Wisdom"></span><div class="header">
|
|
<p>
|
|
Previous: <a href="Words-of-Wisdom_002dSaving-Plans.html" accesskey="p" rel="prev">Words of Wisdom-Saving Plans</a>, Up: <a href="Other-Important-Topics.html" accesskey="u" rel="up">Other Important Topics</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
|
|
</div>
|
|
<hr>
|
|
<span id="Caveats-in-Using-Wisdom-1"></span><h3 class="section">3.4 Caveats in Using Wisdom</h3>
|
|
<span id="index-wisdom_002c-problems-with"></span>
|
|
|
|
<blockquote>
|
|
<i>
|
|
<p>For in much wisdom is much grief, and he that increaseth knowledge
|
|
increaseth sorrow.
|
|
</i>
|
|
[Ecclesiastes 1:18]
|
|
<span id="index-Ecclesiastes"></span>
|
|
</p></blockquote>
|
|
|
|
<span id="index-portability-1"></span>
|
|
<p>There are pitfalls to using wisdom, in that it can negate FFTW’s
|
|
ability to adapt to changing hardware and other conditions. For
|
|
example, it would be perfectly possible to export wisdom from a
|
|
program running on one processor and import it into a program running
|
|
on another processor. Doing so, however, would mean that the second
|
|
program would use plans optimized for the first processor, instead of
|
|
the one it is running on.
|
|
</p>
|
|
<p>It should be safe to reuse wisdom as long as the hardware and program
|
|
binaries remain unchanged. (Actually, the optimal plan may change even
|
|
between runs of the same binary on identical hardware, due to
|
|
differences in the virtual memory environment, etcetera. Users
|
|
seriously interested in performance should worry about this problem,
|
|
too.) It is likely that, if the same wisdom is used for two
|
|
different program binaries, even running on the same machine, the
|
|
plans may be sub-optimal because of differing code alignments. It is
|
|
therefore wise to recreate wisdom every time an application is
|
|
recompiled. The more the underlying hardware and software changes
|
|
between the creation of wisdom and its use, the greater grows
|
|
the risk of sub-optimal plans.
|
|
</p>
|
|
<p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
|
|
using possibly-suboptimal wisdom (created on the same machine, but for a
|
|
different binary), the wisdom is likely to be better. For this reason,
|
|
we provide a function to import wisdom from a standard system-wide
|
|
location (<code>/etc/fftw/wisdom</code> on Unix):
|
|
<span id="index-wisdom_002c-system_002dwide"></span>
|
|
</p>
|
|
<div class="example">
|
|
<pre class="example">int fftw_import_system_wisdom(void);
|
|
</pre></div>
|
|
<span id="index-fftw_005fimport_005fsystem_005fwisdom"></span>
|
|
|
|
<p>FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
|
|
by its own <code>man</code> page on Unix) with which users can create wisdom,
|
|
e.g. for a canonical set of sizes to store in the system wisdom file.
|
|
See <a href="Wisdom-Utilities.html">Wisdom Utilities</a>.
|
|
<span id="index-fftw_002dwisdom-utility"></span>
|
|
</p>
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
Previous: <a href="Words-of-Wisdom_002dSaving-Plans.html" accesskey="p" rel="prev">Words of Wisdom-Saving Plans</a>, Up: <a href="Other-Important-Topics.html" accesskey="u" rel="up">Other Important Topics</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
|
|
</div>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|