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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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Chris@42 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Chris@42 3 <!-- This manual is for FFTW
Chris@42 4 (version 3.3.5, 30 July 2016).
Chris@42 5
Chris@42 6 Copyright (C) 2003 Matteo Frigo.
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Chris@42 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Words of Wisdom-Saving Plans</title>
Chris@42 26
Chris@42 27 <meta name="description" content="FFTW 3.3.5: Words of Wisdom-Saving Plans">
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Chris@42 37 <link href="Caveats-in-Using-Wisdom.html#Caveats-in-Using-Wisdom" rel="next" title="Caveats in Using Wisdom">
Chris@42 38 <link href="Dynamic-Arrays-in-C_002dThe-Wrong-Way.html#Dynamic-Arrays-in-C_002dThe-Wrong-Way" rel="prev" title="Dynamic Arrays in C-The Wrong Way">
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Chris@42 69 </head>
Chris@42 70
Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Words-of-Wisdom_002dSaving-Plans"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 Next: <a href="Caveats-in-Using-Wisdom.html#Caveats-in-Using-Wisdom" accesskey="n" rel="next">Caveats in Using Wisdom</a>, Previous: <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" accesskey="p" rel="prev">Multi-dimensional Array Format</a>, Up: <a href="Other-Important-Topics.html#Other-Important-Topics" accesskey="u" rel="up">Other Important Topics</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="Words-of-Wisdom_002d_002d_002dSaving-Plans"></a>
Chris@42 79 <h3 class="section">3.3 Words of Wisdom&mdash;Saving Plans</h3>
Chris@42 80 <a name="index-wisdom"></a>
Chris@42 81 <a name="index-saving-plans-to-disk"></a>
Chris@42 82
Chris@42 83 <p>FFTW implements a method for saving plans to disk and restoring them.
Chris@42 84 In fact, what FFTW does is more general than just saving and loading
Chris@42 85 plans. The mechanism is called <em>wisdom</em>. Here, we describe
Chris@42 86 this feature at a high level. See <a href="FFTW-Reference.html#FFTW-Reference">FFTW Reference</a>, for a less casual
Chris@42 87 but more complete discussion of how to use wisdom in FFTW.
Chris@42 88 </p>
Chris@42 89 <p>Plans created with the <code>FFTW_MEASURE</code>, <code>FFTW_PATIENT</code>, or
Chris@42 90 <code>FFTW_EXHAUSTIVE</code> options produce near-optimal FFT performance,
Chris@42 91 but may require a long time to compute because FFTW must measure the
Chris@42 92 runtime of many possible plans and select the best one. This setup is
Chris@42 93 designed for the situations where so many transforms of the same size
Chris@42 94 must be computed that the start-up time is irrelevant. For short
Chris@42 95 initialization times, but slower transforms, we have provided
Chris@42 96 <code>FFTW_ESTIMATE</code>. The <code>wisdom</code> mechanism is a way to get the
Chris@42 97 best of both worlds: you compute a good plan once, save it to
Chris@42 98 disk, and later reload it as many times as necessary. The wisdom
Chris@42 99 mechanism can actually save and reload many plans at once, not just
Chris@42 100 one.
Chris@42 101 <a name="index-FFTW_005fMEASURE-1"></a>
Chris@42 102 <a name="index-FFTW_005fPATIENT-1"></a>
Chris@42 103 <a name="index-FFTW_005fEXHAUSTIVE"></a>
Chris@42 104 <a name="index-FFTW_005fESTIMATE-1"></a>
Chris@42 105 </p>
Chris@42 106
Chris@42 107 <p>Whenever you create a plan, the FFTW planner accumulates wisdom, which
Chris@42 108 is information sufficient to reconstruct the plan. After planning,
Chris@42 109 you can save this information to disk by means of the function:
Chris@42 110 </p><div class="example">
Chris@42 111 <pre class="example">int fftw_export_wisdom_to_filename(const char *filename);
Chris@42 112 </pre></div>
Chris@42 113 <a name="index-fftw_005fexport_005fwisdom_005fto_005ffilename"></a>
Chris@42 114 <p>(This function returns non-zero on success.)
Chris@42 115 </p>
Chris@42 116 <p>The next time you run the program, you can restore the wisdom with
Chris@42 117 <code>fftw_import_wisdom_from_filename</code> (which also returns non-zero on success),
Chris@42 118 and then recreate the plan using the same flags as before.
Chris@42 119 </p><div class="example">
Chris@42 120 <pre class="example">int fftw_import_wisdom_from_filename(const char *filename);
Chris@42 121 </pre></div>
Chris@42 122 <a name="index-fftw_005fimport_005fwisdom_005ffrom_005ffilename"></a>
Chris@42 123
Chris@42 124 <p>Wisdom is automatically used for any size to which it is applicable, as
Chris@42 125 long as the planner flags are not more &ldquo;patient&rdquo; than those with which
Chris@42 126 the wisdom was created. For example, wisdom created with
Chris@42 127 <code>FFTW_MEASURE</code> can be used if you later plan with
Chris@42 128 <code>FFTW_ESTIMATE</code> or <code>FFTW_MEASURE</code>, but not with
Chris@42 129 <code>FFTW_PATIENT</code>.
Chris@42 130 </p>
Chris@42 131 <p>The <code>wisdom</code> is cumulative, and is stored in a global, private
Chris@42 132 data structure managed internally by FFTW. The storage space required
Chris@42 133 is minimal, proportional to the logarithm of the sizes the wisdom was
Chris@42 134 generated from. If memory usage is a concern, however, the wisdom can
Chris@42 135 be forgotten and its associated memory freed by calling:
Chris@42 136 </p><div class="example">
Chris@42 137 <pre class="example">void fftw_forget_wisdom(void);
Chris@42 138 </pre></div>
Chris@42 139 <a name="index-fftw_005fforget_005fwisdom"></a>
Chris@42 140
Chris@42 141 <p>Wisdom can be exported to a file, a string, or any other medium.
Chris@42 142 For details, see <a href="Wisdom.html#Wisdom">Wisdom</a>.
Chris@42 143 </p>
Chris@42 144 <hr>
Chris@42 145 <div class="header">
Chris@42 146 <p>
Chris@42 147 Next: <a href="Caveats-in-Using-Wisdom.html#Caveats-in-Using-Wisdom" accesskey="n" rel="next">Caveats in Using Wisdom</a>, Previous: <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" accesskey="p" rel="prev">Multi-dimensional Array Format</a>, Up: <a href="Other-Important-Topics.html#Other-Important-Topics" accesskey="u" rel="up">Other Important Topics</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
Chris@42 148 </div>
Chris@42 149
Chris@42 150
Chris@42 151
Chris@42 152 </body>
Chris@42 153 </html>