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1 <html lang="en">
2 <head>
3 <title>Words of Wisdom-Saving Plans - FFTW 3.3.4</title>
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13 This manual is for FFTW
14 (version 3.3.4, 20 September 2013).
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16 Copyright (C) 2003 Matteo Frigo.
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18 Copyright (C) 2003 Massachusetts Institute of Technology.
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48 <div class="node">
49 <a name="Words-of-Wisdom-Saving-Plans"></a>
50 <a name="Words-of-Wisdom_002dSaving-Plans"></a>
51 <p>
52 Next:&nbsp;<a rel="next" accesskey="n" href="Caveats-in-Using-Wisdom.html#Caveats-in-Using-Wisdom">Caveats in Using Wisdom</a>,
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55 <hr>
56 </div>
57
58 <h3 class="section">3.3 Words of Wisdom&mdash;Saving Plans</h3>
59
60 <p><a name="index-wisdom-124"></a><a name="index-saving-plans-to-disk-125"></a>
61 FFTW implements a method for saving plans to disk and restoring them.
62 In fact, what FFTW does is more general than just saving and loading
63 plans. The mechanism is called <dfn>wisdom</dfn>. Here, we describe
64 this feature at a high level. See <a href="FFTW-Reference.html#FFTW-Reference">FFTW Reference</a>, for a less casual
65 but more complete discussion of how to use wisdom in FFTW.
66
67 <p>Plans created with the <code>FFTW_MEASURE</code>, <code>FFTW_PATIENT</code>, or
68 <code>FFTW_EXHAUSTIVE</code> options produce near-optimal FFT performance,
69 but may require a long time to compute because FFTW must measure the
70 runtime of many possible plans and select the best one. This setup is
71 designed for the situations where so many transforms of the same size
72 must be computed that the start-up time is irrelevant. For short
73 initialization times, but slower transforms, we have provided
74 <code>FFTW_ESTIMATE</code>. The <code>wisdom</code> mechanism is a way to get the
75 best of both worlds: you compute a good plan once, save it to
76 disk, and later reload it as many times as necessary. The wisdom
77 mechanism can actually save and reload many plans at once, not just
78 one.
79 <a name="index-FFTW_005fMEASURE-126"></a><a name="index-FFTW_005fPATIENT-127"></a><a name="index-FFTW_005fEXHAUSTIVE-128"></a><a name="index-FFTW_005fESTIMATE-129"></a>
80
81 <p>Whenever you create a plan, the FFTW planner accumulates wisdom, which
82 is information sufficient to reconstruct the plan. After planning,
83 you can save this information to disk by means of the function:
84 <pre class="example"> int fftw_export_wisdom_to_filename(const char *filename);
85 </pre>
86 <p><a name="index-fftw_005fexport_005fwisdom_005fto_005ffilename-130"></a>(This function returns non-zero on success.)
87
88 <p>The next time you run the program, you can restore the wisdom with
89 <code>fftw_import_wisdom_from_filename</code> (which also returns non-zero on success),
90 and then recreate the plan using the same flags as before.
91 <pre class="example"> int fftw_import_wisdom_from_filename(const char *filename);
92 </pre>
93 <p><a name="index-fftw_005fimport_005fwisdom_005ffrom_005ffilename-131"></a>
94 Wisdom is automatically used for any size to which it is applicable, as
95 long as the planner flags are not more &ldquo;patient&rdquo; than those with which
96 the wisdom was created. For example, wisdom created with
97 <code>FFTW_MEASURE</code> can be used if you later plan with
98 <code>FFTW_ESTIMATE</code> or <code>FFTW_MEASURE</code>, but not with
99 <code>FFTW_PATIENT</code>.
100
101 <p>The <code>wisdom</code> is cumulative, and is stored in a global, private
102 data structure managed internally by FFTW. The storage space required
103 is minimal, proportional to the logarithm of the sizes the wisdom was
104 generated from. If memory usage is a concern, however, the wisdom can
105 be forgotten and its associated memory freed by calling:
106 <pre class="example"> void fftw_forget_wisdom(void);
107 </pre>
108 <p><a name="index-fftw_005fforget_005fwisdom-132"></a>
109 Wisdom can be exported to a file, a string, or any other medium.
110 For details, see <a href="Wisdom.html#Wisdom">Wisdom</a>.
111
112 </body></html>
113