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3 <title>Words of Wisdom-Saving Plans - FFTW 3.2.1</title>
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5 <meta name="description" content="FFTW 3.2.1">
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8 <link rel="up" href="Other-Important-Topics.html#Other-Important-Topics" title="Other Important Topics">
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9 <link rel="prev" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" title="Multi-dimensional Array Format">
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10 <link rel="next" href="Caveats-in-Using-Wisdom.html#Caveats-in-Using-Wisdom" title="Caveats in Using Wisdom">
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12 <!--
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13 This manual is for FFTW
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14 (version 3.2.1, 5 February 2009).
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15
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16 Copyright (C) 2003 Matteo Frigo.
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17
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18 Copyright (C) 2003 Massachusetts Institute of Technology.
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19
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20 Permission is granted to make and distribute verbatim copies of
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21 this manual provided the copyright notice and this permission
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22 notice are preserved on all copies.
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24 Permission is granted to copy and distribute modified versions of
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25 this manual under the conditions for verbatim copying, provided
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26 that the entire resulting derived work is distributed under the
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27 terms of a permission notice identical to this one.
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28
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29 Permission is granted to copy and distribute translations of this
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30 manual into another language, under the above conditions for
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31 modified versions, except that this permission notice may be
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32 stated in a translation approved by the Free Software Foundation.
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33 -->
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45 --></style>
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46 </head>
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47 <body>
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48 <div class="node">
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49 <p>
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50 <a name="Words-of-Wisdom-Saving-Plans"></a>
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51 <a name="Words-of-Wisdom_002dSaving-Plans"></a>
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52 Next: <a rel="next" accesskey="n" href="Caveats-in-Using-Wisdom.html#Caveats-in-Using-Wisdom">Caveats in Using Wisdom</a>,
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53 Previous: <a rel="previous" accesskey="p" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>,
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54 Up: <a rel="up" accesskey="u" href="Other-Important-Topics.html#Other-Important-Topics">Other Important Topics</a>
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55 <hr>
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56 </div>
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57
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58 <h3 class="section">3.3 Words of Wisdom—Saving Plans</h3>
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59
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60 <p><a name="index-wisdom-121"></a><a name="index-saving-plans-to-disk-122"></a>
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61 FFTW implements a method for saving plans to disk and restoring them.
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62 In fact, what FFTW does is more general than just saving and loading
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63 plans. The mechanism is called <dfn>wisdom</dfn>. Here, we describe
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64 this feature at a high level. See <a href="FFTW-Reference.html#FFTW-Reference">FFTW Reference</a>, for a less casual
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65 but more complete discussion of how to use wisdom in FFTW.
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66
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67 <p>Plans created with the <code>FFTW_MEASURE</code>, <code>FFTW_PATIENT</code>, or
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68 <code>FFTW_EXHAUSTIVE</code> options produce near-optimal FFT performance,
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69 but may require a long time to compute because FFTW must measure the
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70 runtime of many possible plans and select the best one. This setup is
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71 designed for the situations where so many transforms of the same size
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72 must be computed that the start-up time is irrelevant. For short
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73 initialization times, but slower transforms, we have provided
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74 <code>FFTW_ESTIMATE</code>. The <code>wisdom</code> mechanism is a way to get the
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75 best of both worlds: you compute a good plan once, save it to
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76 disk, and later reload it as many times as necessary. The wisdom
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77 mechanism can actually save and reload many plans at once, not just
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78 one.
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79 <a name="index-FFTW_005fMEASURE-123"></a><a name="index-FFTW_005fPATIENT-124"></a><a name="index-FFTW_005fEXHAUSTIVE-125"></a><a name="index-FFTW_005fESTIMATE-126"></a>
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80 Whenever you create a plan, the FFTW planner accumulates wisdom, which
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81 is information sufficient to reconstruct the plan. After planning,
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82 you can save this information to disk by means of the function:
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83 <pre class="example"> void fftw_export_wisdom_to_file(FILE *output_file);
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84 </pre>
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85 <p><a name="index-fftw_005fexport_005fwisdom_005fto_005ffile-127"></a>
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86 The next time you run the program, you can restore the wisdom with
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87 <code>fftw_import_wisdom_from_file</code> (which returns non-zero on success),
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88 and then recreate the plan using the same flags as before.
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89 <pre class="example"> int fftw_import_wisdom_from_file(FILE *input_file);
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90 </pre>
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91 <p><a name="index-fftw_005fimport_005fwisdom_005ffrom_005ffile-128"></a>
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92 Wisdom is automatically used for any size to which it is applicable, as
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93 long as the planner flags are not more “patient” than those with which
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94 the wisdom was created. For example, wisdom created with
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95 <code>FFTW_MEASURE</code> can be used if you later plan with
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96 <code>FFTW_ESTIMATE</code> or <code>FFTW_MEASURE</code>, but not with
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97 <code>FFTW_PATIENT</code>.
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98
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99 <p>The <code>wisdom</code> is cumulative, and is stored in a global, private
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100 data structure managed internally by FFTW. The storage space required
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101 is minimal, proportional to the logarithm of the sizes the wisdom was
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102 generated from. If memory usage is a concern, however, the wisdom can
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103 be forgotten and its associated memory freed by calling:
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104 <pre class="example"> void fftw_forget_wisdom(void);
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105 </pre>
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106 <p><a name="index-fftw_005fforget_005fwisdom-129"></a>
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107 Wisdom can be exported to a file, a string, or any other medium.
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108 For details, see <a href="Wisdom.html#Wisdom">Wisdom</a>.
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109
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110 </body></html>
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111
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