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2 <head>
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3 <title>One-dimensional distributions - FFTW 3.2alpha3</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.2alpha3">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="MPI-data-distribution.html#MPI-data-distribution" title="MPI data distribution">
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9 <link rel="prev" href="Transposed-distributions.html#Transposed-distributions" title="Transposed distributions">
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10 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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11 <!--
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12 This manual is for FFTW
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13 (version 3.2alpha3, 14 August 2007).
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14
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15 Copyright (C) 2003 Matteo Frigo.
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16
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17 Copyright (C) 2003 Massachusetts Institute of Technology.
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18
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19 Permission is granted to make and distribute verbatim copies of
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20 this manual provided the copyright notice and this permission
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21 notice are preserved on all copies.
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22
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23 Permission is granted to copy and distribute modified versions of
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24 this manual under the conditions for verbatim copying, provided
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25 that the entire resulting derived work is distributed under the
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26 terms of a permission notice identical to this one.
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27
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28 Permission is granted to copy and distribute translations of this
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29 manual into another language, under the above conditions for
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30 modified versions, except that this permission notice may be
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31 stated in a translation approved by the Free Software Foundation.
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32 -->
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34 <style type="text/css"><!--
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44 --></style>
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45 </head>
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46 <body>
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47 <div class="node">
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48 <p>
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49 <a name="One-dimensional-distributions"></a>
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50 <a name="One_002ddimensional-distributions"></a>
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51 Previous: <a rel="previous" accesskey="p" href="Transposed-distributions.html#Transposed-distributions">Transposed distributions</a>,
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52 Up: <a rel="up" accesskey="u" href="MPI-data-distribution.html#MPI-data-distribution">MPI data distribution</a>
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53 <hr>
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54 </div>
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55
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56 <h4 class="subsection">6.4.4 One-dimensional distributions</h4>
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57
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58 <p>For one-dimensional distributed DFTs using FFTW, matters are slightly
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59 more complicated because the data distribution is more closely tied to
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60 how the algorithm works. In particular, you can no longer pass an
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61 arbitrary block size, and must accept FFTW's default, and the block
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62 sizes may be different for input and output. Also, the data
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63 distribution depends on the flags and transform direction, in order
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64 for forward and backward transforms to work correctly.
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65
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66 <pre class="example"> ptrdiff_t fftw_mpi_local_size_1d(ptrdiff_t n0, MPI_Comm comm,
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67 int sign, unsigned flags,
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68 ptrdiff_t *local_ni, ptrdiff_t *local_i_start,
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69 ptrdiff_t *local_no, ptrdiff_t *local_o_start);
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70 </pre>
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71 <p><a name="index-fftw_005fmpi_005flocal_005fsize_005f1d-363"></a>
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72 This function computes the data distribution for a 1d transform of
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73 size <code>n0</code> with the given transform <code>sign</code> and <code>flags</code>.
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74 Both input and output data use block distributions. The input on the
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75 current process will consist of <code>local_ni</code> numbers starting at
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76 index <code>local_i_start</code>; e.g. if only a single process is used,
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77 then <code>local_ni</code> will be <code>n0</code> and <code>local_i_start</code> will
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78 be <code>0</code>. Similarly for the output, with <code>local_no</code> numbers
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79 starting at index <code>local_o_start</code>. The return value of
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80 <code>fftw_mpi_local_size_1d</code> will be the total number of elements to
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81 allocate on the current process (which might be slightly larger than
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82 the local size due to intermediate steps in the algorithm).
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83
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84 <p>As mentioned above (see <a href="Load-balancing.html#Load-balancing">Load balancing</a>), the data will be divided
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85 equally among the processes if <code>n0</code> is divisible by the
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86 <em>square</em> of the number of processes. In this case,
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87 <code>local_ni</code> will equal <code>local_no</code>. Otherwise, they may be
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88 different.
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89
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90 <p>For some applications, such as convolutions, the order of the output
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91 data is irrelevant. In this case, performance can be improved by
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92 specifying that the output data be stored in an FFTW-defined
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93 “scrambled” format. (In particular, this is the analogue of
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94 transposed output in the multidimensional case: scrambled output saves
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95 a communications step.) If you pass <code>FFTW_MPI_SCRAMBLED_OUT</code> in
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96 the flags, then the output is stored in this (undocumented) scrambled
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97 order. Conversely, to perform the inverse transform of data in
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98 scrambled order, pass the <code>FFTW_MPI_SCRAMBLED_IN</code> flag.
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99 <a name="index-FFTW_005fMPI_005fSCRAMBLED_005fOUT-364"></a><a name="index-FFTW_005fMPI_005fSCRAMBLED_005fIN-365"></a>
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100 In MPI FFTW, only composite sizes <code>n0</code> can be parallelized; we
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101 have not yet implemented a parallel algorithm for large prime sizes.
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102
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103 <!-- -->
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104 </body></html>
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105
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