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1 <html lang="en">
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2 <head>
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3 <title>MPI Data Distribution - FFTW 3.3.3</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.3.3">
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6 <meta name="generator" content="makeinfo 4.13">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" title="Distributed-memory FFTW with MPI">
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9 <link rel="prev" href="2d-MPI-example.html#g_t2d-MPI-example" title="2d MPI example">
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10 <link rel="next" href="Multi_002ddimensional-MPI-DFTs-of-Real-Data.html#Multi_002ddimensional-MPI-DFTs-of-Real-Data" title="Multi-dimensional MPI DFTs of Real Data">
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11 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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12 <!--
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13 This manual is for FFTW
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14 (version 3.3.3, 25 November 2012).
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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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23
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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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34 <meta http-equiv="Content-Style-Type" content="text/css">
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35 <style type="text/css"><!--
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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 <a name="MPI-Data-Distribution"></a>
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50 <p>
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51 Next: <a rel="next" accesskey="n" href="Multi_002ddimensional-MPI-DFTs-of-Real-Data.html#Multi_002ddimensional-MPI-DFTs-of-Real-Data">Multi-dimensional MPI DFTs of Real Data</a>,
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52 Previous: <a rel="previous" accesskey="p" href="2d-MPI-example.html#g_t2d-MPI-example">2d MPI example</a>,
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53 Up: <a rel="up" accesskey="u" href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI">Distributed-memory FFTW with MPI</a>
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54 <hr>
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55 </div>
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56
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57 <h3 class="section">6.4 MPI Data Distribution</h3>
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58
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59 <p><a name="index-data-distribution-368"></a>
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60 The most important concept to understand in using FFTW's MPI interface
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61 is the data distribution. With a serial or multithreaded FFT, all of
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62 the inputs and outputs are stored as a single contiguous chunk of
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63 memory. With a distributed-memory FFT, the inputs and outputs are
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64 broken into disjoint blocks, one per process.
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65
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66 <p>In particular, FFTW uses a <em>1d block distribution</em> of the data,
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67 distributed along the <em>first dimension</em>. For example, if you
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68 want to perform a 100 × 200 complex DFT, distributed over 4
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69 processes, each process will get a 25 × 200 slice of the data.
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70 That is, process 0 will get rows 0 through 24, process 1 will get rows
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71 25 through 49, process 2 will get rows 50 through 74, and process 3
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72 will get rows 75 through 99. If you take the same array but
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73 distribute it over 3 processes, then it is not evenly divisible so the
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74 different processes will have unequal chunks. FFTW's default choice
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75 in this case is to assign 34 rows to processes 0 and 1, and 32 rows to
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76 process 2.
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77 <a name="index-block-distribution-369"></a>
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78
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79 <p>FFTW provides several ‘<samp><span class="samp">fftw_mpi_local_size</span></samp>’ routines that you can
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80 call to find out what portion of an array is stored on the current
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81 process. In most cases, you should use the default block sizes picked
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82 by FFTW, but it is also possible to specify your own block size. For
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83 example, with a 100 × 200 array on three processes, you can
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84 tell FFTW to use a block size of 40, which would assign 40 rows to
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85 processes 0 and 1, and 20 rows to process 2. FFTW's default is to
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86 divide the data equally among the processes if possible, and as best
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87 it can otherwise. The rows are always assigned in “rank order,”
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88 i.e. process 0 gets the first block of rows, then process 1, and so
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89 on. (You can change this by using <code>MPI_Comm_split</code> to create a
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90 new communicator with re-ordered processes.) However, you should
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91 always call the ‘<samp><span class="samp">fftw_mpi_local_size</span></samp>’ routines, if possible,
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92 rather than trying to predict FFTW's distribution choices.
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93
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94 <p>In particular, it is critical that you allocate the storage size that
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95 is returned by ‘<samp><span class="samp">fftw_mpi_local_size</span></samp>’, which is <em>not</em>
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96 necessarily the size of the local slice of the array. The reason is
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97 that intermediate steps of FFTW's algorithms involve transposing the
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98 array and redistributing the data, so at these intermediate steps FFTW
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99 may require more local storage space (albeit always proportional to
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100 the total size divided by the number of processes). The
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101 ‘<samp><span class="samp">fftw_mpi_local_size</span></samp>’ functions know how much storage is required
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102 for these intermediate steps and tell you the correct amount to
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103 allocate.
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104
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105 <ul class="menu">
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106 <li><a accesskey="1" href="Basic-and-advanced-distribution-interfaces.html#Basic-and-advanced-distribution-interfaces">Basic and advanced distribution interfaces</a>
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107 <li><a accesskey="2" href="Load-balancing.html#Load-balancing">Load balancing</a>
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108 <li><a accesskey="3" href="Transposed-distributions.html#Transposed-distributions">Transposed distributions</a>
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109 <li><a accesskey="4" href="One_002ddimensional-distributions.html#One_002ddimensional-distributions">One-dimensional distributions</a>
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110 </ul>
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111
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112 </body></html>
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113
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