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Add null config files
author Chris Cannam
date Mon, 02 Mar 2020 14:03:47 +0000
parents d0c2a83c1364
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Chris@82 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Chris@82 3 <!-- This manual is for FFTW
Chris@82 4 (version 3.3.8, 24 May 2018).
Chris@82 5
Chris@82 6 Copyright (C) 2003 Matteo Frigo.
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Chris@82 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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Chris@82 24 <head>
Chris@82 25 <title>FFTW 3.3.8: One-dimensional distributions</title>
Chris@82 26
Chris@82 27 <meta name="description" content="FFTW 3.3.8: One-dimensional distributions">
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Chris@82 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
Chris@82 36 <link href="MPI-Data-Distribution.html#MPI-Data-Distribution" rel="up" title="MPI Data Distribution">
Chris@82 37 <link href="Multi_002ddimensional-MPI-DFTs-of-Real-Data.html#Multi_002ddimensional-MPI-DFTs-of-Real-Data" rel="next" title="Multi-dimensional MPI DFTs of Real Data">
Chris@82 38 <link href="Transposed-distributions.html#Transposed-distributions" rel="prev" title="Transposed distributions">
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Chris@82 66
Chris@82 67
Chris@82 68 </head>
Chris@82 69
Chris@82 70 <body lang="en">
Chris@82 71 <a name="One_002ddimensional-distributions"></a>
Chris@82 72 <div class="header">
Chris@82 73 <p>
Chris@82 74 Previous: <a href="Transposed-distributions.html#Transposed-distributions" accesskey="p" rel="prev">Transposed distributions</a>, Up: <a href="MPI-Data-Distribution.html#MPI-Data-Distribution" accesskey="u" rel="up">MPI Data Distribution</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@82 75 </div>
Chris@82 76 <hr>
Chris@82 77 <a name="One_002ddimensional-distributions-1"></a>
Chris@82 78 <h4 class="subsection">6.4.4 One-dimensional distributions</h4>
Chris@82 79
Chris@82 80 <p>For one-dimensional distributed DFTs using FFTW, matters are slightly
Chris@82 81 more complicated because the data distribution is more closely tied to
Chris@82 82 how the algorithm works. In particular, you can no longer pass an
Chris@82 83 arbitrary block size and must accept FFTW&rsquo;s default; also, the block
Chris@82 84 sizes may be different for input and output. Also, the data
Chris@82 85 distribution depends on the flags and transform direction, in order
Chris@82 86 for forward and backward transforms to work correctly.
Chris@82 87 </p>
Chris@82 88 <div class="example">
Chris@82 89 <pre class="example">ptrdiff_t fftw_mpi_local_size_1d(ptrdiff_t n0, MPI_Comm comm,
Chris@82 90 int sign, unsigned flags,
Chris@82 91 ptrdiff_t *local_ni, ptrdiff_t *local_i_start,
Chris@82 92 ptrdiff_t *local_no, ptrdiff_t *local_o_start);
Chris@82 93 </pre></div>
Chris@82 94 <a name="index-fftw_005fmpi_005flocal_005fsize_005f1d"></a>
Chris@82 95
Chris@82 96 <p>This function computes the data distribution for a 1d transform of
Chris@82 97 size <code>n0</code> with the given transform <code>sign</code> and <code>flags</code>.
Chris@82 98 Both input and output data use block distributions. The input on the
Chris@82 99 current process will consist of <code>local_ni</code> numbers starting at
Chris@82 100 index <code>local_i_start</code>; e.g. if only a single process is used,
Chris@82 101 then <code>local_ni</code> will be <code>n0</code> and <code>local_i_start</code> will
Chris@82 102 be <code>0</code>. Similarly for the output, with <code>local_no</code> numbers
Chris@82 103 starting at index <code>local_o_start</code>. The return value of
Chris@82 104 <code>fftw_mpi_local_size_1d</code> will be the total number of elements to
Chris@82 105 allocate on the current process (which might be slightly larger than
Chris@82 106 the local size due to intermediate steps in the algorithm).
Chris@82 107 </p>
Chris@82 108 <p>As mentioned above (see <a href="Load-balancing.html#Load-balancing">Load balancing</a>), the data will be divided
Chris@82 109 equally among the processes if <code>n0</code> is divisible by the
Chris@82 110 <em>square</em> of the number of processes. In this case,
Chris@82 111 <code>local_ni</code> will equal <code>local_no</code>. Otherwise, they may be
Chris@82 112 different.
Chris@82 113 </p>
Chris@82 114 <p>For some applications, such as convolutions, the order of the output
Chris@82 115 data is irrelevant. In this case, performance can be improved by
Chris@82 116 specifying that the output data be stored in an FFTW-defined
Chris@82 117 &ldquo;scrambled&rdquo; format. (In particular, this is the analogue of
Chris@82 118 transposed output in the multidimensional case: scrambled output saves
Chris@82 119 a communications step.) If you pass <code>FFTW_MPI_SCRAMBLED_OUT</code> in
Chris@82 120 the flags, then the output is stored in this (undocumented) scrambled
Chris@82 121 order. Conversely, to perform the inverse transform of data in
Chris@82 122 scrambled order, pass the <code>FFTW_MPI_SCRAMBLED_IN</code> flag.
Chris@82 123 <a name="index-FFTW_005fMPI_005fSCRAMBLED_005fOUT"></a>
Chris@82 124 <a name="index-FFTW_005fMPI_005fSCRAMBLED_005fIN"></a>
Chris@82 125 </p>
Chris@82 126
Chris@82 127 <p>In MPI FFTW, only composite sizes <code>n0</code> can be parallelized; we
Chris@82 128 have not yet implemented a parallel algorithm for large prime sizes.
Chris@82 129 </p>
Chris@82 130 <hr>
Chris@82 131 <div class="header">
Chris@82 132 <p>
Chris@82 133 Previous: <a href="Transposed-distributions.html#Transposed-distributions" accesskey="p" rel="prev">Transposed distributions</a>, Up: <a href="MPI-Data-Distribution.html#MPI-Data-Distribution" accesskey="u" rel="up">MPI Data Distribution</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@82 134 </div>
Chris@82 135
Chris@82 136
Chris@82 137
Chris@82 138 </body>
Chris@82 139 </html>