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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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3 <!-- This manual is for FFTW
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25 <title>FFTW 3.3.5: One-dimensional distributions</title>
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72 <a name="One_002ddimensional-distributions"></a>
73 <div class="header">
74 <p>
75 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>
76 </div>
77 <hr>
78 <a name="One_002ddimensional-distributions-1"></a>
79 <h4 class="subsection">6.4.4 One-dimensional distributions</h4>
80
81 <p>For one-dimensional distributed DFTs using FFTW, matters are slightly
82 more complicated because the data distribution is more closely tied to
83 how the algorithm works. In particular, you can no longer pass an
84 arbitrary block size and must accept FFTW&rsquo;s default; also, the block
85 sizes may be different for input and output. Also, the data
86 distribution depends on the flags and transform direction, in order
87 for forward and backward transforms to work correctly.
88 </p>
89 <div class="example">
90 <pre class="example">ptrdiff_t fftw_mpi_local_size_1d(ptrdiff_t n0, MPI_Comm comm,
91 int sign, unsigned flags,
92 ptrdiff_t *local_ni, ptrdiff_t *local_i_start,
93 ptrdiff_t *local_no, ptrdiff_t *local_o_start);
94 </pre></div>
95 <a name="index-fftw_005fmpi_005flocal_005fsize_005f1d"></a>
96
97 <p>This function computes the data distribution for a 1d transform of
98 size <code>n0</code> with the given transform <code>sign</code> and <code>flags</code>.
99 Both input and output data use block distributions. The input on the
100 current process will consist of <code>local_ni</code> numbers starting at
101 index <code>local_i_start</code>; e.g. if only a single process is used,
102 then <code>local_ni</code> will be <code>n0</code> and <code>local_i_start</code> will
103 be <code>0</code>. Similarly for the output, with <code>local_no</code> numbers
104 starting at index <code>local_o_start</code>. The return value of
105 <code>fftw_mpi_local_size_1d</code> will be the total number of elements to
106 allocate on the current process (which might be slightly larger than
107 the local size due to intermediate steps in the algorithm).
108 </p>
109 <p>As mentioned above (see <a href="Load-balancing.html#Load-balancing">Load balancing</a>), the data will be divided
110 equally among the processes if <code>n0</code> is divisible by the
111 <em>square</em> of the number of processes. In this case,
112 <code>local_ni</code> will equal <code>local_no</code>. Otherwise, they may be
113 different.
114 </p>
115 <p>For some applications, such as convolutions, the order of the output
116 data is irrelevant. In this case, performance can be improved by
117 specifying that the output data be stored in an FFTW-defined
118 &ldquo;scrambled&rdquo; format. (In particular, this is the analogue of
119 transposed output in the multidimensional case: scrambled output saves
120 a communications step.) If you pass <code>FFTW_MPI_SCRAMBLED_OUT</code> in
121 the flags, then the output is stored in this (undocumented) scrambled
122 order. Conversely, to perform the inverse transform of data in
123 scrambled order, pass the <code>FFTW_MPI_SCRAMBLED_IN</code> flag.
124 <a name="index-FFTW_005fMPI_005fSCRAMBLED_005fOUT"></a>
125 <a name="index-FFTW_005fMPI_005fSCRAMBLED_005fIN"></a>
126 </p>
127
128 <p>In MPI FFTW, only composite sizes <code>n0</code> can be parallelized; we
129 have not yet implemented a parallel algorithm for large prime sizes.
130 </p>
131 <hr>
132 <div class="header">
133 <p>
134 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>
135 </div>
136
137
138
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