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author Geogaddi\David <d.m.ronan@qmul.ac.uk>
date Thu, 09 Jul 2015 01:12:16 +0100
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1 <html lang="en">
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3 <title>Basic distributed-transpose interface - FFTW 3.2alpha3</title>
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50 <a name="Basic-distributed-transpose-interface"></a>
51 <a name="Basic-distributed_002dtranspose-interface"></a>
52 Next:&nbsp;<a rel="next" accesskey="n" href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface">Advanced distributed-transpose interface</a>,
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55 <hr>
56 </div>
57
58 <h4 class="subsection">6.7.1 Basic distributed-transpose interface</h4>
59
60 <p>In particular, suppose that we have an <code>n0</code> by <code>n1</code> array in
61 row-major order, block-distributed across the <code>n0</code> dimension. To
62 transpose this into an <code>n1</code> by <code>n0</code> array block-distributed
63 across the <code>n1</code> dimension, we would create a plan by calling the
64 following function:
65
66 <pre class="example"> fftw_plan fftw_mpi_plan_transpose(ptrdiff_t n0, ptrdiff_t n1,
67 double *in, double *out,
68 MPI_Comm comm, unsigned flags);
69 </pre>
70 <p><a name="index-fftw_005fmpi_005fplan_005ftranspose-367"></a>
71 The input and output arrays (<code>in</code> and <code>out</code>) can be the
72 same. The transpose is actually executed by calling
73 <code>fftw_execute</code> on the plan, as usual.
74 <a name="index-fftw_005fexecute-368"></a>
75 The <code>flags</code> are the usual FFTW planner flags, but support
76 two additional flags: <code>FFTW_MPI_TRANSPOSED_OUT</code> and/or
77 <code>FFTW_MPI_TRANSPOSED_IN</code>. What these flags indicate, for
78 transpose plans, is that the output and/or input, respectively, are
79 <em>locally</em> transposed. That is, on each process input data is
80 normally stored as a <code>local_n0</code> by <code>n1</code> array in row-major
81 order, but for an <code>FFTW_MPI_TRANSPOSED_IN</code> plan the input data is
82 stored as <code>n1</code> by <code>local_n0</code> in row-major order. Similarly,
83 <code>FFTW_MPI_TRANSPOSED_OUT</code> means that the output is <code>n0</code> by
84 <code>local_n1</code> instead of <code>local_n1</code> by <code>n0</code>.
85 <a name="index-FFTW_005fMPI_005fTRANSPOSED_005fOUT-369"></a><a name="index-FFTW_005fMPI_005fTRANSPOSED_005fIN-370"></a>
86 To determine the local size of the array on each process before and
87 after the transpose, as well as the amount of storage that must be
88 allocated, one should call <code>fftw_mpi_local_size_2d_transposed</code>,
89 just as for a 2d DFT as described in the previous section:
90 <a name="index-data-distribution-371"></a>
91 <pre class="example"> ptrdiff_t fftw_mpi_local_size_2d_transposed
92 (ptrdiff_t n0, ptrdiff_t n1, MPI_Comm comm,
93 ptrdiff_t *local_n0, ptrdiff_t *local_0_start,
94 ptrdiff_t *local_n1, ptrdiff_t *local_1_start);
95 </pre>
96 <p><a name="index-fftw_005fmpi_005flocal_005fsize_005f2d_005ftransposed-372"></a>
97
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99