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author Chris Cannam <cannam@all-day-breakfast.com>
date Wed, 20 Mar 2013 15:35:50 +0000
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3 <title>Basic distributed-transpose interface - FFTW 3.3.3</title>
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49 <a name="Basic-distributed-transpose-interface"></a>
50 <a name="Basic-distributed_002dtranspose-interface"></a>
51 <p>
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-397"></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-398"></a>
75
76 <p>The <code>flags</code> are the usual FFTW planner flags, but support
77 two additional flags: <code>FFTW_MPI_TRANSPOSED_OUT</code> and/or
78 <code>FFTW_MPI_TRANSPOSED_IN</code>. What these flags indicate, for
79 transpose plans, is that the output and/or input, respectively, are
80 <em>locally</em> transposed. That is, on each process input data is
81 normally stored as a <code>local_n0</code> by <code>n1</code> array in row-major
82 order, but for an <code>FFTW_MPI_TRANSPOSED_IN</code> plan the input data is
83 stored as <code>n1</code> by <code>local_n0</code> in row-major order. Similarly,
84 <code>FFTW_MPI_TRANSPOSED_OUT</code> means that the output is <code>n0</code> by
85 <code>local_n1</code> instead of <code>local_n1</code> by <code>n0</code>.
86 <a name="index-FFTW_005fMPI_005fTRANSPOSED_005fOUT-399"></a><a name="index-FFTW_005fMPI_005fTRANSPOSED_005fIN-400"></a>
87
88 <p>To determine the local size of the array on each process before and
89 after the transpose, as well as the amount of storage that must be
90 allocated, one should call <code>fftw_mpi_local_size_2d_transposed</code>,
91 just as for a 2d DFT as described in the previous section:
92 <a name="index-data-distribution-401"></a>
93 <pre class="example"> ptrdiff_t fftw_mpi_local_size_2d_transposed
94 (ptrdiff_t n0, ptrdiff_t n1, MPI_Comm comm,
95 ptrdiff_t *local_n0, ptrdiff_t *local_0_start,
96 ptrdiff_t *local_n1, ptrdiff_t *local_1_start);
97 </pre>
98 <p><a name="index-fftw_005fmpi_005flocal_005fsize_005f2d_005ftransposed-402"></a>
99 Again, the return value is the local storage to allocate, which in
100 this case is the number of <em>real</em> (<code>double</code>) values rather
101 than complex numbers as in the previous examples.
102
103 </body></html>
104