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author Chris Cannam
date Wed, 07 Sep 2016 10:40:32 +0100
parents 26056e866c29
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Chris@19 1 <html lang="en">
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Chris@19 3 <title>Basic distributed-transpose interface - FFTW 3.3.4</title>
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Chris@19 48 <div class="node">
Chris@19 49 <a name="Basic-distributed-transpose-interface"></a>
Chris@19 50 <a name="Basic-distributed_002dtranspose-interface"></a>
Chris@19 51 <p>
Chris@19 52 Next:&nbsp;<a rel="next" accesskey="n" href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface">Advanced distributed-transpose interface</a>,
Chris@19 53 Previous:&nbsp;<a rel="previous" accesskey="p" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes">FFTW MPI Transposes</a>,
Chris@19 54 Up:&nbsp;<a rel="up" accesskey="u" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes">FFTW MPI Transposes</a>
Chris@19 55 <hr>
Chris@19 56 </div>
Chris@19 57
Chris@19 58 <h4 class="subsection">6.7.1 Basic distributed-transpose interface</h4>
Chris@19 59
Chris@19 60 <p>In particular, suppose that we have an <code>n0</code> by <code>n1</code> array in
Chris@19 61 row-major order, block-distributed across the <code>n0</code> dimension. To
Chris@19 62 transpose this into an <code>n1</code> by <code>n0</code> array block-distributed
Chris@19 63 across the <code>n1</code> dimension, we would create a plan by calling the
Chris@19 64 following function:
Chris@19 65
Chris@19 66 <pre class="example"> fftw_plan fftw_mpi_plan_transpose(ptrdiff_t n0, ptrdiff_t n1,
Chris@19 67 double *in, double *out,
Chris@19 68 MPI_Comm comm, unsigned flags);
Chris@19 69 </pre>
Chris@19 70 <p><a name="index-fftw_005fmpi_005fplan_005ftranspose-400"></a>
Chris@19 71 The input and output arrays (<code>in</code> and <code>out</code>) can be the
Chris@19 72 same. The transpose is actually executed by calling
Chris@19 73 <code>fftw_execute</code> on the plan, as usual.
Chris@19 74 <a name="index-fftw_005fexecute-401"></a>
Chris@19 75
Chris@19 76 <p>The <code>flags</code> are the usual FFTW planner flags, but support
Chris@19 77 two additional flags: <code>FFTW_MPI_TRANSPOSED_OUT</code> and/or
Chris@19 78 <code>FFTW_MPI_TRANSPOSED_IN</code>. What these flags indicate, for
Chris@19 79 transpose plans, is that the output and/or input, respectively, are
Chris@19 80 <em>locally</em> transposed. That is, on each process input data is
Chris@19 81 normally stored as a <code>local_n0</code> by <code>n1</code> array in row-major
Chris@19 82 order, but for an <code>FFTW_MPI_TRANSPOSED_IN</code> plan the input data is
Chris@19 83 stored as <code>n1</code> by <code>local_n0</code> in row-major order. Similarly,
Chris@19 84 <code>FFTW_MPI_TRANSPOSED_OUT</code> means that the output is <code>n0</code> by
Chris@19 85 <code>local_n1</code> instead of <code>local_n1</code> by <code>n0</code>.
Chris@19 86 <a name="index-FFTW_005fMPI_005fTRANSPOSED_005fOUT-402"></a><a name="index-FFTW_005fMPI_005fTRANSPOSED_005fIN-403"></a>
Chris@19 87
Chris@19 88 <p>To determine the local size of the array on each process before and
Chris@19 89 after the transpose, as well as the amount of storage that must be
Chris@19 90 allocated, one should call <code>fftw_mpi_local_size_2d_transposed</code>,
Chris@19 91 just as for a 2d DFT as described in the previous section:
Chris@19 92 <a name="index-data-distribution-404"></a>
Chris@19 93 <pre class="example"> ptrdiff_t fftw_mpi_local_size_2d_transposed
Chris@19 94 (ptrdiff_t n0, ptrdiff_t n1, MPI_Comm comm,
Chris@19 95 ptrdiff_t *local_n0, ptrdiff_t *local_0_start,
Chris@19 96 ptrdiff_t *local_n1, ptrdiff_t *local_1_start);
Chris@19 97 </pre>
Chris@19 98 <p><a name="index-fftw_005fmpi_005flocal_005fsize_005f2d_005ftransposed-405"></a>
Chris@19 99 Again, the return value is the local storage to allocate, which in
Chris@19 100 this case is the number of <em>real</em> (<code>double</code>) values rather
Chris@19 101 than complex numbers as in the previous examples.
Chris@19 102
Chris@19 103 </body></html>
Chris@19 104