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Chris@42: <title>FFTW 3.3.5: Basic distributed-transpose interface</title>
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Chris@42: <a name="Basic-distributed_002dtranspose-interface"></a>
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Chris@42: <p>
Chris@42: Next: <a href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface" accesskey="n" rel="next">Advanced distributed-transpose interface</a>, Previous: <a href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" accesskey="p" rel="prev">FFTW MPI Transposes</a>, Up: <a href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" accesskey="u" rel="up">FFTW MPI Transposes</a> [<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@42: </div>
Chris@42: <hr>
Chris@42: <a name="Basic-distributed_002dtranspose-interface-1"></a>
Chris@42: <h4 class="subsection">6.7.1 Basic distributed-transpose interface</h4>
Chris@42:
Chris@42: <p>In particular, suppose that we have an <code>n0</code> by <code>n1</code> array in
Chris@42: row-major order, block-distributed across the <code>n0</code> dimension. To
Chris@42: transpose this into an <code>n1</code> by <code>n0</code> array block-distributed
Chris@42: across the <code>n1</code> dimension, we would create a plan by calling the
Chris@42: following function:
Chris@42: </p>
Chris@42: <div class="example">
Chris@42: <pre class="example">fftw_plan fftw_mpi_plan_transpose(ptrdiff_t n0, ptrdiff_t n1,
Chris@42: double *in, double *out,
Chris@42: MPI_Comm comm, unsigned flags);
Chris@42: </pre></div>
Chris@42: <a name="index-fftw_005fmpi_005fplan_005ftranspose"></a>
Chris@42:
Chris@42: <p>The input and output arrays (<code>in</code> and <code>out</code>) can be the
Chris@42: same. The transpose is actually executed by calling
Chris@42: <code>fftw_execute</code> on the plan, as usual.
Chris@42: <a name="index-fftw_005fexecute-3"></a>
Chris@42: </p>
Chris@42:
Chris@42: <p>The <code>flags</code> are the usual FFTW planner flags, but support
Chris@42: two additional flags: <code>FFTW_MPI_TRANSPOSED_OUT</code> and/or
Chris@42: <code>FFTW_MPI_TRANSPOSED_IN</code>. What these flags indicate, for
Chris@42: transpose plans, is that the output and/or input, respectively, are
Chris@42: <em>locally</em> transposed. That is, on each process input data is
Chris@42: normally stored as a <code>local_n0</code> by <code>n1</code> array in row-major
Chris@42: order, but for an <code>FFTW_MPI_TRANSPOSED_IN</code> plan the input data is
Chris@42: stored as <code>n1</code> by <code>local_n0</code> in row-major order. Similarly,
Chris@42: <code>FFTW_MPI_TRANSPOSED_OUT</code> means that the output is <code>n0</code> by
Chris@42: <code>local_n1</code> instead of <code>local_n1</code> by <code>n0</code>.
Chris@42: <a name="index-FFTW_005fMPI_005fTRANSPOSED_005fOUT-1"></a>
Chris@42: <a name="index-FFTW_005fMPI_005fTRANSPOSED_005fIN-1"></a>
Chris@42: </p>
Chris@42:
Chris@42: <p>To determine the local size of the array on each process before and
Chris@42: after the transpose, as well as the amount of storage that must be
Chris@42: allocated, one should call <code>fftw_mpi_local_size_2d_transposed</code>,
Chris@42: just as for a 2d DFT as described in the previous section:
Chris@42: <a name="index-data-distribution-4"></a>
Chris@42: </p>
Chris@42: <div class="example">
Chris@42: <pre class="example">ptrdiff_t fftw_mpi_local_size_2d_transposed
Chris@42: (ptrdiff_t n0, ptrdiff_t n1, MPI_Comm comm,
Chris@42: ptrdiff_t *local_n0, ptrdiff_t *local_0_start,
Chris@42: ptrdiff_t *local_n1, ptrdiff_t *local_1_start);
Chris@42: </pre></div>
Chris@42: <a name="index-fftw_005fmpi_005flocal_005fsize_005f2d_005ftransposed"></a>
Chris@42:
Chris@42: <p>Again, the return value is the local storage to allocate, which in
Chris@42: this case is the number of <em>real</em> (<code>double</code>) values rather
Chris@42: than complex numbers as in the previous examples.
Chris@42: </p>
Chris@42: <hr>
Chris@42: <div class="header">
Chris@42: <p>
Chris@42: Next: <a href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface" accesskey="n" rel="next">Advanced distributed-transpose interface</a>, Previous: <a href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" accesskey="p" rel="prev">FFTW MPI Transposes</a>, Up: <a href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" accesskey="u" rel="up">FFTW MPI Transposes</a> [<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>
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