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130 lines
6.3 KiB
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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<title>Basic distributed-transpose interface (FFTW 3.3.10)</title>
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="FFTW-MPI-Transposes.html" rel="up" title="FFTW MPI Transposes">
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<span id="Basic-distributed_002dtranspose-interface"></span><div class="header">
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<p>
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Next: <a href="Advanced-distributed_002dtranspose-interface.html" accesskey="n" rel="next">Advanced distributed-transpose interface</a>, Previous: <a href="FFTW-MPI-Transposes.html" accesskey="p" rel="prev">FFTW MPI Transposes</a>, Up: <a href="FFTW-MPI-Transposes.html" 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" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Basic-distributed_002dtranspose-interface-1"></span><h4 class="subsection">6.7.1 Basic distributed-transpose interface</h4>
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<p>In particular, suppose that we have an <code>n0</code> by <code>n1</code> array in
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row-major order, block-distributed across the <code>n0</code> dimension. To
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transpose this into an <code>n1</code> by <code>n0</code> array block-distributed
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across the <code>n1</code> dimension, we would create a plan by calling the
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following function:
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</p>
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<div class="example">
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<pre class="example">fftw_plan fftw_mpi_plan_transpose(ptrdiff_t n0, ptrdiff_t n1,
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double *in, double *out,
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MPI_Comm comm, unsigned flags);
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</pre></div>
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<span id="index-fftw_005fmpi_005fplan_005ftranspose"></span>
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<p>The input and output arrays (<code>in</code> and <code>out</code>) can be the
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same. The transpose is actually executed by calling
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<code>fftw_execute</code> on the plan, as usual.
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<span id="index-fftw_005fexecute-3"></span>
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</p>
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<p>The <code>flags</code> are the usual FFTW planner flags, but support
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two additional flags: <code>FFTW_MPI_TRANSPOSED_OUT</code> and/or
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<code>FFTW_MPI_TRANSPOSED_IN</code>. What these flags indicate, for
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transpose plans, is that the output and/or input, respectively, are
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<em>locally</em> transposed. That is, on each process input data is
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normally stored as a <code>local_n0</code> by <code>n1</code> array in row-major
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order, but for an <code>FFTW_MPI_TRANSPOSED_IN</code> plan the input data is
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stored as <code>n1</code> by <code>local_n0</code> in row-major order. Similarly,
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<code>FFTW_MPI_TRANSPOSED_OUT</code> means that the output is <code>n0</code> by
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<code>local_n1</code> instead of <code>local_n1</code> by <code>n0</code>.
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<span id="index-FFTW_005fMPI_005fTRANSPOSED_005fOUT-1"></span>
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<span id="index-FFTW_005fMPI_005fTRANSPOSED_005fIN-1"></span>
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</p>
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<p>To determine the local size of the array on each process before and
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after the transpose, as well as the amount of storage that must be
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allocated, one should call <code>fftw_mpi_local_size_2d_transposed</code>,
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just as for a 2d DFT as described in the previous section:
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<span id="index-data-distribution-4"></span>
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</p>
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<div class="example">
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<pre class="example">ptrdiff_t fftw_mpi_local_size_2d_transposed
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(ptrdiff_t n0, ptrdiff_t n1, MPI_Comm comm,
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ptrdiff_t *local_n0, ptrdiff_t *local_0_start,
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ptrdiff_t *local_n1, ptrdiff_t *local_1_start);
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</pre></div>
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<span id="index-fftw_005fmpi_005flocal_005fsize_005f2d_005ftransposed"></span>
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<p>Again, the return value is the local storage to allocate, which in
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this case is the number of <em>real</em> (<code>double</code>) values rather
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than complex numbers as in the previous examples.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Advanced-distributed_002dtranspose-interface.html" accesskey="n" rel="next">Advanced distributed-transpose interface</a>, Previous: <a href="FFTW-MPI-Transposes.html" accesskey="p" rel="prev">FFTW MPI Transposes</a>, Up: <a href="FFTW-MPI-Transposes.html" 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" title="Index" rel="index">Index</a>]</p>
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</div>
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