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<html>
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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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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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approved by the Free Software Foundation. -->
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<head>
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<title>MPI Data Distribution Functions (FFTW 3.3.10)</title>
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="FFTW-MPI-Reference.html" rel="up" title="FFTW MPI Reference">
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<link href="MPI-Plan-Creation.html" rel="next" title="MPI Plan Creation">
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</head>
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<body lang="en">
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<span id="MPI-Data-Distribution-Functions"></span><div class="header">
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<p>
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Next: <a href="MPI-Plan-Creation.html" accesskey="n" rel="next">MPI Plan Creation</a>, Previous: <a href="Using-MPI-Plans.html" accesskey="p" rel="prev">Using MPI Plans</a>, Up: <a href="FFTW-MPI-Reference.html" accesskey="u" rel="up">FFTW MPI Reference</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="MPI-Data-Distribution-Functions-1"></span><h4 class="subsection">6.12.4 MPI Data Distribution Functions</h4>
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<span id="index-data-distribution-5"></span>
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<p>As described above (see <a href="MPI-Data-Distribution.html">MPI Data Distribution</a>), in order to
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allocate your arrays, <em>before</em> creating a plan, you must first
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call one of the following routines to determine the required
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allocation size and the portion of the array locally stored on a given
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process. The <code>MPI_Comm</code> communicator passed here must be
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equivalent to the communicator used below for plan creation.
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</p>
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<p>The basic interface for multidimensional transforms consists of the
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functions:
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</p>
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<span id="index-fftw_005fmpi_005flocal_005fsize_005f2d-2"></span>
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<span id="index-fftw_005fmpi_005flocal_005fsize_005f3d"></span>
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<span id="index-fftw_005fmpi_005flocal_005fsize"></span>
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<span id="index-fftw_005fmpi_005flocal_005fsize_005f2d_005ftransposed-1"></span>
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<span id="index-fftw_005fmpi_005flocal_005fsize_005f3d_005ftransposed-1"></span>
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<span id="index-fftw_005fmpi_005flocal_005fsize_005ftransposed"></span>
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<div class="example">
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<pre class="example">ptrdiff_t fftw_mpi_local_size_2d(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 fftw_mpi_local_size_3d(ptrdiff_t n0, ptrdiff_t n1, ptrdiff_t n2,
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MPI_Comm comm,
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ptrdiff_t *local_n0, ptrdiff_t *local_0_start);
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ptrdiff_t fftw_mpi_local_size(int rnk, const ptrdiff_t *n, MPI_Comm comm,
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ptrdiff_t *local_n0, ptrdiff_t *local_0_start);
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ptrdiff_t fftw_mpi_local_size_2d_transposed(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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ptrdiff_t fftw_mpi_local_size_3d_transposed(ptrdiff_t n0, ptrdiff_t n1, ptrdiff_t n2,
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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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ptrdiff_t fftw_mpi_local_size_transposed(int rnk, const ptrdiff_t *n, 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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<p>These functions return the number of elements to allocate (complex
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numbers for DFT/r2c/c2r plans, real numbers for r2r plans), whereas
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the <code>local_n0</code> and <code>local_0_start</code> return the portion
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(<code>local_0_start</code> to <code>local_0_start + local_n0 - 1</code>) of the
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first dimension of an n<sub>0</sub> × n<sub>1</sub> × n<sub>2</sub> × … × n<sub>d-1</sub>
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array that is stored on the local
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process. See <a href="Basic-and-advanced-distribution-interfaces.html">Basic and advanced distribution interfaces</a>. For
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<code>FFTW_MPI_TRANSPOSED_OUT</code> plans, the ‘<samp>_transposed</samp>’ variants
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are useful in order to also return the local portion of the first
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dimension in the n<sub>1</sub> × n<sub>0</sub> × n<sub>2</sub> ×…× n<sub>d-1</sub>
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transposed output.
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See <a href="Transposed-distributions.html">Transposed distributions</a>.
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The advanced interface for multidimensional transforms is:
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</p>
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<span id="index-advanced-interface-5"></span>
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<span id="index-fftw_005fmpi_005flocal_005fsize_005fmany-1"></span>
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<span id="index-fftw_005fmpi_005flocal_005fsize_005fmany_005ftransposed-1"></span>
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<div class="example">
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<pre class="example">ptrdiff_t fftw_mpi_local_size_many(int rnk, const ptrdiff_t *n, ptrdiff_t howmany,
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ptrdiff_t block0, MPI_Comm comm,
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ptrdiff_t *local_n0, ptrdiff_t *local_0_start);
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ptrdiff_t fftw_mpi_local_size_many_transposed(int rnk, const ptrdiff_t *n, ptrdiff_t howmany,
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ptrdiff_t block0, ptrdiff_t block1, 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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<p>These differ from the basic interface in only two ways. First, they
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allow you to specify block sizes <code>block0</code> and <code>block1</code> (the
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latter for the transposed output); you can pass
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<code>FFTW_MPI_DEFAULT_BLOCK</code> to use FFTW’s default block size as in
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the basic interface. Second, you can pass a <code>howmany</code> parameter,
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corresponding to the advanced planning interface below: this is for
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transforms of contiguous <code>howmany</code>-tuples of numbers
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(<code>howmany = 1</code> in the basic interface).
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</p>
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<p>The corresponding basic and advanced routines for one-dimensional
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transforms (currently only complex DFTs) are:
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</p>
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<span id="index-fftw_005fmpi_005flocal_005fsize_005f1d-1"></span>
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<span id="index-fftw_005fmpi_005flocal_005fsize_005fmany_005f1d"></span>
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<div class="example">
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<pre class="example">ptrdiff_t fftw_mpi_local_size_1d(
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ptrdiff_t n0, MPI_Comm comm, int sign, unsigned flags,
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ptrdiff_t *local_ni, ptrdiff_t *local_i_start,
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ptrdiff_t *local_no, ptrdiff_t *local_o_start);
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ptrdiff_t fftw_mpi_local_size_many_1d(
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ptrdiff_t n0, ptrdiff_t howmany,
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MPI_Comm comm, int sign, unsigned flags,
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ptrdiff_t *local_ni, ptrdiff_t *local_i_start,
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ptrdiff_t *local_no, ptrdiff_t *local_o_start);
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</pre></div>
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<span id="index-FFTW_005fMPI_005fSCRAMBLED_005fOUT-1"></span>
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<span id="index-FFTW_005fMPI_005fSCRAMBLED_005fIN-1"></span>
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<p>As above, the return value is the number of elements to allocate
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(complex numbers, for complex DFTs). The <code>local_ni</code> and
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<code>local_i_start</code> arguments return the portion
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(<code>local_i_start</code> to <code>local_i_start + local_ni - 1</code>) of the
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1d array that is stored on this process for the transform
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<em>input</em>, and <code>local_no</code> and <code>local_o_start</code> are the
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corresponding quantities for the input. The <code>sign</code>
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(<code>FFTW_FORWARD</code> or <code>FFTW_BACKWARD</code>) and <code>flags</code> must
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match the arguments passed when creating a plan. Although the inputs
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and outputs have different data distributions in general, it is
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guaranteed that the <em>output</em> data distribution of an
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<code>FFTW_FORWARD</code> plan will match the <em>input</em> data distribution
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of an <code>FFTW_BACKWARD</code> plan and vice versa; similarly for the
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<code>FFTW_MPI_SCRAMBLED_OUT</code> and <code>FFTW_MPI_SCRAMBLED_IN</code> flags.
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See <a href="One_002ddimensional-distributions.html">One-dimensional distributions</a>.
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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="MPI-Plan-Creation.html" accesskey="n" rel="next">MPI Plan Creation</a>, Previous: <a href="Using-MPI-Plans.html" accesskey="p" rel="prev">Using MPI Plans</a>, Up: <a href="FFTW-MPI-Reference.html" accesskey="u" rel="up">FFTW MPI Reference</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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</body>
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</html>
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