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178 lines
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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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<title>Basic and advanced distribution interfaces (FFTW 3.3.10)</title>
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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="MPI-Data-Distribution.html" rel="up" title="MPI Data Distribution">
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<span id="Basic-and-advanced-distribution-interfaces"></span><div class="header">
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<p>
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Next: <a href="Load-balancing.html" accesskey="n" rel="next">Load balancing</a>, Previous: <a href="MPI-Data-Distribution.html" accesskey="p" rel="prev">MPI Data Distribution</a>, Up: <a href="MPI-Data-Distribution.html" accesskey="u" rel="up">MPI Data Distribution</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-and-advanced-distribution-interfaces-1"></span><h4 class="subsection">6.4.1 Basic and advanced distribution interfaces</h4>
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<p>As with the planner interface, the ‘<samp>fftw_mpi_local_size</samp>’
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distribution interface is broken into basic and advanced
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(‘<samp>_many</samp>’) interfaces, where the latter allows you to specify the
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block size manually and also to request block sizes when computing
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multiple transforms simultaneously. These functions are documented
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more exhaustively by the FFTW MPI Reference, but we summarize the
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basic ideas here using a couple of two-dimensional examples.
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</p>
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<p>For the 100 × 200
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complex-DFT example, above, we would find
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the distribution by calling the following function in the basic
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interface:
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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(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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</pre></div>
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<span id="index-fftw_005fmpi_005flocal_005fsize_005f2d-1"></span>
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<p>Given the total size of the data to be transformed (here, <code>n0 =
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100</code> and <code>n1 = 200</code>) and an MPI communicator (<code>comm</code>), this
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function provides three numbers.
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</p>
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<p>First, it describes the shape of the local data: the current process
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should store a <code>local_n0</code> by <code>n1</code> slice of the overall
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dataset, in row-major order (<code>n1</code> dimension contiguous), starting
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at index <code>local_0_start</code>. That is, if the total dataset is
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viewed as a <code>n0</code> by <code>n1</code> matrix, the current process should
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store the rows <code>local_0_start</code> to
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<code>local_0_start+local_n0-1</code>. Obviously, if you are running with
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only a single MPI process, that process will store the entire array:
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<code>local_0_start</code> will be zero and <code>local_n0</code> will be
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<code>n0</code>. See <a href="Row_002dmajor-Format.html">Row-major Format</a>.
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<span id="index-row_002dmajor-4"></span>
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</p>
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<p>Second, the return value is the total number of data elements (e.g.,
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complex numbers for a complex DFT) that should be allocated for the
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input and output arrays on the current process (ideally with
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<code>fftw_malloc</code> or an ‘<samp>fftw_alloc</samp>’ function, to ensure optimal
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alignment). It might seem that this should always be equal to
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<code>local_n0 * n1</code>, but this is <em>not</em> the case. FFTW’s
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distributed FFT algorithms require data redistributions at
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intermediate stages of the transform, and in some circumstances this
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may require slightly larger local storage. This is discussed in more
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detail below, under <a href="Load-balancing.html">Load balancing</a>.
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<span id="index-fftw_005fmalloc-5"></span>
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<span id="index-fftw_005falloc_005fcomplex-3"></span>
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</p>
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<span id="index-advanced-interface-4"></span>
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<p>The advanced-interface ‘<samp>local_size</samp>’ function for multidimensional
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transforms returns the same three things (<code>local_n0</code>,
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<code>local_0_start</code>, and the total number of elements to allocate),
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but takes more inputs:
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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_many(int rnk, const ptrdiff_t *n,
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ptrdiff_t howmany,
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ptrdiff_t block0,
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MPI_Comm comm,
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ptrdiff_t *local_n0,
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ptrdiff_t *local_0_start);
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</pre></div>
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<span id="index-fftw_005fmpi_005flocal_005fsize_005fmany"></span>
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<p>The two-dimensional case above corresponds to <code>rnk = 2</code> and an
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array <code>n</code> of length 2 with <code>n[0] = n0</code> and <code>n[1] = n1</code>.
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This routine is for any <code>rnk > 1</code>; one-dimensional transforms
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have their own interface because they work slightly differently, as
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discussed below.
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</p>
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<p>First, the advanced interface allows you to perform multiple
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transforms at once, of interleaved data, as specified by the
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<code>howmany</code> parameter. (<code>hoamany</code> is 1 for a single
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transform.)
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</p>
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<p>Second, here you can specify your desired block size in the <code>n0</code>
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dimension, <code>block0</code>. To use FFTW’s default block size, pass
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<code>FFTW_MPI_DEFAULT_BLOCK</code> (0) for <code>block0</code>. Otherwise, on
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<code>P</code> processes, FFTW will return <code>local_n0</code> equal to
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<code>block0</code> on the first <code>P / block0</code> processes (rounded down),
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return <code>local_n0</code> equal to <code>n0 - block0 * (P / block0)</code> on
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the next process, and <code>local_n0</code> equal to zero on any remaining
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processes. In general, we recommend using the default block size
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(which corresponds to <code>n0 / P</code>, rounded up).
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<span id="index-FFTW_005fMPI_005fDEFAULT_005fBLOCK"></span>
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<span id="index-block-distribution-1"></span>
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</p>
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<p>For example, suppose you have <code>P = 4</code> processes and <code>n0 =
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21</code>. The default will be a block size of <code>6</code>, which will give
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<code>local_n0 = 6</code> on the first three processes and <code>local_n0 =
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3</code> on the last process. Instead, however, you could specify
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<code>block0 = 5</code> if you wanted, which would give <code>local_n0 = 5</code>
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on processes 0 to 2, <code>local_n0 = 6</code> on process 3. (This choice,
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while it may look superficially more “balanced,” has the same
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critical path as FFTW’s default but requires more communications.)
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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="Load-balancing.html" accesskey="n" rel="next">Load balancing</a>, Previous: <a href="MPI-Data-Distribution.html" accesskey="p" rel="prev">MPI Data Distribution</a>, Up: <a href="MPI-Data-Distribution.html" accesskey="u" rel="up">MPI Data Distribution</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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