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Current fftw source
| author | Chris Cannam <cannam@all-day-breakfast.com> |
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| date | Tue, 18 Oct 2016 13:40:26 +0100 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.5/doc/html/Basic-and-advanced-distribution-interfaces.html Tue Oct 18 13:40:26 2016 +0100 @@ -0,0 +1,188 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This manual is for FFTW +(version 3.3.5, 30 July 2016). + +Copyright (C) 2003 Matteo Frigo. + +Copyright (C) 2003 Massachusetts Institute of Technology. + +Permission is granted to make and distribute verbatim copies of this +manual provided the copyright notice and this permission notice are +preserved on all copies. + +Permission is granted to copy and distribute modified versions of this +manual under the conditions for verbatim copying, provided that the +entire resulting derived work is distributed under the terms of a +permission notice identical to this one. + +Permission is granted to copy and distribute translations of this manual +into another language, under the above conditions for modified versions, +except that this permission notice may be stated in a translation +approved by the Free Software Foundation. --> +<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> +<head> +<title>FFTW 3.3.5: Basic and advanced distribution interfaces</title> + +<meta name="description" content="FFTW 3.3.5: Basic and advanced distribution interfaces"> +<meta name="keywords" content="FFTW 3.3.5: Basic and advanced distribution interfaces"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="MPI-Data-Distribution.html#MPI-Data-Distribution" rel="up" title="MPI Data Distribution"> +<link href="Load-balancing.html#Load-balancing" rel="next" title="Load balancing"> +<link href="MPI-Data-Distribution.html#MPI-Data-Distribution" rel="prev" title="MPI Data Distribution"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Basic-and-advanced-distribution-interfaces"></a> +<div class="header"> +<p> +Next: <a href="Load-balancing.html#Load-balancing" accesskey="n" rel="next">Load balancing</a>, Previous: <a href="MPI-Data-Distribution.html#MPI-Data-Distribution" accesskey="p" rel="prev">MPI Data Distribution</a>, Up: <a href="MPI-Data-Distribution.html#MPI-Data-Distribution" 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#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Basic-and-advanced-distribution-interfaces-1"></a> +<h4 class="subsection">6.4.1 Basic and advanced distribution interfaces</h4> + +<p>As with the planner interface, the ‘<samp>fftw_mpi_local_size</samp>’ +distribution interface is broken into basic and advanced +(‘<samp>_many</samp>’) interfaces, where the latter allows you to specify the +block size manually and also to request block sizes when computing +multiple transforms simultaneously. These functions are documented +more exhaustively by the FFTW MPI Reference, but we summarize the +basic ideas here using a couple of two-dimensional examples. +</p> +<p>For the 100 × 200 complex-DFT example, above, we would find +the distribution by calling the following function in the basic +interface: +</p> +<div class="example"> +<pre class="example">ptrdiff_t fftw_mpi_local_size_2d(ptrdiff_t n0, ptrdiff_t n1, MPI_Comm comm, + ptrdiff_t *local_n0, ptrdiff_t *local_0_start); +</pre></div> +<a name="index-fftw_005fmpi_005flocal_005fsize_005f2d-1"></a> + +<p>Given the total size of the data to be transformed (here, <code>n0 = +100</code> and <code>n1 = 200</code>) and an MPI communicator (<code>comm</code>), this +function provides three numbers. +</p> +<p>First, it describes the shape of the local data: the current process +should store a <code>local_n0</code> by <code>n1</code> slice of the overall +dataset, in row-major order (<code>n1</code> dimension contiguous), starting +at index <code>local_0_start</code>. That is, if the total dataset is +viewed as a <code>n0</code> by <code>n1</code> matrix, the current process should +store the rows <code>local_0_start</code> to +<code>local_0_start+local_n0-1</code>. Obviously, if you are running with +only a single MPI process, that process will store the entire array: +<code>local_0_start</code> will be zero and <code>local_n0</code> will be +<code>n0</code>. See <a href="Row_002dmajor-Format.html#Row_002dmajor-Format">Row-major Format</a>. +<a name="index-row_002dmajor-4"></a> +</p> + +<p>Second, the return value is the total number of data elements (e.g., +complex numbers for a complex DFT) that should be allocated for the +input and output arrays on the current process (ideally with +<code>fftw_malloc</code> or an ‘<samp>fftw_alloc</samp>’ function, to ensure optimal +alignment). It might seem that this should always be equal to +<code>local_n0 * n1</code>, but this is <em>not</em> the case. FFTW’s +distributed FFT algorithms require data redistributions at +intermediate stages of the transform, and in some circumstances this +may require slightly larger local storage. This is discussed in more +detail below, under <a href="Load-balancing.html#Load-balancing">Load balancing</a>. +<a name="index-fftw_005fmalloc-5"></a> +<a name="index-fftw_005falloc_005fcomplex-3"></a> +</p> + +<a name="index-advanced-interface-4"></a> +<p>The advanced-interface ‘<samp>local_size</samp>’ function for multidimensional +transforms returns the same three things (<code>local_n0</code>, +<code>local_0_start</code>, and the total number of elements to allocate), +but takes more inputs: +</p> +<div class="example"> +<pre class="example">ptrdiff_t fftw_mpi_local_size_many(int rnk, const ptrdiff_t *n, + ptrdiff_t howmany, + ptrdiff_t block0, + MPI_Comm comm, + ptrdiff_t *local_n0, + ptrdiff_t *local_0_start); +</pre></div> +<a name="index-fftw_005fmpi_005flocal_005fsize_005fmany"></a> + +<p>The two-dimensional case above corresponds to <code>rnk = 2</code> and an +array <code>n</code> of length 2 with <code>n[0] = n0</code> and <code>n[1] = n1</code>. +This routine is for any <code>rnk > 1</code>; one-dimensional transforms +have their own interface because they work slightly differently, as +discussed below. +</p> +<p>First, the advanced interface allows you to perform multiple +transforms at once, of interleaved data, as specified by the +<code>howmany</code> parameter. (<code>hoamany</code> is 1 for a single +transform.) +</p> +<p>Second, here you can specify your desired block size in the <code>n0</code> +dimension, <code>block0</code>. To use FFTW’s default block size, pass +<code>FFTW_MPI_DEFAULT_BLOCK</code> (0) for <code>block0</code>. Otherwise, on +<code>P</code> processes, FFTW will return <code>local_n0</code> equal to +<code>block0</code> on the first <code>P / block0</code> processes (rounded down), +return <code>local_n0</code> equal to <code>n0 - block0 * (P / block0)</code> on +the next process, and <code>local_n0</code> equal to zero on any remaining +processes. In general, we recommend using the default block size +(which corresponds to <code>n0 / P</code>, rounded up). +<a name="index-FFTW_005fMPI_005fDEFAULT_005fBLOCK"></a> +<a name="index-block-distribution-1"></a> +</p> + +<p>For example, suppose you have <code>P = 4</code> processes and <code>n0 = +21</code>. The default will be a block size of <code>6</code>, which will give +<code>local_n0 = 6</code> on the first three processes and <code>local_n0 = +3</code> on the last process. Instead, however, you could specify +<code>block0 = 5</code> if you wanted, which would give <code>local_n0 = 5</code> +on processes 0 to 2, <code>local_n0 = 6</code> on process 3. (This choice, +while it may look superficially more “balanced,” has the same +critical path as FFTW’s default but requires more communications.) +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Load-balancing.html#Load-balancing" accesskey="n" rel="next">Load balancing</a>, Previous: <a href="MPI-Data-Distribution.html#MPI-Data-Distribution" accesskey="p" rel="prev">MPI Data Distribution</a>, Up: <a href="MPI-Data-Distribution.html#MPI-Data-Distribution" 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#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html>