Mercurial > hg > sv-dependency-builds
diff src/fftw-3.3.3/doc/html/Basic-and-advanced-distribution-interfaces.html @ 95:89f5e221ed7b
Add FFTW3
| author | Chris Cannam <cannam@all-day-breakfast.com> |
|---|---|
| date | Wed, 20 Mar 2013 15:35:50 +0000 |
| parents | |
| children |
line wrap: on
line diff
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.3/doc/html/Basic-and-advanced-distribution-interfaces.html Wed Mar 20 15:35:50 2013 +0000 @@ -0,0 +1,148 @@ +<html lang="en"> +<head> +<title>Basic and advanced distribution interfaces - FFTW 3.3.3</title> +<meta http-equiv="Content-Type" content="text/html"> +<meta name="description" content="FFTW 3.3.3"> +<meta name="generator" content="makeinfo 4.13"> +<link title="Top" rel="start" href="index.html#Top"> +<link rel="up" href="MPI-Data-Distribution.html#MPI-Data-Distribution" title="MPI Data Distribution"> +<link rel="prev" href="MPI-Data-Distribution.html#MPI-Data-Distribution" title="MPI Data Distribution"> +<link rel="next" href="Load-balancing.html#Load-balancing" title="Load balancing"> +<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> +<!-- +This manual is for FFTW +(version 3.3.3, 25 November 2012). + +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. + --> +<meta http-equiv="Content-Style-Type" content="text/css"> +<style type="text/css"><!-- + pre.display { font-family:inherit } + pre.format { font-family:inherit } + pre.smalldisplay { font-family:inherit; font-size:smaller } + pre.smallformat { font-family:inherit; font-size:smaller } + pre.smallexample { font-size:smaller } + pre.smalllisp { font-size:smaller } + span.sc { font-variant:small-caps } + span.roman { font-family:serif; font-weight:normal; } + span.sansserif { font-family:sans-serif; font-weight:normal; } +--></style> +</head> +<body> +<div class="node"> +<a name="Basic-and-advanced-distribution-interfaces"></a> +<p> +Next: <a rel="next" accesskey="n" href="Load-balancing.html#Load-balancing">Load balancing</a>, +Previous: <a rel="previous" accesskey="p" href="MPI-Data-Distribution.html#MPI-Data-Distribution">MPI Data Distribution</a>, +Up: <a rel="up" accesskey="u" href="MPI-Data-Distribution.html#MPI-Data-Distribution">MPI Data Distribution</a> +<hr> +</div> + +<h4 class="subsection">6.4.1 Basic and advanced distribution interfaces</h4> + +<p>As with the planner interface, the ‘<samp><span class="samp">fftw_mpi_local_size</span></samp>’ +distribution interface is broken into basic and advanced +(‘<samp><span class="samp">_many</span></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>For the 100 × 200 complex-DFT example, above, we would find +the distribution by calling the following function in the basic +interface: + +<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> + <p><a name="index-fftw_005fmpi_005flocal_005fsize_005f2d-370"></a> +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>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-371"></a> + + <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><span class="samp">fftw_alloc</span></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-372"></a><a name="index-fftw_005falloc_005fcomplex-373"></a> + + <p><a name="index-advanced-interface-374"></a>The advanced-interface ‘<samp><span class="samp">local_size</span></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: + +<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> + <p><a name="index-fftw_005fmpi_005flocal_005fsize_005fmany-375"></a> +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>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>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-376"></a><a name="index-block-distribution-377"></a> + + <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.) + + </body></html> +