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diff Lib/fftw-3.2.1/doc/html/.svn/text-base/MPI-data-distribution.html.svn-base @ 0:25bf17994ef1
First commit. VS2013, Codeblocks and Mac OSX configuration
| author | Geogaddi\David <d.m.ronan@qmul.ac.uk> |
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| date | Thu, 09 Jul 2015 01:12:16 +0100 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/Lib/fftw-3.2.1/doc/html/.svn/text-base/MPI-data-distribution.html.svn-base Thu Jul 09 01:12:16 2015 +0100 @@ -0,0 +1,101 @@ +<html lang="en"> +<head> +<title>MPI data distribution - FFTW 3.2alpha3</title> +<meta http-equiv="Content-Type" content="text/html"> +<meta name="description" content="FFTW 3.2alpha3"> +<meta name="generator" content="makeinfo 4.8"> +<link title="Top" rel="start" href="index.html#Top"> +<link rel="up" href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" title="Distributed-memory FFTW with MPI"> +<link rel="prev" href="Simple-MPI-example.html#Simple-MPI-example" title="Simple MPI example"> +<link rel="next" href="Multi_002ddimensional-MPI-DFT-of-Real-Data.html#Multi_002ddimensional-MPI-DFT-of-Real-Data" title="Multi-dimensional MPI DFT of Real Data"> +<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> +<!-- +This manual is for FFTW +(version 3.2alpha3, 14 August 2007). + +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"> +<p> +<a name="MPI-data-distribution"></a> +Next: <a rel="next" accesskey="n" href="Multi_002ddimensional-MPI-DFT-of-Real-Data.html#Multi_002ddimensional-MPI-DFT-of-Real-Data">Multi-dimensional MPI DFT of Real Data</a>, +Previous: <a rel="previous" accesskey="p" href="Simple-MPI-example.html#Simple-MPI-example">Simple MPI example</a>, +Up: <a rel="up" accesskey="u" href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI">Distributed-memory FFTW with MPI</a> +<hr> +</div> + +<h3 class="section">6.4 MPI data distribution</h3> + +<p><a name="index-data-distribution-350"></a> +The most important concept to understand in using FFTW's MPI interface +is the data distribution. With a serial or multithreaded FFT, all of +the input and outputs are stored as a single contiguous chunk of +memory. With a distributed-memory FFT, the inputs and outputs are +broken into disjoint blocks, one per process. + + <p>In particular, FFTW uses a <em>1d block distribution</em> of the data, +distributed along the <em>first dimension</em>. For example, if you +want to perform a 100 × 200 complex DFT, distributed over 4 +processes, each process will get a 25 × 200 slice of the data. +That is, process 0 will get rows 0 through 24, process 1 will get rows +25 through 49, process 2 will get rows 50 through 74, and process 3 +will get rows 75 through 99. If you take the same array but +distribute it over 3 processes, then it is not evenly divisible so the +different processes will have unequal chunks. FFTW's default choice +in this case is to assign 34 rows to processes 0 and 1, and 32 rows to +process 2. +<a name="index-block-distribution-351"></a> +FFTW provides several `<samp><span class="samp">fftw_mpi_local_size</span></samp>' routines that you can +call to find out what portion of an array is stored on the current +process. In most cases, you should use the default block sizes picked +by FFTW, but it is also possible to specify your own block size. For +example, with a 100 × 200 array on three processes, you can +tell FFTW to use a block size of 40, which would assign 40 rows to +processes 0 and 1, and 20 rows to process 2. FFTW's default is to +divide the data equally among the processes if possible, and as best +it can otherwise. The rows are always assigned in “rank order,” +i.e. process 0 gets the first block of rows, then process 1, and so +on. (You can change this by using <code>MPI_Comm_split</code> to create a +new communicator with re-ordered processes.) However, you should +always call the `<samp><span class="samp">fftw_mpi_local_size</span></samp>' routines, if possible, +rather than trying to predict FFTW's distribution choices. + +<ul class="menu"> +<li><a accesskey="1" href="Basic-and-advanced-distribution-interfaces.html#Basic-and-advanced-distribution-interfaces">Basic and advanced distribution interfaces</a> +<li><a accesskey="2" href="Load-balancing.html#Load-balancing">Load balancing</a> +<li><a accesskey="3" href="Transposed-distributions.html#Transposed-distributions">Transposed distributions</a> +<li><a accesskey="4" href="One_002ddimensional-distributions.html#One_002ddimensional-distributions">One-dimensional distributions</a> +</ul> + + </body></html> +