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diff src/fftw-3.3.3/doc/html/MPI-Data-Distribution.html @ 95:89f5e221ed7b
Add FFTW3
| author | Chris Cannam <cannam@all-day-breakfast.com> |
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| date | Wed, 20 Mar 2013 15:35:50 +0000 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.3/doc/html/MPI-Data-Distribution.html Wed Mar 20 15:35:50 2013 +0000 @@ -0,0 +1,113 @@ +<html lang="en"> +<head> +<title>MPI Data Distribution - 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="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" title="Distributed-memory FFTW with MPI"> +<link rel="prev" href="2d-MPI-example.html#g_t2d-MPI-example" title="2d MPI example"> +<link rel="next" href="Multi_002ddimensional-MPI-DFTs-of-Real-Data.html#Multi_002ddimensional-MPI-DFTs-of-Real-Data" title="Multi-dimensional MPI DFTs of Real Data"> +<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="MPI-Data-Distribution"></a> +<p> +Next: <a rel="next" accesskey="n" href="Multi_002ddimensional-MPI-DFTs-of-Real-Data.html#Multi_002ddimensional-MPI-DFTs-of-Real-Data">Multi-dimensional MPI DFTs of Real Data</a>, +Previous: <a rel="previous" accesskey="p" href="2d-MPI-example.html#g_t2d-MPI-example">2d 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-368"></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 inputs 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-369"></a> + + <p>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. + + <p>In particular, it is critical that you allocate the storage size that +is returned by ‘<samp><span class="samp">fftw_mpi_local_size</span></samp>’, which is <em>not</em> +necessarily the size of the local slice of the array. The reason is +that intermediate steps of FFTW's algorithms involve transposing the +array and redistributing the data, so at these intermediate steps FFTW +may require more local storage space (albeit always proportional to +the total size divided by the number of processes). The +‘<samp><span class="samp">fftw_mpi_local_size</span></samp>’ functions know how much storage is required +for these intermediate steps and tell you the correct amount to +allocate. + +<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> +