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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/MPI-Data-Distribution.html Tue Oct 18 13:40:26 2016 +0100 @@ -0,0 +1,148 @@ +<!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: MPI Data Distribution</title> + +<meta name="description" content="FFTW 3.3.5: MPI Data Distribution"> +<meta name="keywords" content="FFTW 3.3.5: MPI Data Distribution"> +<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="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="up" title="Distributed-memory FFTW with MPI"> +<link href="Basic-and-advanced-distribution-interfaces.html#Basic-and-advanced-distribution-interfaces" rel="next" title="Basic and advanced distribution interfaces"> +<link href="2d-MPI-example.html#g_t2d-MPI-example" rel="prev" title="2d MPI example"> +<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="MPI-Data-Distribution"></a> +<div class="header"> +<p> +Next: <a href="Multi_002ddimensional-MPI-DFTs-of-Real-Data.html#Multi_002ddimensional-MPI-DFTs-of-Real-Data" accesskey="n" rel="next">Multi-dimensional MPI DFTs of Real Data</a>, Previous: <a href="2d-MPI-example.html#g_t2d-MPI-example" accesskey="p" rel="prev">2d MPI example</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</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="MPI-Data-Distribution-1"></a> +<h3 class="section">6.4 MPI Data Distribution</h3> +<a name="index-data-distribution-2"></a> + +<p>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> +<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"></a> +</p> + +<p>FFTW provides several ‘<samp>fftw_mpi_local_size</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>fftw_mpi_local_size</samp>’ routines, if possible, +rather than trying to predict FFTW’s distribution choices. +</p> +<p>In particular, it is critical that you allocate the storage size that +is returned by ‘<samp>fftw_mpi_local_size</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>fftw_mpi_local_size</samp>’ functions know how much storage is required +for these intermediate steps and tell you the correct amount to +allocate. +</p> +<table class="menu" border="0" cellspacing="0"> +<tr><td align="left" valign="top">• <a href="Basic-and-advanced-distribution-interfaces.html#Basic-and-advanced-distribution-interfaces" accesskey="1">Basic and advanced distribution interfaces</a>:</td><td> </td><td align="left" valign="top"> +</td></tr> +<tr><td align="left" valign="top">• <a href="Load-balancing.html#Load-balancing" accesskey="2">Load balancing</a>:</td><td> </td><td align="left" valign="top"> +</td></tr> +<tr><td align="left" valign="top">• <a href="Transposed-distributions.html#Transposed-distributions" accesskey="3">Transposed distributions</a>:</td><td> </td><td align="left" valign="top"> +</td></tr> +<tr><td align="left" valign="top">• <a href="One_002ddimensional-distributions.html#One_002ddimensional-distributions" accesskey="4">One-dimensional distributions</a>:</td><td> </td><td align="left" valign="top"> +</td></tr> +</table> + +<hr> +<div class="header"> +<p> +Next: <a href="Multi_002ddimensional-MPI-DFTs-of-Real-Data.html#Multi_002ddimensional-MPI-DFTs-of-Real-Data" accesskey="n" rel="next">Multi-dimensional MPI DFTs of Real Data</a>, Previous: <a href="2d-MPI-example.html#g_t2d-MPI-example" accesskey="p" rel="prev">2d MPI example</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</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>