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| 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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| 1 <html lang="en"> | |
| 2 <head> | |
| 3 <title>MPI data distribution - FFTW 3.2alpha3</title> | |
| 4 <meta http-equiv="Content-Type" content="text/html"> | |
| 5 <meta name="description" content="FFTW 3.2alpha3"> | |
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| 7 <link title="Top" rel="start" href="index.html#Top"> | |
| 8 <link rel="up" href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" title="Distributed-memory FFTW with MPI"> | |
| 9 <link rel="prev" href="Simple-MPI-example.html#Simple-MPI-example" title="Simple MPI example"> | |
| 10 <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"> | |
| 11 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> | |
| 12 <!-- | |
| 13 This manual is for FFTW | |
| 14 (version 3.2alpha3, 14 August 2007). | |
| 15 | |
| 16 Copyright (C) 2003 Matteo Frigo. | |
| 17 | |
| 18 Copyright (C) 2003 Massachusetts Institute of Technology. | |
| 19 | |
| 20 Permission is granted to make and distribute verbatim copies of | |
| 21 this manual provided the copyright notice and this permission | |
| 22 notice are preserved on all copies. | |
| 23 | |
| 24 Permission is granted to copy and distribute modified versions of | |
| 25 this manual under the conditions for verbatim copying, provided | |
| 26 that the entire resulting derived work is distributed under the | |
| 27 terms of a permission notice identical to this one. | |
| 28 | |
| 29 Permission is granted to copy and distribute translations of this | |
| 30 manual into another language, under the above conditions for | |
| 31 modified versions, except that this permission notice may be | |
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| 46 </head> | |
| 47 <body> | |
| 48 <div class="node"> | |
| 49 <p> | |
| 50 <a name="MPI-data-distribution"></a> | |
| 51 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>, | |
| 52 Previous: <a rel="previous" accesskey="p" href="Simple-MPI-example.html#Simple-MPI-example">Simple MPI example</a>, | |
| 53 Up: <a rel="up" accesskey="u" href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI">Distributed-memory FFTW with MPI</a> | |
| 54 <hr> | |
| 55 </div> | |
| 56 | |
| 57 <h3 class="section">6.4 MPI data distribution</h3> | |
| 58 | |
| 59 <p><a name="index-data-distribution-350"></a> | |
| 60 The most important concept to understand in using FFTW's MPI interface | |
| 61 is the data distribution. With a serial or multithreaded FFT, all of | |
| 62 the input and outputs are stored as a single contiguous chunk of | |
| 63 memory. With a distributed-memory FFT, the inputs and outputs are | |
| 64 broken into disjoint blocks, one per process. | |
| 65 | |
| 66 <p>In particular, FFTW uses a <em>1d block distribution</em> of the data, | |
| 67 distributed along the <em>first dimension</em>. For example, if you | |
| 68 want to perform a 100 × 200 complex DFT, distributed over 4 | |
| 69 processes, each process will get a 25 × 200 slice of the data. | |
| 70 That is, process 0 will get rows 0 through 24, process 1 will get rows | |
| 71 25 through 49, process 2 will get rows 50 through 74, and process 3 | |
| 72 will get rows 75 through 99. If you take the same array but | |
| 73 distribute it over 3 processes, then it is not evenly divisible so the | |
| 74 different processes will have unequal chunks. FFTW's default choice | |
| 75 in this case is to assign 34 rows to processes 0 and 1, and 32 rows to | |
| 76 process 2. | |
| 77 <a name="index-block-distribution-351"></a> | |
| 78 FFTW provides several `<samp><span class="samp">fftw_mpi_local_size</span></samp>' routines that you can | |
| 79 call to find out what portion of an array is stored on the current | |
| 80 process. In most cases, you should use the default block sizes picked | |
| 81 by FFTW, but it is also possible to specify your own block size. For | |
| 82 example, with a 100 × 200 array on three processes, you can | |
| 83 tell FFTW to use a block size of 40, which would assign 40 rows to | |
| 84 processes 0 and 1, and 20 rows to process 2. FFTW's default is to | |
| 85 divide the data equally among the processes if possible, and as best | |
| 86 it can otherwise. The rows are always assigned in “rank order,” | |
| 87 i.e. process 0 gets the first block of rows, then process 1, and so | |
| 88 on. (You can change this by using <code>MPI_Comm_split</code> to create a | |
| 89 new communicator with re-ordered processes.) However, you should | |
| 90 always call the `<samp><span class="samp">fftw_mpi_local_size</span></samp>' routines, if possible, | |
| 91 rather than trying to predict FFTW's distribution choices. | |
| 92 | |
| 93 <ul class="menu"> | |
| 94 <li><a accesskey="1" href="Basic-and-advanced-distribution-interfaces.html#Basic-and-advanced-distribution-interfaces">Basic and advanced distribution interfaces</a> | |
| 95 <li><a accesskey="2" href="Load-balancing.html#Load-balancing">Load balancing</a> | |
| 96 <li><a accesskey="3" href="Transposed-distributions.html#Transposed-distributions">Transposed distributions</a> | |
| 97 <li><a accesskey="4" href="One_002ddimensional-distributions.html#One_002ddimensional-distributions">One-dimensional distributions</a> | |
| 98 </ul> | |
| 99 | |
| 100 </body></html> | |
| 101 |