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| author | Chris Cannam |
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| date | Wed, 20 Mar 2013 15:35:50 +0000 |
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| 1 <html lang="en"> | |
| 2 <head> | |
| 3 <title>Load balancing - FFTW 3.3.3</title> | |
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| 5 <meta name="description" content="FFTW 3.3.3"> | |
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| 12 <!-- | |
| 13 This manual is for FFTW | |
| 14 (version 3.3.3, 25 November 2012). | |
| 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. | |
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| 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. | |
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| 29 Permission is granted to copy and distribute translations of this | |
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| 49 <a name="Load-balancing"></a> | |
| 50 <p> | |
| 51 Next: <a rel="next" accesskey="n" href="Transposed-distributions.html#Transposed-distributions">Transposed distributions</a>, | |
| 52 Previous: <a rel="previous" accesskey="p" href="Basic-and-advanced-distribution-interfaces.html#Basic-and-advanced-distribution-interfaces">Basic and advanced distribution interfaces</a>, | |
| 53 Up: <a rel="up" accesskey="u" href="MPI-Data-Distribution.html#MPI-Data-Distribution">MPI Data Distribution</a> | |
| 54 <hr> | |
| 55 </div> | |
| 56 | |
| 57 <h4 class="subsection">6.4.2 Load balancing</h4> | |
| 58 | |
| 59 <p><a name="index-load-balancing-378"></a> | |
| 60 Ideally, when you parallelize a transform over some P | |
| 61 processes, each process should end up with work that takes equal time. | |
| 62 Otherwise, all of the processes end up waiting on whichever process is | |
| 63 slowest. This goal is known as “load balancing.” In this section, | |
| 64 we describe the circumstances under which FFTW is able to load-balance | |
| 65 well, and in particular how you should choose your transform size in | |
| 66 order to load balance. | |
| 67 | |
| 68 <p>Load balancing is especially difficult when you are parallelizing over | |
| 69 heterogeneous machines; for example, if one of your processors is a | |
| 70 old 486 and another is a Pentium IV, obviously you should give the | |
| 71 Pentium more work to do than the 486 since the latter is much slower. | |
| 72 FFTW does not deal with this problem, however—it assumes that your | |
| 73 processes run on hardware of comparable speed, and that the goal is | |
| 74 therefore to divide the problem as equally as possible. | |
| 75 | |
| 76 <p>For a multi-dimensional complex DFT, FFTW can divide the problem | |
| 77 equally among the processes if: (i) the <em>first</em> dimension | |
| 78 <code>n0</code> is divisible by P; and (ii), the <em>product</em> of | |
| 79 the subsequent dimensions is divisible by P. (For the advanced | |
| 80 interface, where you can specify multiple simultaneous transforms via | |
| 81 some “vector” length <code>howmany</code>, a factor of <code>howmany</code> is | |
| 82 included in the product of the subsequent dimensions.) | |
| 83 | |
| 84 <p>For a one-dimensional complex DFT, the length <code>N</code> of the data | |
| 85 should be divisible by P <em>squared</em> to be able to divide | |
| 86 the problem equally among the processes. | |
| 87 | |
| 88 </body></html> | |
| 89 |