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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>Cell Caveats - FFTW 3.2.1</title> | |
| 4 <meta http-equiv="Content-Type" content="text/html"> | |
| 5 <meta name="description" content="FFTW 3.2.1"> | |
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| 7 <link title="Top" rel="start" href="index.html#Top"> | |
| 8 <link rel="up" href="FFTW-on-the-Cell-Processor.html#FFTW-on-the-Cell-Processor" title="FFTW on the Cell Processor"> | |
| 9 <link rel="prev" href="Cell-Installation.html#Cell-Installation" title="Cell Installation"> | |
| 10 <link rel="next" href="FFTW-Accuracy-on-Cell.html#FFTW-Accuracy-on-Cell" title="FFTW Accuracy on Cell"> | |
| 11 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> | |
| 12 <!-- | |
| 13 This manual is for FFTW | |
| 14 (version 3.2.1, 5 February 2009). | |
| 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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| 45 --></style> | |
| 46 </head> | |
| 47 <body> | |
| 48 <div class="node"> | |
| 49 <p> | |
| 50 <a name="Cell-Caveats"></a> | |
| 51 Next: <a rel="next" accesskey="n" href="FFTW-Accuracy-on-Cell.html#FFTW-Accuracy-on-Cell">FFTW Accuracy on Cell</a>, | |
| 52 Previous: <a rel="previous" accesskey="p" href="Cell-Installation.html#Cell-Installation">Cell Installation</a>, | |
| 53 Up: <a rel="up" accesskey="u" href="FFTW-on-the-Cell-Processor.html#FFTW-on-the-Cell-Processor">FFTW on the Cell Processor</a> | |
| 54 <hr> | |
| 55 </div> | |
| 56 | |
| 57 <h3 class="section">6.2 Cell Caveats</h3> | |
| 58 | |
| 59 <ul> | |
| 60 <li>The FFTW benchmark program allocates memory using malloc() or | |
| 61 equivalent library calls, reflecting the common usage of the FFTW | |
| 62 library. However, you can sometimes improve performance significantly | |
| 63 by allocating memory in system-specific large TLB pages. E.g., we | |
| 64 have seen 39 GFLOPS/s for a 256 × 256 × 256 problem using | |
| 65 large pages, whereas the speed is about 25 GFLOPS/s with normal pages. | |
| 66 YMMV. | |
| 67 | |
| 68 <li>FFTW hoards all available SPEs for itself. You can optionally | |
| 69 choose a different number of SPEs by calling the undocumented | |
| 70 function <code>fftw_cell_set_nspe(n)</code>, where <code>n</code> is the number of desired | |
| 71 SPEs. Expect this interface to go away once we figure out how to | |
| 72 make FFTW play nicely with other Cell software. | |
| 73 | |
| 74 <p>In particular, if you try to link both the single and double precision | |
| 75 of FFTW in the same program (which you can do), they will both try | |
| 76 to grab all SPEs and the second one will hang. | |
| 77 | |
| 78 <li>The SPEs demand that data be stored in contiguous arrays aligned at | |
| 79 16-byte boundaries. If you instruct FFTW to operate on | |
| 80 noncontiguous or nonaligned data, the SPEs will not be used, | |
| 81 resulting in slow execution. See <a href="Data-Alignment.html#Data-Alignment">Data Alignment</a>. | |
| 82 | |
| 83 <li>The <code>FFTW_ESTIMATE</code> mode may produce seriously suboptimal plans, and | |
| 84 it becomes particularly confused if you enable both the SPEs and | |
| 85 Altivec. If you care about performance, please use <code>FFTW_MEASURE</code> | |
| 86 or <code>FFTW_PATIENT</code> until we figure out a more reliable performance model. | |
| 87 | |
| 88 </ul> | |
| 89 | |
| 90 <!-- --> | |
| 91 </body></html> | |
| 92 |