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3 <title>Combining MPI and Threads - FFTW 3.2alpha3</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.2alpha3">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" title="Distributed-memory FFTW with MPI">
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9 <link rel="prev" href="FFTW-MPI-Performance-Tips.html#FFTW-MPI-Performance-Tips" title="FFTW MPI Performance Tips">
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10 <link rel="next" href="FFTW-MPI-Reference.html#FFTW-MPI-Reference" title="FFTW MPI Reference">
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12 <!--
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13 This manual is for FFTW
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14 (version 3.2alpha3, 14 August 2007).
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15
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16 Copyright (C) 2003 Matteo Frigo.
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17
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18 Copyright (C) 2003 Massachusetts Institute of Technology.
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19
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20 Permission is granted to make and distribute verbatim copies of
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21 this manual provided the copyright notice and this permission
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22 notice are preserved on all copies.
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23
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24 Permission is granted to copy and distribute modified versions of
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25 this manual under the conditions for verbatim copying, provided
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26 that the entire resulting derived work is distributed under the
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27 terms of a permission notice identical to this one.
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28
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29 Permission is granted to copy and distribute translations of this
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30 manual into another language, under the above conditions for
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31 modified versions, except that this permission notice may be
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32 stated in a translation approved by the Free Software Foundation.
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46 </head>
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47 <body>
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48 <div class="node">
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49 <p>
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50 <a name="Combining-MPI-and-Threads"></a>
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51 Next: <a rel="next" accesskey="n" href="FFTW-MPI-Reference.html#FFTW-MPI-Reference">FFTW MPI Reference</a>,
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52 Previous: <a rel="previous" accesskey="p" href="FFTW-MPI-Performance-Tips.html#FFTW-MPI-Performance-Tips">FFTW MPI Performance Tips</a>,
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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>
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54 <hr>
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55 </div>
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56
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57 <h3 class="section">6.11 Combining MPI and Threads</h3>
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58
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59 <p><a name="index-threads-398"></a>
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60 In certain cases, it may be advantageous to combine MPI
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61 (distributed-memory) and threads (shared-memory) parallelization.
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62 FFTW supports this, with certain caveats. For example, if you have a
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63 cluster of 4-processor shared-memory nodes, you may want to use
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64 threads within the nodes and MPI between the nodes, instead of MPI for
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65 all parallelization. FFTW's MPI code can also transparently use
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66 FFTW's Cell processor support (e.g. for clusters of Cell processors).
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67 <a name="index-Cell-processor-399"></a>
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68 In particular, it is possible to seamlessly combine the MPI FFTW
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69 routines with the multi-threaded FFTW routines (see <a href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW">Multi-threaded FFTW</a>). In this case, you will begin your program by calling both
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70 <code>fftw_mpi_init()</code> and <code>fftw_init_threads()</code>. Then, if you
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71 call <code>fftw_plan_with_nthreads(N)</code>, then <em>every</em> MPI process
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72 will launch <code>N</code> threads to parallelize its transforms.
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73 <a name="index-fftw_005fmpi_005finit-400"></a><a name="index-fftw_005finit_005fthreads-401"></a><a name="index-fftw_005fplan_005fwith_005fnthreads-402"></a>
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74 For example, in the hypothetical cluster of 4-processor nodes, you
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75 might wish to launch only a single MPI process per node, and then call
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76 <code>fftw_plan_with_nthreads(4)</code> on each process to use all
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77 processors in the nodes.
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78
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79 <p>This may or may not be faster than simply using as many MPI processes
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80 as you have processors, however. On the one hand, using threads within a
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81 node eliminates the need for explicit message passing within the node.
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82 On the other hand, FFTW's transpose routines are not multi-threaded,
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83 and this means that the communications that do take place will not
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84 benefit from parallelization within the node. Moreover, many MPI
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85 implementations already have optimizations to exploit shared memory
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86 when it is available.
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87 <a name="index-transpose-403"></a>
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88 (Note that this is quite independent of whether MPI itself is
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89 thread-safe or multi-threaded: regardless of how many threads you
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90 specify with <code>fftw_plan_with_nthreads</code>, FFTW will perform all of
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91 its MPI communication only from the parent process.)
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92 <a name="index-thread-safety-404"></a>
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93 <!-- -->
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94
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95 </body></html>
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96
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