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Current fftw source
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 18 Oct 2016 13:40:26 +0100
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cannam@127 3 <!-- This manual is for FFTW
cannam@127 4 (version 3.3.5, 30 July 2016).
cannam@127 5
cannam@127 6 Copyright (C) 2003 Matteo Frigo.
cannam@127 7
cannam@127 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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cannam@127 24 <head>
cannam@127 25 <title>FFTW 3.3.5: Combining MPI and Threads</title>
cannam@127 26
cannam@127 27 <meta name="description" content="FFTW 3.3.5: Combining MPI and Threads">
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cannam@127 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
cannam@127 36 <link href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="up" title="Distributed-memory FFTW with MPI">
cannam@127 37 <link href="FFTW-MPI-Reference.html#FFTW-MPI-Reference" rel="next" title="FFTW MPI Reference">
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cannam@127 69 </head>
cannam@127 70
cannam@127 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
cannam@127 72 <a name="Combining-MPI-and-Threads"></a>
cannam@127 73 <div class="header">
cannam@127 74 <p>
cannam@127 75 Next: <a href="FFTW-MPI-Reference.html#FFTW-MPI-Reference" accesskey="n" rel="next">FFTW MPI Reference</a>, Previous: <a href="FFTW-MPI-Performance-Tips.html#FFTW-MPI-Performance-Tips" accesskey="p" rel="prev">FFTW MPI Performance Tips</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> &nbsp; [<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>
cannam@127 76 </div>
cannam@127 77 <hr>
cannam@127 78 <a name="Combining-MPI-and-Threads-1"></a>
cannam@127 79 <h3 class="section">6.11 Combining MPI and Threads</h3>
cannam@127 80 <a name="index-threads-2"></a>
cannam@127 81
cannam@127 82 <p>In certain cases, it may be advantageous to combine MPI
cannam@127 83 (distributed-memory) and threads (shared-memory) parallelization.
cannam@127 84 FFTW supports this, with certain caveats. For example, if you have a
cannam@127 85 cluster of 4-processor shared-memory nodes, you may want to use
cannam@127 86 threads within the nodes and MPI between the nodes, instead of MPI for
cannam@127 87 all parallelization.
cannam@127 88 </p>
cannam@127 89 <p>In particular, it is possible to seamlessly combine the MPI FFTW
cannam@127 90 routines with the multi-threaded FFTW routines (see <a href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW">Multi-threaded FFTW</a>). However, some care must be taken in the initialization code,
cannam@127 91 which should look something like this:
cannam@127 92 </p>
cannam@127 93 <div class="example">
cannam@127 94 <pre class="example">int threads_ok;
cannam@127 95
cannam@127 96 int main(int argc, char **argv)
cannam@127 97 {
cannam@127 98 int provided;
cannam@127 99 MPI_Init_thread(&amp;argc, &amp;argv, MPI_THREAD_FUNNELED, &amp;provided);
cannam@127 100 threads_ok = provided &gt;= MPI_THREAD_FUNNELED;
cannam@127 101
cannam@127 102 if (threads_ok) threads_ok = fftw_init_threads();
cannam@127 103 fftw_mpi_init();
cannam@127 104
cannam@127 105 ...
cannam@127 106 if (threads_ok) fftw_plan_with_nthreads(...);
cannam@127 107 ...
cannam@127 108
cannam@127 109 MPI_Finalize();
cannam@127 110 }
cannam@127 111 </pre></div>
cannam@127 112 <a name="index-fftw_005fmpi_005finit-3"></a>
cannam@127 113 <a name="index-fftw_005finit_005fthreads-2"></a>
cannam@127 114 <a name="index-fftw_005fplan_005fwith_005fnthreads-1"></a>
cannam@127 115
cannam@127 116 <p>First, note that instead of calling <code>MPI_Init</code>, you should call
cannam@127 117 <code>MPI_Init_threads</code>, which is the initialization routine defined
cannam@127 118 by the MPI-2 standard to indicate to MPI that your program will be
cannam@127 119 multithreaded. We pass <code>MPI_THREAD_FUNNELED</code>, which indicates
cannam@127 120 that we will only call MPI routines from the main thread. (FFTW will
cannam@127 121 launch additional threads internally, but the extra threads will not
cannam@127 122 call MPI code.) (You may also pass <code>MPI_THREAD_SERIALIZED</code> or
cannam@127 123 <code>MPI_THREAD_MULTIPLE</code>, which requests additional multithreading
cannam@127 124 support from the MPI implementation, but this is not required by
cannam@127 125 FFTW.) The <code>provided</code> parameter returns what level of threads
cannam@127 126 support is actually supported by your MPI implementation; this
cannam@127 127 <em>must</em> be at least <code>MPI_THREAD_FUNNELED</code> if you want to call
cannam@127 128 the FFTW threads routines, so we define a global variable
cannam@127 129 <code>threads_ok</code> to record this. You should only call
cannam@127 130 <code>fftw_init_threads</code> or <code>fftw_plan_with_nthreads</code> if
cannam@127 131 <code>threads_ok</code> is true. For more information on thread safety in
cannam@127 132 MPI, see the
cannam@127 133 <a href="http://www.mpi-forum.org/docs/mpi-20-html/node162.htm">MPI and
cannam@127 134 Threads</a> section of the MPI-2 standard.
cannam@127 135 <a name="index-thread-safety-2"></a>
cannam@127 136 </p>
cannam@127 137
cannam@127 138 <p>Second, we must call <code>fftw_init_threads</code> <em>before</em>
cannam@127 139 <code>fftw_mpi_init</code>. This is critical for technical reasons having
cannam@127 140 to do with how FFTW initializes its list of algorithms.
cannam@127 141 </p>
cannam@127 142 <p>Then, if you call <code>fftw_plan_with_nthreads(N)</code>, <em>every</em> MPI
cannam@127 143 process will launch (up to) <code>N</code> threads to parallelize its transforms.
cannam@127 144 </p>
cannam@127 145 <p>For example, in the hypothetical cluster of 4-processor nodes, you
cannam@127 146 might wish to launch only a single MPI process per node, and then call
cannam@127 147 <code>fftw_plan_with_nthreads(4)</code> on each process to use all
cannam@127 148 processors in the nodes.
cannam@127 149 </p>
cannam@127 150 <p>This may or may not be faster than simply using as many MPI processes
cannam@127 151 as you have processors, however. On the one hand, using threads
cannam@127 152 within a node eliminates the need for explicit message passing within
cannam@127 153 the node. On the other hand, FFTW&rsquo;s transpose routines are not
cannam@127 154 multi-threaded, and this means that the communications that do take
cannam@127 155 place will not benefit from parallelization within the node.
cannam@127 156 Moreover, many MPI implementations already have optimizations to
cannam@127 157 exploit shared memory when it is available, so adding the
cannam@127 158 multithreaded FFTW on top of this may be superfluous.
cannam@127 159 <a name="index-transpose-4"></a>
cannam@127 160 </p>
cannam@127 161 <hr>
cannam@127 162 <div class="header">
cannam@127 163 <p>
cannam@127 164 Next: <a href="FFTW-MPI-Reference.html#FFTW-MPI-Reference" accesskey="n" rel="next">FFTW MPI Reference</a>, Previous: <a href="FFTW-MPI-Performance-Tips.html#FFTW-MPI-Performance-Tips" accesskey="p" rel="prev">FFTW MPI Performance Tips</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> &nbsp; [<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>
cannam@127 165 </div>
cannam@127 166
cannam@127 167
cannam@127 168
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cannam@127 170 </html>