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Add FFTW 3.3.8 source, and a Linux build
author Chris Cannam
date Tue, 19 Nov 2019 14:52:55 +0000
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Chris@82 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Chris@82 3 <!-- This manual is for FFTW
Chris@82 4 (version 3.3.8, 24 May 2018).
Chris@82 5
Chris@82 6 Copyright (C) 2003 Matteo Frigo.
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Chris@82 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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Chris@82 24 <head>
Chris@82 25 <title>FFTW 3.3.8: Combining MPI and Threads</title>
Chris@82 26
Chris@82 27 <meta name="description" content="FFTW 3.3.8: Combining MPI and Threads">
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Chris@82 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
Chris@82 36 <link href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="up" title="Distributed-memory FFTW with MPI">
Chris@82 37 <link href="FFTW-MPI-Reference.html#FFTW-MPI-Reference" rel="next" title="FFTW MPI Reference">
Chris@82 38 <link href="FFTW-MPI-Performance-Tips.html#FFTW-MPI-Performance-Tips" rel="prev" title="FFTW MPI Performance Tips">
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Chris@82 68 </head>
Chris@82 69
Chris@82 70 <body lang="en">
Chris@82 71 <a name="Combining-MPI-and-Threads"></a>
Chris@82 72 <div class="header">
Chris@82 73 <p>
Chris@82 74 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>
Chris@82 75 </div>
Chris@82 76 <hr>
Chris@82 77 <a name="Combining-MPI-and-Threads-1"></a>
Chris@82 78 <h3 class="section">6.11 Combining MPI and Threads</h3>
Chris@82 79 <a name="index-threads-2"></a>
Chris@82 80
Chris@82 81 <p>In certain cases, it may be advantageous to combine MPI
Chris@82 82 (distributed-memory) and threads (shared-memory) parallelization.
Chris@82 83 FFTW supports this, with certain caveats. For example, if you have a
Chris@82 84 cluster of 4-processor shared-memory nodes, you may want to use
Chris@82 85 threads within the nodes and MPI between the nodes, instead of MPI for
Chris@82 86 all parallelization.
Chris@82 87 </p>
Chris@82 88 <p>In particular, it is possible to seamlessly combine the MPI FFTW
Chris@82 89 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,
Chris@82 90 which should look something like this:
Chris@82 91 </p>
Chris@82 92 <div class="example">
Chris@82 93 <pre class="example">int threads_ok;
Chris@82 94
Chris@82 95 int main(int argc, char **argv)
Chris@82 96 {
Chris@82 97 int provided;
Chris@82 98 MPI_Init_thread(&amp;argc, &amp;argv, MPI_THREAD_FUNNELED, &amp;provided);
Chris@82 99 threads_ok = provided &gt;= MPI_THREAD_FUNNELED;
Chris@82 100
Chris@82 101 if (threads_ok) threads_ok = fftw_init_threads();
Chris@82 102 fftw_mpi_init();
Chris@82 103
Chris@82 104 ...
Chris@82 105 if (threads_ok) fftw_plan_with_nthreads(...);
Chris@82 106 ...
Chris@82 107
Chris@82 108 MPI_Finalize();
Chris@82 109 }
Chris@82 110 </pre></div>
Chris@82 111 <a name="index-fftw_005fmpi_005finit-3"></a>
Chris@82 112 <a name="index-fftw_005finit_005fthreads-2"></a>
Chris@82 113 <a name="index-fftw_005fplan_005fwith_005fnthreads-1"></a>
Chris@82 114
Chris@82 115 <p>First, note that instead of calling <code>MPI_Init</code>, you should call
Chris@82 116 <code>MPI_Init_threads</code>, which is the initialization routine defined
Chris@82 117 by the MPI-2 standard to indicate to MPI that your program will be
Chris@82 118 multithreaded. We pass <code>MPI_THREAD_FUNNELED</code>, which indicates
Chris@82 119 that we will only call MPI routines from the main thread. (FFTW will
Chris@82 120 launch additional threads internally, but the extra threads will not
Chris@82 121 call MPI code.) (You may also pass <code>MPI_THREAD_SERIALIZED</code> or
Chris@82 122 <code>MPI_THREAD_MULTIPLE</code>, which requests additional multithreading
Chris@82 123 support from the MPI implementation, but this is not required by
Chris@82 124 FFTW.) The <code>provided</code> parameter returns what level of threads
Chris@82 125 support is actually supported by your MPI implementation; this
Chris@82 126 <em>must</em> be at least <code>MPI_THREAD_FUNNELED</code> if you want to call
Chris@82 127 the FFTW threads routines, so we define a global variable
Chris@82 128 <code>threads_ok</code> to record this. You should only call
Chris@82 129 <code>fftw_init_threads</code> or <code>fftw_plan_with_nthreads</code> if
Chris@82 130 <code>threads_ok</code> is true. For more information on thread safety in
Chris@82 131 MPI, see the
Chris@82 132 <a href="http://www.mpi-forum.org/docs/mpi-20-html/node162.htm">MPI and
Chris@82 133 Threads</a> section of the MPI-2 standard.
Chris@82 134 <a name="index-thread-safety-2"></a>
Chris@82 135 </p>
Chris@82 136
Chris@82 137 <p>Second, we must call <code>fftw_init_threads</code> <em>before</em>
Chris@82 138 <code>fftw_mpi_init</code>. This is critical for technical reasons having
Chris@82 139 to do with how FFTW initializes its list of algorithms.
Chris@82 140 </p>
Chris@82 141 <p>Then, if you call <code>fftw_plan_with_nthreads(N)</code>, <em>every</em> MPI
Chris@82 142 process will launch (up to) <code>N</code> threads to parallelize its transforms.
Chris@82 143 </p>
Chris@82 144 <p>For example, in the hypothetical cluster of 4-processor nodes, you
Chris@82 145 might wish to launch only a single MPI process per node, and then call
Chris@82 146 <code>fftw_plan_with_nthreads(4)</code> on each process to use all
Chris@82 147 processors in the nodes.
Chris@82 148 </p>
Chris@82 149 <p>This may or may not be faster than simply using as many MPI processes
Chris@82 150 as you have processors, however. On the one hand, using threads
Chris@82 151 within a node eliminates the need for explicit message passing within
Chris@82 152 the node. On the other hand, FFTW&rsquo;s transpose routines are not
Chris@82 153 multi-threaded, and this means that the communications that do take
Chris@82 154 place will not benefit from parallelization within the node.
Chris@82 155 Moreover, many MPI implementations already have optimizations to
Chris@82 156 exploit shared memory when it is available, so adding the
Chris@82 157 multithreaded FFTW on top of this may be superfluous.
Chris@82 158 <a name="index-transpose-4"></a>
Chris@82 159 </p>
Chris@82 160 <hr>
Chris@82 161 <div class="header">
Chris@82 162 <p>
Chris@82 163 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>
Chris@82 164 </div>
Chris@82 165
Chris@82 166
Chris@82 167
Chris@82 168 </body>
Chris@82 169 </html>