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Add FFTW 3.3.8 source, and a Linux build
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 19 Nov 2019 14:52:55 +0000
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cannam@167 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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cannam@167 3 <!-- This manual is for FFTW
cannam@167 4 (version 3.3.8, 24 May 2018).
cannam@167 5
cannam@167 6 Copyright (C) 2003 Matteo Frigo.
cannam@167 7
cannam@167 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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cannam@167 10 Permission is granted to make and distribute verbatim copies of this
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cannam@167 23 <!-- Created by GNU Texinfo 6.3, http://www.gnu.org/software/texinfo/ -->
cannam@167 24 <head>
cannam@167 25 <title>FFTW 3.3.8: Combining MPI and Threads</title>
cannam@167 26
cannam@167 27 <meta name="description" content="FFTW 3.3.8: Combining MPI and Threads">
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cannam@167 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
cannam@167 36 <link href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="up" title="Distributed-memory FFTW with MPI">
cannam@167 37 <link href="FFTW-MPI-Reference.html#FFTW-MPI-Reference" rel="next" title="FFTW MPI Reference">
cannam@167 38 <link href="FFTW-MPI-Performance-Tips.html#FFTW-MPI-Performance-Tips" rel="prev" title="FFTW MPI Performance Tips">
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cannam@167 66
cannam@167 67
cannam@167 68 </head>
cannam@167 69
cannam@167 70 <body lang="en">
cannam@167 71 <a name="Combining-MPI-and-Threads"></a>
cannam@167 72 <div class="header">
cannam@167 73 <p>
cannam@167 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>
cannam@167 75 </div>
cannam@167 76 <hr>
cannam@167 77 <a name="Combining-MPI-and-Threads-1"></a>
cannam@167 78 <h3 class="section">6.11 Combining MPI and Threads</h3>
cannam@167 79 <a name="index-threads-2"></a>
cannam@167 80
cannam@167 81 <p>In certain cases, it may be advantageous to combine MPI
cannam@167 82 (distributed-memory) and threads (shared-memory) parallelization.
cannam@167 83 FFTW supports this, with certain caveats. For example, if you have a
cannam@167 84 cluster of 4-processor shared-memory nodes, you may want to use
cannam@167 85 threads within the nodes and MPI between the nodes, instead of MPI for
cannam@167 86 all parallelization.
cannam@167 87 </p>
cannam@167 88 <p>In particular, it is possible to seamlessly combine the MPI FFTW
cannam@167 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,
cannam@167 90 which should look something like this:
cannam@167 91 </p>
cannam@167 92 <div class="example">
cannam@167 93 <pre class="example">int threads_ok;
cannam@167 94
cannam@167 95 int main(int argc, char **argv)
cannam@167 96 {
cannam@167 97 int provided;
cannam@167 98 MPI_Init_thread(&amp;argc, &amp;argv, MPI_THREAD_FUNNELED, &amp;provided);
cannam@167 99 threads_ok = provided &gt;= MPI_THREAD_FUNNELED;
cannam@167 100
cannam@167 101 if (threads_ok) threads_ok = fftw_init_threads();
cannam@167 102 fftw_mpi_init();
cannam@167 103
cannam@167 104 ...
cannam@167 105 if (threads_ok) fftw_plan_with_nthreads(...);
cannam@167 106 ...
cannam@167 107
cannam@167 108 MPI_Finalize();
cannam@167 109 }
cannam@167 110 </pre></div>
cannam@167 111 <a name="index-fftw_005fmpi_005finit-3"></a>
cannam@167 112 <a name="index-fftw_005finit_005fthreads-2"></a>
cannam@167 113 <a name="index-fftw_005fplan_005fwith_005fnthreads-1"></a>
cannam@167 114
cannam@167 115 <p>First, note that instead of calling <code>MPI_Init</code>, you should call
cannam@167 116 <code>MPI_Init_threads</code>, which is the initialization routine defined
cannam@167 117 by the MPI-2 standard to indicate to MPI that your program will be
cannam@167 118 multithreaded. We pass <code>MPI_THREAD_FUNNELED</code>, which indicates
cannam@167 119 that we will only call MPI routines from the main thread. (FFTW will
cannam@167 120 launch additional threads internally, but the extra threads will not
cannam@167 121 call MPI code.) (You may also pass <code>MPI_THREAD_SERIALIZED</code> or
cannam@167 122 <code>MPI_THREAD_MULTIPLE</code>, which requests additional multithreading
cannam@167 123 support from the MPI implementation, but this is not required by
cannam@167 124 FFTW.) The <code>provided</code> parameter returns what level of threads
cannam@167 125 support is actually supported by your MPI implementation; this
cannam@167 126 <em>must</em> be at least <code>MPI_THREAD_FUNNELED</code> if you want to call
cannam@167 127 the FFTW threads routines, so we define a global variable
cannam@167 128 <code>threads_ok</code> to record this. You should only call
cannam@167 129 <code>fftw_init_threads</code> or <code>fftw_plan_with_nthreads</code> if
cannam@167 130 <code>threads_ok</code> is true. For more information on thread safety in
cannam@167 131 MPI, see the
cannam@167 132 <a href="http://www.mpi-forum.org/docs/mpi-20-html/node162.htm">MPI and
cannam@167 133 Threads</a> section of the MPI-2 standard.
cannam@167 134 <a name="index-thread-safety-2"></a>
cannam@167 135 </p>
cannam@167 136
cannam@167 137 <p>Second, we must call <code>fftw_init_threads</code> <em>before</em>
cannam@167 138 <code>fftw_mpi_init</code>. This is critical for technical reasons having
cannam@167 139 to do with how FFTW initializes its list of algorithms.
cannam@167 140 </p>
cannam@167 141 <p>Then, if you call <code>fftw_plan_with_nthreads(N)</code>, <em>every</em> MPI
cannam@167 142 process will launch (up to) <code>N</code> threads to parallelize its transforms.
cannam@167 143 </p>
cannam@167 144 <p>For example, in the hypothetical cluster of 4-processor nodes, you
cannam@167 145 might wish to launch only a single MPI process per node, and then call
cannam@167 146 <code>fftw_plan_with_nthreads(4)</code> on each process to use all
cannam@167 147 processors in the nodes.
cannam@167 148 </p>
cannam@167 149 <p>This may or may not be faster than simply using as many MPI processes
cannam@167 150 as you have processors, however. On the one hand, using threads
cannam@167 151 within a node eliminates the need for explicit message passing within
cannam@167 152 the node. On the other hand, FFTW&rsquo;s transpose routines are not
cannam@167 153 multi-threaded, and this means that the communications that do take
cannam@167 154 place will not benefit from parallelization within the node.
cannam@167 155 Moreover, many MPI implementations already have optimizations to
cannam@167 156 exploit shared memory when it is available, so adding the
cannam@167 157 multithreaded FFTW on top of this may be superfluous.
cannam@167 158 <a name="index-transpose-4"></a>
cannam@167 159 </p>
cannam@167 160 <hr>
cannam@167 161 <div class="header">
cannam@167 162 <p>
cannam@167 163 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>
cannam@167 164 </div>
cannam@167 165
cannam@167 166
cannam@167 167
cannam@167 168 </body>
cannam@167 169 </html>