159 lines
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159 lines
7.3 KiB
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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<title>Combining MPI and Threads (FFTW 3.3.10)</title>
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Distributed_002dmemory-FFTW-with-MPI.html" rel="up" title="Distributed-memory FFTW with MPI">
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<link href="FFTW-MPI-Reference.html" rel="next" title="FFTW MPI Reference">
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<link href="FFTW-MPI-Performance-Tips.html" rel="prev" title="FFTW MPI Performance Tips">
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<body lang="en">
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<span id="Combining-MPI-and-Threads"></span><div class="header">
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<p>
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Next: <a href="FFTW-MPI-Reference.html" accesskey="n" rel="next">FFTW MPI Reference</a>, Previous: <a href="FFTW-MPI-Performance-Tips.html" accesskey="p" rel="prev">FFTW MPI Performance Tips</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Combining-MPI-and-Threads-1"></span><h3 class="section">6.11 Combining MPI and Threads</h3>
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<span id="index-threads-2"></span>
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<p>In certain cases, it may be advantageous to combine MPI
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(distributed-memory) and threads (shared-memory) parallelization.
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FFTW supports this, with certain caveats. For example, if you have a
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cluster of 4-processor shared-memory nodes, you may want to use
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threads within the nodes and MPI between the nodes, instead of MPI for
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all parallelization.
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</p>
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<p>In particular, it is possible to seamlessly combine the MPI FFTW
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routines with the multi-threaded FFTW routines (see <a href="Multi_002dthreaded-FFTW.html">Multi-threaded FFTW</a>). However, some care must be taken in the initialization code,
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which should look something like this:
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</p>
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<div class="example">
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<pre class="example">int threads_ok;
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int main(int argc, char **argv)
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{
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int provided;
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MPI_Init_thread(&argc, &argv, MPI_THREAD_FUNNELED, &provided);
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threads_ok = provided >= MPI_THREAD_FUNNELED;
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if (threads_ok) threads_ok = fftw_init_threads();
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fftw_mpi_init();
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...
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if (threads_ok) fftw_plan_with_nthreads(...);
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...
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MPI_Finalize();
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}
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</pre></div>
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<span id="index-fftw_005fmpi_005finit-3"></span>
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<span id="index-fftw_005finit_005fthreads-2"></span>
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<span id="index-fftw_005fplan_005fwith_005fnthreads-1"></span>
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<p>First, note that instead of calling <code>MPI_Init</code>, you should call
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<code>MPI_Init_threads</code>, which is the initialization routine defined
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by the MPI-2 standard to indicate to MPI that your program will be
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multithreaded. We pass <code>MPI_THREAD_FUNNELED</code>, which indicates
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that we will only call MPI routines from the main thread. (FFTW will
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launch additional threads internally, but the extra threads will not
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call MPI code.) (You may also pass <code>MPI_THREAD_SERIALIZED</code> or
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<code>MPI_THREAD_MULTIPLE</code>, which requests additional multithreading
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support from the MPI implementation, but this is not required by
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FFTW.) The <code>provided</code> parameter returns what level of threads
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support is actually supported by your MPI implementation; this
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<em>must</em> be at least <code>MPI_THREAD_FUNNELED</code> if you want to call
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the FFTW threads routines, so we define a global variable
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<code>threads_ok</code> to record this. You should only call
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<code>fftw_init_threads</code> or <code>fftw_plan_with_nthreads</code> if
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<code>threads_ok</code> is true. For more information on thread safety in
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MPI, see the
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<a href="http://www.mpi-forum.org/docs/mpi-20-html/node162.htm">MPI and
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Threads</a> section of the MPI-2 standard.
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<span id="index-thread-safety-2"></span>
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</p>
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<p>Second, we must call <code>fftw_init_threads</code> <em>before</em>
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<code>fftw_mpi_init</code>. This is critical for technical reasons having
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to do with how FFTW initializes its list of algorithms.
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</p>
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<p>Then, if you call <code>fftw_plan_with_nthreads(N)</code>, <em>every</em> MPI
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process will launch (up to) <code>N</code> threads to parallelize its transforms.
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</p>
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<p>For example, in the hypothetical cluster of 4-processor nodes, you
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might wish to launch only a single MPI process per node, and then call
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<code>fftw_plan_with_nthreads(4)</code> on each process to use all
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processors in the nodes.
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</p>
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<p>This may or may not be faster than simply using as many MPI processes
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as you have processors, however. On the one hand, using threads
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within a node eliminates the need for explicit message passing within
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the node. On the other hand, FFTW’s transpose routines are not
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multi-threaded, and this means that the communications that do take
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place will not benefit from parallelization within the node.
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Moreover, many MPI implementations already have optimizations to
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exploit shared memory when it is available, so adding the
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multithreaded FFTW on top of this may be superfluous.
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<span id="index-transpose-4"></span>
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="FFTW-MPI-Reference.html" accesskey="n" rel="next">FFTW MPI Reference</a>, Previous: <a href="FFTW-MPI-Performance-Tips.html" accesskey="p" rel="prev">FFTW MPI Performance Tips</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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