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Tidy up for ROLI
author Geogaddi\David <d.m.ronan@qmul.ac.uk>
date Tue, 17 May 2016 18:50:19 +0100
parents 636c989477e7
children
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--- a/Lib/fftw-3.2.1/doc/html/.svn/text-base/Combining-MPI-and-Threads.html.svn-base	Wed May 04 11:02:59 2016 +0100
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-<title>Combining MPI and Threads - FFTW 3.2alpha3</title>
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-
-<h3 class="section">6.11 Combining MPI and Threads</h3>
-
-<p><a name="index-threads-398"></a>
-In certain cases, it may be advantageous to combine MPI
-(distributed-memory) and threads (shared-memory) parallelization. 
-FFTW supports this, with certain caveats.  For example, if you have a
-cluster of 4-processor shared-memory nodes, you may want to use
-threads within the nodes and MPI between the nodes, instead of MPI for
-all parallelization.  FFTW's MPI code can also transparently use
-FFTW's Cell processor support (e.g. for clusters of Cell processors). 
-<a name="index-Cell-processor-399"></a>
-In particular, it is possible to seamlessly combine the MPI FFTW
-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
-<code>fftw_mpi_init()</code> and <code>fftw_init_threads()</code>.  Then, if you
-call <code>fftw_plan_with_nthreads(N)</code>, then <em>every</em> MPI process
-will launch <code>N</code> threads to parallelize its transforms. 
-<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>
-For example, in the hypothetical cluster of 4-processor nodes, you
-might wish to launch only a single MPI process per node, and then call
-<code>fftw_plan_with_nthreads(4)</code> on each process to use all
-processors in the nodes.
-
-   <p>This may or may not be faster than simply using as many MPI processes
-as you have processors, however.  On the one hand, using threads within a
-node eliminates the need for explicit message passing within the node. 
-On the other hand, FFTW's transpose routines are not multi-threaded,
-and this means that the communications that do take place will not
-benefit from parallelization within the node.  Moreover, many MPI
-implementations already have optimizations to exploit shared memory
-when it is available. 
-<a name="index-transpose-403"></a>
-(Note that this is quite independent of whether MPI itself is
-thread-safe or multi-threaded: regardless of how many threads you
-specify with <code>fftw_plan_with_nthreads</code>, FFTW will perform all of
-its MPI communication only from the parent process.) 
-<a name="index-thread-safety-404"></a>
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