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author Chris Cannam <cannam@all-day-breakfast.com>
date Wed, 20 Mar 2013 15:35:50 +0000
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3 <title>Distributed-memory FFTW with MPI - FFTW 3.3.3</title>
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56
57 <h2 class="chapter">6 Distributed-memory FFTW with MPI</h2>
58
59 <p><a name="index-MPI-344"></a>
60 <a name="index-parallel-transform-345"></a>In this chapter we document the parallel FFTW routines for parallel
61 systems supporting the MPI message-passing interface. Unlike the
62 shared-memory threads described in the previous chapter, MPI allows
63 you to use <em>distributed-memory</em> parallelism, where each CPU has
64 its own separate memory, and which can scale up to clusters of many
65 thousands of processors. This capability comes at a price, however:
66 each process only stores a <em>portion</em> of the data to be
67 transformed, which means that the data structures and
68 programming-interface are quite different from the serial or threads
69 versions of FFTW.
70 <a name="index-data-distribution-346"></a>
71
72 <p>Distributed-memory parallelism is especially useful when you are
73 transforming arrays so large that they do not fit into the memory of a
74 single processor. The storage per-process required by FFTW's MPI
75 routines is proportional to the total array size divided by the number
76 of processes. Conversely, distributed-memory parallelism can easily
77 pose an unacceptably high communications overhead for small problems;
78 the threshold problem size for which parallelism becomes advantageous
79 will depend on the precise problem you are interested in, your
80 hardware, and your MPI implementation.
81
82 <p>A note on terminology: in MPI, you divide the data among a set of
83 &ldquo;processes&rdquo; which each run in their own memory address space.
84 Generally, each process runs on a different physical processor, but
85 this is not required. A set of processes in MPI is described by an
86 opaque data structure called a &ldquo;communicator,&rdquo; the most common of
87 which is the predefined communicator <code>MPI_COMM_WORLD</code> which
88 refers to <em>all</em> processes. For more information on these and
89 other concepts common to all MPI programs, we refer the reader to the
90 documentation at <a href="http://www.mcs.anl.gov/research/projects/mpi/">the MPI home page</a>.
91 <a name="index-MPI-communicator-347"></a><a name="index-MPI_005fCOMM_005fWORLD-348"></a>
92
93 <p>We assume in this chapter that the reader is familiar with the usage
94 of the serial (uniprocessor) FFTW, and focus only on the concepts new
95 to the MPI interface.
96
97 <ul class="menu">
98 <li><a accesskey="1" href="FFTW-MPI-Installation.html#FFTW-MPI-Installation">FFTW MPI Installation</a>
99 <li><a accesskey="2" href="Linking-and-Initializing-MPI-FFTW.html#Linking-and-Initializing-MPI-FFTW">Linking and Initializing MPI FFTW</a>
100 <li><a accesskey="3" href="2d-MPI-example.html#g_t2d-MPI-example">2d MPI example</a>
101 <li><a accesskey="4" href="MPI-Data-Distribution.html#MPI-Data-Distribution">MPI Data Distribution</a>
102 <li><a accesskey="5" href="Multi_002ddimensional-MPI-DFTs-of-Real-Data.html#Multi_002ddimensional-MPI-DFTs-of-Real-Data">Multi-dimensional MPI DFTs of Real Data</a>
103 <li><a accesskey="6" href="Other-Multi_002ddimensional-Real_002ddata-MPI-Transforms.html#Other-Multi_002ddimensional-Real_002ddata-MPI-Transforms">Other Multi-dimensional Real-data MPI Transforms</a>
104 <li><a accesskey="7" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes">FFTW MPI Transposes</a>
105 <li><a accesskey="8" href="FFTW-MPI-Wisdom.html#FFTW-MPI-Wisdom">FFTW MPI Wisdom</a>
106 <li><a accesskey="9" href="Avoiding-MPI-Deadlocks.html#Avoiding-MPI-Deadlocks">Avoiding MPI Deadlocks</a>
107 <li><a href="FFTW-MPI-Performance-Tips.html#FFTW-MPI-Performance-Tips">FFTW MPI Performance Tips</a>
108 <li><a href="Combining-MPI-and-Threads.html#Combining-MPI-and-Threads">Combining MPI and Threads</a>
109 <li><a href="FFTW-MPI-Reference.html#FFTW-MPI-Reference">FFTW MPI Reference</a>
110 <li><a href="FFTW-MPI-Fortran-Interface.html#FFTW-MPI-Fortran-Interface">FFTW MPI Fortran Interface</a>
111 </ul>
112
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