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This manual is for FFTW
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(version 3.2alpha3, 14 August 2007).
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Copyright (C) 2003 Matteo Frigo.
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<h3 class="section">6.10 FFTW MPI Performance Tips</h3>
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<p>In this section, we collect a few tips on getting the best performance
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out of FFTW's MPI transforms.
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<p>First, because of the 1d block distribution, FFTW's parallelization is
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currently limited by the size of the first dimension.
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(Multidimensional block distributions may be supported by a future
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version.) More generally, you should ideally arrange the dimensions so
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that FFTW can divide them equally among the processes. See <a href="Load-balancing.html#Load-balancing">Load balancing</a>.
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<a name="index-block-distribution-394"></a><a name="index-load-balancing-395"></a>
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Second, if it is not too inconvenient, you should consider working
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with transposed output for multidimensional plans, as this saves a
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considerable amount of communications. See <a href="Transposed-distributions.html#Transposed-distributions">Transposed distributions</a>.
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<a name="index-transpose-396"></a>
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Third, the fastest choices are generally either an in-place transform
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or an out-of-place transform with the <code>FFTW_DESTROY_INPUT</code> flag
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(which allows the input array to be used as scratch space). In-place
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is especially beneficial if the amount of data per process is large.
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<a name="index-FFTW_005fDESTROY_005fINPUT-397"></a>
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Fourth, if you have multiple arrays to transform at once, rather than
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calling FFTW's MPI transforms several times it usually seems to be
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faster to interleave the data and use the advanced interface. (This
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groups the communications together instead of requiring separate
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messages for each transform.)
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