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<title>Cell Caveats - FFTW 3.2.2</title>
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<h3 class="section">6.2 Cell Caveats</h3>
<ul>
<li>The FFTW benchmark program allocates memory using malloc() or
equivalent library calls, reflecting the common usage of the FFTW
library. However, you can sometimes improve performance significantly
by allocating memory in system-specific large TLB pages. E.g., we
have seen 39 GFLOPS/s for a 256 × 256 × 256 problem using
large pages, whereas the speed is about 25 GFLOPS/s with normal pages.
YMMV.
<li>FFTW hoards all available SPEs for itself. You can optionally
choose a different number of SPEs by calling the undocumented
function <code>fftw_cell_set_nspe(n)</code>, where <code>n</code> is the number of desired
SPEs. Expect this interface to go away once we figure out how to
make FFTW play nicely with other Cell software.
<p>In particular, if you try to link both the single and double precision
of FFTW in the same program (which you can do), they will both try
to grab all SPEs and the second one will hang.
<li>The SPEs demand that data be stored in contiguous arrays aligned at
16-byte boundaries. If you instruct FFTW to operate on
noncontiguous or nonaligned data, the SPEs will not be used,
resulting in slow execution. See <a href="Data-Alignment.html#Data-Alignment">Data Alignment</a>.
<li>The <code>FFTW_ESTIMATE</code> mode may produce seriously suboptimal plans, and
it becomes particularly confused if you enable both the SPEs and
Altivec. If you care about performance, please use <code>FFTW_MEASURE</code>
or <code>FFTW_PATIENT</code> until we figure out a more reliable performance model.
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