[sldev] [patch] OpenJPEG DWT optimizations

[Callum Lerwick]

On Wed, 2007-04-04 at 00:30 -0700, Dzonatas wrote:
> This means over 50% of the time spent in the the OpenJPEG DWT is in the 
> overhead of data manipulation.

One thing I've noticed is OpenJPEG will eat a hell of a lot of RAM,
roughly proportionate to the image size. Whereas KDU doesn't even make
my RAM meter budge, even on the biggest image I have! OpenJPEG has a bit
of a problem with allocating huge data structures all over the place,
and that means cache thrashing any way you slice it. The DWT seems to
thrash the cache quite badly, and this is probably the source of much of
the slowness you're seeing.

Check out my latest oprofile run of slviewer. This is from the end of
dwt_decode_real:

 * CPU: PIII, speed 1328.96 MHz (estimated)
 * Counted CPU_CLK_UNHALTED events (clocks processor is not halted)
 * Counted DCU_LINES_IN events (total lines allocated in the DCU)

   1666  0.0120    64  0.0861 : for (j = 0; j < rw; ++j) {
                              :    aj = a + j;
   2825  0.0203    60  0.0807 :    dwt_interleave_v(aj, bj, dn, rh1, w, cas_row);
   1567  0.0113    47  0.0632 :    dwt_decode_1_real(bj, dn, rh1, cas_row);
1865412 13.4263 11704 15.7464 :    for (k = 0; k < rh; ++k)
   9018  0.0649   294  0.3955 :       aj[k * w] = bj[k];
                              :    }
                              : }

Second Life hits this loop hard. Its just a data copy loop! This needs
to be fixed one way or another, via rearranging data structures and just
plain cutting down the memory footprint of the working data buffers.
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