[Haskell-cafe] announce: Glome.hs-0.3 (Haskell raytracer)
dons at galois.com
Fri Apr 18 19:25:53 EDT 2008
> David Roundy wrote:
> >On Fri, Apr 18, 2008 at 02:09:28PM -0700, Jim Snow wrote:
> >>On a particular scene with one instance of the single-threaded renderer
> >>running, it takes about 19 seconds to render an image. With two
> >>instances running, they each take about 23 seconds. This is on an
> >>Athlon-64 3800+ dual core, with 512kB of L2 cache per core. So, it
> >>seems my memory really is slowing things down noticeably.
> >This doesn't mean there's no hope, it just means that you'll need to be
> >extra-clever if you're to get a speedup that is close to optimal. The key
> >to overcoming memory bandwidth issues is to think about cache use and
> >figure out how to improve it. For instance, O(N^3) matrix multiplication
> >can be done in close to O(N^2) time provided it's memory-limited, by
> >blocking memory accesses so that you access less memory at once.
> >In the case of ray-tracing I've little idea where or how you could improve
> >memory access patterns, but this is what you should be thinking about (if
> >you want to optimize this code). Of course, improving overall scaling is
> >best (e.g. avoiding using lists if you need random access). Next I'd ask
> >if there are more efficent or compact data structures that you could be
> >using. If your program uses less memory, a greater fraction of that
> >will fit into cache. Switching to stricter data structures and turning on
> >-funbox-strict-fields (or whatever it's called) may help localize your
> >memory access. Even better if you could manage to use unboxed arrays, so
> >that your memory accesses really would be localized (assuming that you
> >actually do have localize memory-access patterns).
> >Of course, also ask yourself how much memory your program is using in
> >total. If it's not much more than 512kB, for instance, we may have
> >misdiagnosed your problem.
> Interestingly, switching between "Float" and "Double" doesn't make any
> noticeable difference in speed (though I see more rendering artifacts
> with Float). Transformation matrices are memory hogs, at 24 floats each
> (a 4x4 matrix and its inverse with the bottom rows omitted (they're
> always 0 0 0 1)). This may be one reason why many real-time ray tracers
> just stick with triangles; a triangle can be transformed by transforming
> its verticies, and then you can throw the matrix away.
The only differences I'd expect to see here would
be with -fvia-C -fexcess-precision -O2 -optc-O2
which might trigger some SSE stuff from the C compiler
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