On Fri, 2008-04-18 at 14:34 -0700, 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 memory > 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.
Ingo Wald's work on interactive coherent raytracing springs to mind. http://www.sci.utah.edu/~wald/Publications/ "Interactive Rendering with Coherent Raytracing" http://graphics.cs.uni-sb.de/~wald/Publications/EG2001_IRCRT/InteractiveRenderingWithCoherentRayTracing.pdf is a decent, if dated, introduction. He clearly has much more newer stuff as well. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
