On 2 August 2010 15:41, David Simcha <[email protected]> wrote: > Unfortunately I just downloaded the benchmark program for FFTW and my FFT is > a ton slower, depending on how you look at it. Using size 2^20 as my > benchmark, FFTW takes about 131 seconds to create its plan, even using > -oestimate, the fastest planner. However, the plan can be reused if > performing multiple FFTs of the same size, and once the plan is created, it > can do an FFT of size 2^20 in about 53 milliseconds (which I find almost > unbelievable because even sorting an array of size 2^20 using a > well-optimized quick sort takes almost that long, and FFT seems like it > should should have a much larger constant than quick sort), compared to my > latest improvements to around 730 on the hardware I'm benchmarking on. > > On the other hand, for one-off use cases, the lack of needing to create a > plan is a big win, both from a speed and a simplicity of API point of view. > Benchmarking against R, which doesn't appear to use plans, making the > comparison somewhat more relevant, things look better for my FFT: R takes > about 610 milliseconds for a size 2^20 pure real FFT.
All you're seeing is the L2 cache. Did you see the link I posted to the NG about the internals of FFTW? The graph at the top shows FFTW is 40 times faster than the 'numerical recipes' code for 2^^20. So your factor of 13 isn't so bad. Based on that graph, if you reduce it to say 2^^15, the factor should drop significantly. Adding a little bit of cache awareness (using core.cpuid) should be able to avoid the performance cliff. Also, DMD's floating point optimiser is so primitive, you lose up to a factor of two immediately. _______________________________________________ phobos mailing list [email protected] http://lists.puremagic.com/mailman/listinfo/phobos
