Hey, 2011/8/16 Borislav Petkov : > On Mon, Aug 15, 2011 at 10:34:35PM -0400, Valdis.Kletnieks@vt.edu wrote: >> On Sun, 14 Aug 2011 11:59:10 +0200, Borislav Petkov said: >> >> > Benchmarking with 10000 iterations, average results: >> > size XM MM speedup >> > 119 540.58 449.491 0.8314969419 >> >> > 12273 2307.86 4042.88 1.751787902 >> > 13924 2431.8 4224.48 1.737184756 >> > 14335 2469.4 4218.82 1.708440514 >> > 15018 2675.67 1904.07 0.711622886 >> > 16374 2989.75 5296.26 1.771470902 >> > 24564 4262.15 7696.86 1.805863077 >> > 27852 4362.53 3347.72 0.7673805572 >> > 28672 5122.8 7113.14 1.388524413 >> > 30033 4874.62 8740.04 1.792967931 >> >> The numbers for 15018 and 27852 are *way* odd for the MM case. I don't feel >> really good about this till we understand what happened for those two cases. > > Yep. > >> Also, anytime I see "10000 iterations", I ask myself if the benchmark >> rigging took proper note of hot/cold cache issues. That *may* explain >> the two oddball results we see above - but not knowing more about how >> it was benched, it's hard to say. > > Yeah, the more scrutiny this gets the better. So I've cleaned up my > setup and have attached it. > > xm_mem.c does the benchmarking and in bench_memcpy() there's the > sse_memcpy call which is the SSE memcpy implementation using inline asm. > It looks like gcc produces pretty crappy code here because if I replace > the sse_memcpy call with xm_memcpy() from xm_memcpy.S - this is the > same function but in pure asm - I get much better numbers, sometimes > even over 2x. It all depends on the alignment of the buffers though. > Also, those numbers don't include the context saving/restoring which the > kernel does for us. > > 7491 1509.89 2346.94 1.554378381 > 8170 2166.81 2857.78 1.318890326 > 12277 2659.03 4179.31 1.571744176 > 13907 2571.24 4125.7 1.604558427 > 14319 2638.74 5799.67 2.19789466 <---- > 14993 2752.42 4413.85 1.603625603 > 16371 3479.11 5562.65 1.59887055 This work intrigued me, in some cases kernel memcpy was a lot faster than sse memcpy, and I finally figured out why. I also extended the test to an optimized avx memcpy, but I think the kernel memcpy will always win in the aligned case. Those numbers you posted aren't right it seems. It depends a lot on the alignment, for example if both are aligned to 64 relative to each other, kernel memcpy will win from avx memcpy on my machine. I replaced the malloc calls with memalign(65536, size + 256) so I could toy around with the alignments a little. This explains why for some sizes, kernel memcpy was faster than sse memcpy in the test results you had. When (src & 63 == dst & 63), it seems that kernel memcpy always wins, otherwise avx memcpy might. If you want to speed up memcpy, I think your best bet is to find out why it's so much slower when src and dst aren't 64-byte aligned compared to each other. Cheers, Maarten --- Attached: my modified version of the sse memcpy you posted. I changed it a bit, and used avx, but some of the other changes might be better for your sse memcpy too.