On Thu, Jul 31, 2014 at 03:01:17PM -0400, Neil Horman wrote: > On Thu, Jul 31, 2014 at 11:36:32AM -0700, Bruce Richardson wrote: > > > > I think a good first step here that I can't see anyone objecting to is > > to enable the ixgbe driver to use the vector code path for a generic > > x86_64 build. I've run a quick test here, and changing "_mm_popcnt_u64" > > to "__builtin_popcountll" [and the include from nmmintrin to tmmintrin] > > allows a compile for machine type default, and testpmd can still forward > > packets at a good rate (roughly perf down about 10% vs native compile on > > SNB). > > The ACL is a tougher nut to crack, but anyone see any issues with that > > two-line change to ixgbe_rxtx_vec.c? [Neil, since you started the patch > > set thread, do you want to submit an official patch here, or would you > > prefer I > > do so?] > > > > I'm happy to do so, Though 10% performance degradation vs. using the sse4.2 > instructions in that path seems significant, isn't it? Given that performance > delta, it seems like it would still be preferable to have a path that used the > sse4.2 instructions when they're available. Or am I misreading what you mean > when you say down 10% > > Neil > Ok, I did a little bit more testing here. Using the vector pmd compiled for generic x86_64 and using __builtin_popcountll is approx 35% faster for packet IO than the existing fast-path functions. It is also 7% (a bit lower than ~10% as I originally stated) slower than the existing native-compiled vpmd on a Sandy Bridge platform.
I then ran an extra test, using EXTRA_CFLAGS='-msse4.2' to turn on the extra instructions. The ~7% performance drop went to ~3%, so we would gain a little more with using SSE4.2, but compared to the gain from having the vector driver at all, it's not that much. [I don't have a system handy with AVX2 support to see what boosts might come from compiling with that instruction set enabled.] Because of this, I'd take the ~35% speed boost for now, and try and find what would be the best general way to solve this problem across all libraries. Also, I think that anyone who needs that extra 4% performance probably wants the other 3% too, and so will compile up the code from source using the "native" compilation target. :-) /Bruce
