Re: [dpdk-dev] [PATCH 0/2] dpdk: Allow for dynamic enablement of some isolated features

[Neil Horman <nhorman@tuxdriver.com>]

On Thu, Jul 31, 2014 at 01:19:50PM -0700, Bruce Richardson wrote:
> 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. :-)
> 


Wait a moment, I'm not entirely sure what you did here.  I understand that you
replaced the _mm_popcnt_u64 call in the ixgbe pmd vector receive path with
__builtin_popcnt, which is good, but ixgbe also uses the __mm_shuffle_epi8
intrinsic which is only available with sse4.2 from what I can see. did you
replace those calls with a __builtin_shuffle variant?  Otherwise, how did you
get the pmd to build?  I'm asking because this is what I tried in the first pass
and Konstantin gave some pretty convicing evidence that this was an unworkable
solution:
http://dpdk.org/ml/archives/dev/2014-July/004443.html

Neil