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

[Neil Horman <nhorman@tuxdriver.com>]

On Fri, Aug 01, 2014 at 01:56:24PM +0000, Ananyev, Konstantin wrote:
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Neil Horman
> > Sent: Friday, August 01, 2014 2:37 PM
> > To: Richardson, Bruce
> > Cc: dev@dpdk.org
> > Subject: Re: [dpdk-dev] [PATCH 0/2] dpdk: Allow for dynamic enablement of some isolated features
> > 
> > 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
> > 
> 
> I think that _mm_shuffle_epi8 (PSHUFB) is available starting from SSE3.
> So I presume, there is no need for replacement.
Ah, I see, its just because we're using the nmmintrinsic.h header.  We need to
replace the popcount instruction and change the include header to tmmintrins.h
to avoid the #error from the failed sse4.2 check

Thanks!
Neil

> Konstantin
>