On Sun, Oct 09, 2016 at 12:09:07PM +0000, Wang, Zhihong wrote:
> > > > Tested with testpmd, host: txonly, guest: rxonly
> > > > size (bytes) improvement (%)
> > > > 64 4.12
> > > > 128 6
> > > > 256 2.65
> > > > 512 -1.12
> > > > 1024 -7.02
> > >
> > > There is a difference between Zhihong's code and the old I spotted in
> > > the first time: Zhihong removed the avail_idx prefetch. I understand
> > > the prefetch becomes a bit tricky when mrg-rx code path is considered;
> > > thus, I didn't comment on that.
> > >
> > > That's one of the difference that, IMO, could drop a regression. I then
> > > finally got a chance to add it back.
> > >
> > > A rough test shows it improves the performance of 1400B packet size
> > greatly
> > > in the "txonly in host and rxonly in guest" case: +33% is the number I get
> > > with my test server (Ivybridge).
> >
> > Thanks Yuanhan! I'll validate this on x86.
>
> Hi Yuanhan,
>
> Seems your code doesn't perform correctly. I write a new version
> of avail idx prefetch but didn't see any perf benefit.
>
> To be honest I doubt the benefit of this idea. The previous mrg_off
> code has this method but doesn't give any benefits.
Good point. I thought of that before, too. But you know that I made it
in rush, that I didn't think further and test more.
I looked the code a bit closer this time, and spotted a bug: the prefetch
actually didn't happen, due to following code piece:
if (vq->next_avail_idx >= NR_AVAIL_IDX_PREFETCH) {
prefetch_avail_idx(vq);
...
}
Since vq->next_avail_idx is set to 0 at the entrance of enqueue path,
prefetch_avail_idx() will be called. The fix is easy though: just put
prefetch_avail_idx before invoking enqueue_packet.
In summary, Zhihong is right, I see no more gains with that fix :(
However, as stated, that's kind of the only difference I found between
yours and the old code, that maybe it's still worthwhile to have a
test on ARM, Jianbo?
--yliu
> Even if this is useful, the benefits should be more significant for
> small packets, it's unlikely this simple idx prefetch could bring
> over 30% perf gain for large packets like 1400B ones.
>
> But if you really do work it out like that I'll be very glad to see.
>
> Thanks
> Zhihong
>
> >
> > >
> > > I guess this might/would help your case as well. Mind to have a test
> > > and tell me the results?
> > >
> > > BTW, I made it in rush; I haven't tested the mrg-rx code path yet.
> > >
> > > Thanks.
> > >
> > > --yliu
