On 10 October 2016 at 14:22, Wang, Zhihong <zhihong.wang at intel.com> wrote: > > >> -----Original Message----- >> From: Jianbo Liu [mailto:jianbo.liu at linaro.org] >> Sent: Monday, October 10, 2016 1:32 PM >> To: Yuanhan Liu <yuanhan.liu at linux.intel.com> >> Cc: Wang, Zhihong <zhihong.wang at intel.com>; Maxime Coquelin >> <maxime.coquelin at redhat.com>; dev at dpdk.org >> Subject: Re: [dpdk-dev] [PATCH v3 0/5] vhost: optimize enqueue >> >> On 10 October 2016 at 10:44, Yuanhan Liu <yuanhan.liu at linux.intel.com> >> wrote: >> > 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? >> > >> I haven't tested it, but I think it could be no improvement for ARM either. >> >> A smalll suggestion for enqueue_packet: >> >> ..... >> + /* start copy from mbuf to desc */ >> + while (mbuf_avail || mbuf->next) { >> ..... >> >> Considering pkt_len is in the first cache line (same as data_len), >> while next pointer is in the second cache line, >> is it better to check the total packet len, instead of the last mbuf's >> next pointer to jump out of while loop and avoid possible cache miss? > > Jianbo, > > Thanks for the reply! > > This idea sounds good, but it won't help the general perf in my > opinion, since the 2nd cache line is accessed anyway prior in > virtio_enqueue_offload. > Yes, you are right. I'm thinking of prefetching beforehand. And if it's a chained mbuf, virtio_enqueue_offload will not be called in next loop.
> Also this would bring a NULL check when actually access mbuf->next. > > BTW, could you please publish the number of: > > 1. mrg_rxbuf=on, comparison between original and original + this patch > > 2. mrg_rxbuf=off, comparison between original and original + this patch > > So we can have a whole picture of how this patch impact on ARM platform. > I think you already have got many results in my previous emails. Sorry I can't test right now and busy with other things.
