Messages in this thread | | | Subject | Re: [PATCH RFC 1/2] virtio-net: bql support | From | Jason Wang <> | Date | Mon, 7 Jan 2019 11:51:55 +0800 |
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On 2019/1/7 上午11:17, Michael S. Tsirkin wrote: > On Mon, Jan 07, 2019 at 10:14:37AM +0800, Jason Wang wrote: >> On 2019/1/2 下午9:59, Michael S. Tsirkin wrote: >>> On Wed, Jan 02, 2019 at 11:28:43AM +0800, Jason Wang wrote: >>>> On 2018/12/31 上午2:45, Michael S. Tsirkin wrote: >>>>> On Thu, Dec 27, 2018 at 06:00:36PM +0800, Jason Wang wrote: >>>>>> On 2018/12/26 下午11:19, Michael S. Tsirkin wrote: >>>>>>> On Thu, Dec 06, 2018 at 04:17:36PM +0800, Jason Wang wrote: >>>>>>>> On 2018/12/6 上午6:54, Michael S. Tsirkin wrote: >>>>>>>>> When use_napi is set, let's enable BQLs. Note: some of the issues are >>>>>>>>> similar to wifi. It's worth considering whether something similar to >>>>>>>>> commit 36148c2bbfbe ("mac80211: Adjust TSQ pacing shift") might be >>>>>>>>> benefitial. >>>>>>>> I've played a similar patch several days before. The tricky part is the mode >>>>>>>> switching between napi and no napi. We should make sure when the packet is >>>>>>>> sent and trakced by BQL, it should be consumed by BQL as well. I did it by >>>>>>>> tracking it through skb->cb. And deal with the freeze by reset the BQL >>>>>>>> status. Patch attached. >>>>>>>> >>>>>>>> But when testing with vhost-net, I don't very a stable performance, >>>>>>> So how about increasing TSQ pacing shift then? >>>>>> I can test this. But changing default TCP value is much more than a >>>>>> virtio-net specific thing. >>>>> Well same logic as wifi applies. Unpredictable latencies related >>>>> to radio in one case, to host scheduler in the other. >>>>> >>>>>>>> it was >>>>>>>> probably because we batch the used ring updating so tx interrupt may come >>>>>>>> randomly. We probably need to implement time bounded coalescing mechanism >>>>>>>> which could be configured from userspace. >>>>>>> I don't think it's reasonable to expect userspace to be that smart ... >>>>>>> Why do we need time bounded? used ring is always updated when ring >>>>>>> becomes empty. >>>>>> We don't add used when means BQL may not see the consumed packet in time. >>>>>> And the delay varies based on the workload since we count packets not bytes >>>>>> or time before doing the batched updating. >>>>>> >>>>>> Thanks >>>>> Sorry I still don't get it. >>>>> When nothing is outstanding then we do update the used. >>>>> So if BQL stops userspace from sending packets then >>>>> we get an interrupt and packets start flowing again. >>>> Yes, but how about the cases of multiple flows. That's where I see unstable >>>> results. >>>> >>>> >>>>> It might be suboptimal, we might need to tune it but I doubt running >>>>> timers is a solution, timer interrupts cause VM exits. >>>> Probably not a timer but a time counter (or event byte counter) in vhost to >>>> add used and signal guest if it exceeds a value instead of waiting the >>>> number of packets. >>>> >>>> >>>> Thanks >>> Well we already have VHOST_NET_WEIGHT - is it too big then? >> >> I'm not sure, it might be too big. >> >> >>> And maybe we should expose the "MORE" flag in the descriptor - >>> do you think that will help? >>> >> I don't know. But how a "more" flag can help here? >> >> Thanks > It sounds like we should be a bit more aggressive in updating used ring. > But if we just do it naively we will harm performance for sure as that > is how we are doing batching right now.
I agree but the problem is to balance the PPS and throughput. More batching helps for PPS but may damage TCP throughput.
> Instead we could make guest > control batching using the more flag - if that's not set we write out > the used ring.
It's under the control of guest, so I'm afraid we still need some more guard (e.g time/bytes counters) on host.
Thanks
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