Messages in this thread | | | Subject | Re: [PATCH RFC 1/2] virtio-net: bql support | From | Jason Wang <> | Date | Mon, 7 Jan 2019 10:14:37 +0800 |
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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
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