Messages in this thread | | | Subject | Re: [PATCH RFC 1/2] virtio-net: bql support | From | Jason Wang <> | Date | Tue, 8 Jan 2019 18:06:45 +0800 |
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On 2019/1/7 下午10:19, Michael S. Tsirkin wrote: > On Mon, Jan 07, 2019 at 02:31:47PM +0800, Jason Wang wrote: >> On 2019/1/7 下午12:01, Michael S. Tsirkin wrote: >>> On Mon, Jan 07, 2019 at 11:51:55AM +0800, Jason Wang wrote: >>>> 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. >>> That is what more flag is supposed to be I think - it is only set if >>> there's a socket that actually needs the skb freed in order to go on. >> >> I'm not quite sure I get, but is this something similar to what you want? >> >> https://lists.linuxfoundation.org/pipermail/virtualization/2014-October/027667.html >> >> Which enables tx interrupt for TCP packets, and you want to add used more >> aggressively for those sockets? >> >> >> Thanks > That's the idea. > But then you said we can just play with event index > instead. I think the answer to why not do that is that it's tricky to do > without races.
We don't do batched used ring update at that time. We can check whether or not guest asking for a interrupt and add used immediately. Actually, I've played a patch to do this. It helps a little but damage the PPS. This is probably because we need more userspace memory accesses.
> > > We need to think about the exact semantics: e.g. I think it is better to > keep interrupts on and then saying "I promise sending more buffers even > if you do not use any buffers so using this one is not urgent" rather > than as your patches do keeping them off and then saying "this one is > urgent". > > The reason being is that "I promise to send more" is > more informative and can allow better batching for the > host.
Just to make sure I understand, you mean set batch flag for e.g non TCP socket?
Thanks
> >>>>> 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 >>> Point is if guest does not care about the skb being freed, then there is no >>> rush host side to mark buffer used. >>> >>>
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