Re: [PATCH RFC 1/2] virtio-net: bql support

["Michael S. Tsirkin" <mst@redhat.com>]

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.

> 
> >   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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