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

From: Jason Wang
Date: Tue Jan 08 2019 - 05:07:06 EST


On 2019/1/7 äå10:19, Michael S. Tsirkin wrote:
On Mon, Jan 07, 2019 at 02:31:47PM +0800, Jason Wang wrote:
On 2019/1/7 äå12:01, Michael S. Tsirkin wrote:
On Mon, Jan 07, 2019 at 11:51:55AM +0800, Jason Wang wrote:
On 2019/1/7 äå11:17, Michael S. Tsirkin wrote:
On Mon, Jan 07, 2019 at 10:14:37AM +0800, Jason Wang wrote:
On 2019/1/2 äå9:59, Michael S. Tsirkin wrote:
On Wed, Jan 02, 2019 at 11:28:43AM +0800, Jason Wang wrote:
On 2018/12/31 äå2:45, Michael S. Tsirkin wrote:
On Thu, Dec 27, 2018 at 06:00:36PM +0800, Jason Wang wrote:
On 2018/12/26 äå11:19, Michael S. Tsirkin wrote:
On Thu, Dec 06, 2018 at 04:17:36PM +0800, Jason Wang wrote:
On 2018/12/6 äå6:54, 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.