Re: [Bloat] virtio_net: BQL?

[Dave Taht <dave.taht@gmail.com>]

On Mon, May 17, 2021 at 4:00 PM Stephen Hemminger
<stephen@networkplumber.org> wrote:
>
> On Mon, 17 May 2021 14:48:46 -0700
> Dave Taht <dave.taht@gmail.com> wrote:
>
> > On Mon, May 17, 2021 at 1:23 PM Willem de Bruijn
> > <willemdebruijn.kernel@gmail.com> wrote:
> > >
> > > On Mon, May 17, 2021 at 2:44 PM Dave Taht <dave.taht@gmail.com> wrote:
> > > >
> > > > Not really related to this patch, but is there some reason why virtio
> > > > has no support for BQL?
> > >
> > > There have been a few attempts to add it over the years.
> > >
> > > Most recently, https://lore.kernel.org/lkml/20181205225323.12555-2-mst@redhat.com/
> > >
> > > That thread has a long discussion. I think the key open issue remains
> > >
> > > "The tricky part is the mode switching between napi and no napi."
> >
> > Oy, vey.
> >
> > I didn't pay any attention to that discussion, sadly enough.
> >
> > It's been about that long (2018) since I paid any attention to
> > bufferbloat in the cloud and my cloudy provider (linode) switched to
> > using virtio when I wasn't looking. For over a year now, I'd been
> > getting reports saying that comcast's pie rollout wasn't working as
> > well as expected, that evenroute's implementation of sch_cake and sqm
> > on inbound wasn't working right, nor pf_sense's and numerous other
> > issues at Internet scale.
> >
> > Last week I ran a string of benchmarks against starlink's new services
> > and was really aghast at what I found there, too. but the problem
> > seemed deeper than in just the dishy...
> >
> > Without BQL, there's no backpressure for fq_codel to do its thing.
> > None. My measurement servers aren't FQ-codeling
> > no matter how much load I put on them. Since that qdisc is the default
> > now in most linux distributions, I imagine that the bulk of the cloud
> > is now behaving as erratically as linux was in 2011 with enormous
> > swings in throughput and latency from GSO/TSO hitting overlarge rx/tx
> > rings, [1], breaking various rate estimators in codel, pie and the tcp
> > stack itself.
> >
> > See:
> >
> > http://fremont.starlink.taht.net/~d/virtio_nobql/rrul_-_evenroute_v3_server_fq_codel.png
> >
> > See the swings in latency there? that's symptomatic of tx/rx rings
> > filling and emptying.
> >
> > it wasn't until I switched my measurement server temporarily over to
> > sch_fq that I got a rrul result that was close to the results we used
> > to get from the virtualized e1000e drivers we were using in 2014.
> >
> > http://fremont.starlink.taht.net/~d/virtio_nobql/rrul_-_evenroute_v3_server_fq.png
> >
> > While I have long supported the use of sch_fq for tcp-heavy workloads,
> > it still behaves better with bql in place, and fq_codel is better for
> > generic workloads... but needs bql based backpressure to kick in.
> >
> > [1] I really hope I'm overreacting but, um, er, could someone(s) spin
> > up a new patch that does bql in some way even half right for this
> > driver and help test it? I haven't built a kernel in a while.
> >
>
> The Azure network driver (netvsc) also does not have BQL. Several years ago
> I tried adding it but it benchmarked worse and there is the added complexity
> of handling the accelerated networking VF path.

I certainly agree it adds complexity, but the question is what sort of
network behavior resulted without backpressure inside the
vm?

What sorts of benchmarks did you do?

I will get setup to do some testing of this that is less adhoc.


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Dave Täht CTO, TekLibre, LLC