Re: virtio_net: BQL?

[Dave Taht]

On Mon, May 17, 2021 at 1:23 PM Willem de Bruijn
<willemdebruijn.kernel@xxxxxxxxx> wrote:
>
> On Mon, May 17, 2021 at 2:44 PM Dave Taht <dave.taht@xxxxxxxxx> 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@xxxxxxxxxx/
>
> 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.


> > On Mon, May 17, 2021 at 11:41 AM Xianting Tian
> > <xianting.tian@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > BUG_ON() uses unlikely in if(), which can be optimized at compile time.
> > >
> > > Signed-off-by: Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx>
> > > ---
> > >   drivers/net/virtio_net.c | 5 ++---
> > >   1 file changed, 2 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index c921ebf3ae82..212d52204884 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -1646,10 +1646,9 @@ static int xmit_skb(struct send_queue *sq, struct
> > > sk_buff *skb)
> > >         else
> > >                 hdr = skb_vnet_hdr(skb);
> > >
> > > -       if (virtio_net_hdr_from_skb(skb, &hdr->hdr,
> > > +       BUG_ON(virtio_net_hdr_from_skb(skb, &hdr->hdr,
> > >                                     virtio_is_little_endian(vi->vdev), false,
> > > -                                   0))
> > > -               BUG();
> > > +                                   0));
> > >
> > >         if (vi->mergeable_rx_bufs)
> > >                 hdr->num_buffers = 0;
> > > --
> > > 2.17.1
> > >
> >
> >
> > --
> > Latest Podcast:
> > https://www.linkedin.com/feed/update/urn:li:activity:6791014284936785920/
> >
> > Dave Täht CTO, TekLibre, LLC



--
Latest Podcast:
https://www.linkedin.com/feed/update/urn:li:activity:6791014284936785920/

Dave Täht CTO, TekLibre, LLC