On Mon, May 17, 2021 at 4:00 PM Stephen Hemminger
[off-list ref] wrote:

On Mon, 17 May 2021 14:48:46 -0700
Dave Taht [off-list ref] wrote:

quoted

On Mon, May 17, 2021 at 1:23 PM Willem de Bruijn
[off-list ref] wrote:

quoted

On Mon, May 17, 2021 at 2:44 PM Dave Taht [off-list ref] wrote:

quoted

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/ (local)

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