[Cake] Cake in mac80211

Dave Taht dave.taht at gmail.com
Tue Feb 4 12:07:25 EST 2020

On Tue, Feb 4, 2020 at 7:25 AM Jonathan Morton <chromatix99 at gmail.com> wrote:
> > On 4 Feb, 2020, at 5:20 pm, Bjørn Ivar Teigen <bjorn at domos.no> wrote:
> >
> > Are there any plans, work or just comments on the idea of implementing cake in mac80211 as was done with fq_codel?
> To consider doing that, there'd have to be a concrete benefit to doing so.

Research is research! :) Everything is worth trying! There's got to be
some better ideas out there, and we have a long list of things we
could have done to keep improving wifi had funding not run out.

We barely scratched the surface of this list.


> Most of Cake's most useful features, beyond what fq_codel already supports, are actually implied or even done better by the WiFi environment and the mac80211 layer adaptation (particularly airtime fairness).

In my opinion(s)

A) I think ack-filtering will help somewhat on 802.11n, but it's not
worth the added cpu cost on an AP and I'd prefer hosts reduce their
ack load in the tcp stack (IMHO, others may differ, it's worth trying)
B) The underlying wifi scheduler essentially does per host fq better
than cake can (because it's layer 2 vs layer 3), as per jonathan's
comment above
C) Instead of using a 8 way set associative hash and 1024 queues,
fq_codel for wifi uses 4096 with a disambiguation pointer for
collisions. Seems good enough.
D) "cobalt" is proving out better in several respects than pure codel,
and folding in some of that makes sense, except I don't know which
things are the most valuable considering wifi's other problems
E) I'd like to dynamically increase the quantum size as a function of
load or number of flows.

I'd really like benchmarks of the proprietary versions coming out.
Qualcomm has their own fq_codelish thing baked into their firmware
now... I have no idea what broadcom is doing... fq-pie?

The librerouter is now available. I'd like to try that.

Recently I benchmarked red rock cafe in mountain view, which had the
best bufferbloat and rrul score of any cybercafe I'd ever tried - they
have a mojo networks AP, which arista bought a while back. It was
lovely.... I have no idea what they do,
but whatever it was it was *good*. I'm really happy see bufferbloat
getting fixes everywhere, but really need to add quic to the benchmark
suite somehow in order to feel better about people not rewriring tcp
headers to do what they want.

more importantly:

Would really like to get cracking on a wifi 6 version. So far, all the
vendors are lying, there is no OFDMA support in anything we've played
with. There are some new outer limits there (1000+ devices), a need to
do gang scheduling, and per-station firmware, and I'm
profoundly unimpressed with proprietary vendor's efforts so far and
wish they'd open up their firmware more so more of us could take a
crack at it....

I'd really like to get the intel  (iwl) version, especially the ax200
chips, ported over to the AQL + fq_codel interfaces, at least.  The
first attempt went badly, last quarter. Needs eyeballs and time...
Would like to find some other wifi chip worth fixing - raspi 4? Some
android wifi chip? what?
Don't know how the ath11k effort is going...

In mainline...
 I'd like to get the wifi codel target on 5ghz down from 20ms (too
much) to 10ms, (or as I run it here to 8ms) in mainline, or at least
openwrt, but that would require some benchmarking by multiple folk,
and I was waiting for the ath10k ATF code to go upstream first. At
least make it tunable.

Overall, reducing hw retries to sanity would be a nice thing to
attempt in the ath9k, at least. Although the ongoing SCE work (gradual
rate reduction) is interesting, I tend to think reducing hardware
retries (with increased loss) would have a more dramatic effect on
reducing wifi latencies.
Presently with the codel target of 20ms in both directions, I get
60-80ms tcp latencies (still better than most fiber!) over wifi with a
20ms target at 70mbits. What happens at 300+, no idea. cynically I
think much of the internet is essentially running at a max rwind or
swind rather than within athe sawtooth.

doing something more sane to rate limit multicast would be good also.
It was quite the long list in that google document, back in the day we
thought the wifi industry might decide to collaborate in order to meet
the 5G threat.

> a Cake instance to the wifi interface as well, if you have a need to do so.

It certainly is feasible to do that. I do that now on several 802.11ac
devices that don't have the fq_codel for wifi hooks, preferring to
rate limit them well below capacity so as to ensure consistent low
latency. It's really neat to see people able to play world of warcraft
and other games over
the wifi here. ( started deploying ubnt's uap mesh products, reflashed
with openwrt, along portions of my wifi backbone . Looking forward to
the AQL backport for those, but I hope someone else does it)

>  - Jonathan Morton
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Make Music, Not War

Dave Täht
CTO, TekLibre, LLC
Tel: 1-831-435-0729

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