[Cake] Understanding the Achieved Rate Multiplication Effect in FlowQueue-based AQM Bottleneck

David P. Reed dpreed at deepplum.com
Sat Dec 4 18:01:50 EST 2021

I agree with your broad assessment, Jonathan.
The self-interference problem within a host isn't just a network problem. It's a user-space scheduler problem as well.
There are lots of interactions between user-space scheduler (in the case of Linux, the "Completely Fair Scheduler" and its quantum, which is set by the HZ variable at boot) and the network stack in the kernel. This interactions have non-trivial effects when mutliple flows are independently created by concurrent processes).
Lately, I've been studying, for reasons related to my day job, the complex interactions of timing at sub-millisecond scale among threads and processes on a single system in Linux. I/O driven by threads become highly correlated, and so assuming "independence" among flow timing  is just not a good assumption.
The paper observes the results of "dependencies" that couple/resonate.
On Friday, December 3, 2021 7:09pm, "Jonathan Morton" <chromatix99 at gmail.com> said:

> > On 4 Dec, 2021, at 12:27 am, Dave Taht <dave.taht at gmail.com> wrote:
> >
> >
> https://jonathankua.github.io/preprints/jkua-ieeelcn2021_understanding_ar_preprint-20jul2021.pdf
> >
> > I would love it if somehow the measured effects of chunklets against cake's
> per-host/per flow fq was examined one day.
> I haven't actually measured it, but based on what the above paper says, I can make
> some firm predictions:
> 1: When competing against traffic to the same local host, the performance effects
> they describe will be present.
> 2: When competing against traffic to a different local-network host, the
> performance effects they describe will be attenuated or even entirely absent.
> 3: They noted one or two cases of observable effects of hash collisions in their
> tests with FQ-Codel. These will be greatly reduced in prevalence with Cake, due
> to the set-associative hash function which specifically addresses that phenomenon.
> - Jonathan Morton
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