[Codel] Do codel or fq-codel need to know the link bandwidth?

[Dave Taht]

"variable bandwidth" requires that the queuing below fq_codel be
minimal. For example 5g has enormous device buffers all over the place
which we haven't found an answer to aside from nagging the 5g folk to
fix it, and "cake-autorate"

Linux WiFi had similar problems, fixed now.

https://www.cs.kau.se/tohojo/airtime-fairness/



On Sat, Dec 7, 2024 at 2:25 PM Dave Taht <dave.taht at gmail.com> wrote:
>
> Concurrent with the rise of codel was the bql algorithm in linux, and
> the combination of the two led to sufficient backpressure for it to
> operate at the native rate of the interface. (AQL does a similar job
> for wifi). This keeps queuing in the device ring down to what can be
> serviced in a single interrupt.
>
> "Bufferbloat Systematic Analysis":
> https://d1wqtxts1xzle7.cloudfront.net/109517412/its2014_bb-libre.pdf
> Byte queue limits: https://lwn.net/Articles/469652/
>
> BSD has not evolved an equivalent mechanism. Theoretically you could
> run it natively but it would have a lot of jitter from a per-packet
> device ring.
>
> Further most ISPs use a non-native rate for their customer interfaces,
> either using a policer or FIFO shaper and thus the rise of combatting
> that with shaping via fq_codel to slightly below the ISPs' rate to
> move the bottleneck to your own hardware.
>
> We designed CAKE to use a deficit based shaper to be (roughly) the
> inverse of a token bucket shaper to get very close to the ISP rate yet
> shape well
>
> LibreQos (Preseem, Paraqum, Bequant) give ISPs a fq_codel or cake based shaper.
> The rightest answer was to have the isp correctly shape the download,
> and the uplink device shape the upload.
>
> The bufferbloat project has never had the resources to tackle BSD.
>
> These days all of Linux (and OSX) run fq_codel natively. The FIFO is dead.



-- 
Dave Täht CSO, LibreQos