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

[Dave Taht]

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.