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

[Evan Mesterhazy]

Thanks Dave, your reply is very informative. I definitely understand that
FreeBSD is somewhat less sophisticated than Linux when it comes to this
since I've read through a few of the bugs and other messages you've posted
regarding its implementation. However the Unifi routers run on Linux, and
they still require the user to specify the ISP's service bandwidth. I think
this paragraph from your reply may be the crux of it:

> 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.

If the router is able to keep pushing bytes over the ISP's link even in
excess of the allowed bandwidth, then the local router's packets probably
aren't sitting in their queues long enough to be dropped by fq-codel. It
sounds like proper traffic shaping needs to happen wherever the bottleneck
is (I suppose this should have been obvious to me), which in the case of a
300 Mbp/s connection over GPON is somewhere at the ISP. I guess if the ISP
does proper traffic shaping the end user shouldn't need to do anything.

Assuming I didn't say anything inaccurate in my reply, I think I understand
why setting a bandwidth limit is necessary to make fq-codel work properly
on a router with a less than line speed connection from the ISP.

Thank you!

On Sat, Dec 7, 2024 at 5:32 PM Dave Taht <dave.taht at gmail.com> wrote:

> "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
>
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