[Starlink] on BQL -- Byte Queue Limits

[Dave Taht]

oops: broadcom preso here: http://www.taht.net/~d/broadcom_aug9_2018.pdf

On Mon, Jun 21, 2021 at 2:01 AM Dave Taht <dave.taht at gmail.com> wrote:
>
> Have been in the linux kernel since version 3.3, but it requires about
> 6 lines of calls within each
> device driver to effectively enable. What BQL does is supply one
> interrupt's worth of data backpressure
> to the overlying queuing discipline (qdisc) to do more magic, where we
> had first put latency sensitive
> qdiscs like fq_codel. At 100Mbit (without TSO), this is typically 3-8k
> of data living on the device tx ring
> where prior to the development of BQL and awareness of bufferbloat as
> a problem, this had hit numbers as large as 64k*4096...
>
> Anything that can supply backpressure to the qdisc before it enters the device
> driver's ring buffers can be used however - high/low watermarks are
> still pretty common - but because
> BQL uses byte limits, rather than variably sized (64-64k) packet
> limits, and bytes == time on most media, BQL is the best thing going
> "down there" in the Linux kernel. BSD has no equivalent functionality
> as yet.
>
> Linux AQL - Airtime queue limits - is closely related, but for wifi.
>
> There is an extension, called XMIT_MORE that helps on batchier
> transfers from the qdisc.
>
> We never wrote BQL up in "bufferbloat and beyond" because it was
> already well established when toke
> started his MsC thesis, even though it was the breakthrough algorithm
> that made all our debloating
> work feasible.
>
> There's a link to a paper and a tutorial about BQL in this old
> presentation to broadcom here:
>
> And a detailed, often ranty but chock full of links to other good old
> papers, over here:
> https://conferences.sigcomm.org/sigcomm/2014/doc/slides/137.pdf
>
> IPQ8014 does appear to have a BQL'd ethernet driver, but haven't
> looked deeply yet.
>
> PS Writing this stuff up because this list is largely a new audience
> for the bufferbloat related APIs in
> the linux kernel
>
> --
> Latest Podcast:
> https://www.linkedin.com/feed/update/urn:li:activity:6791014284936785920/
>
> Dave Täht CTO, TekLibre, LLC



-- 
Latest Podcast:
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Dave Täht CTO, TekLibre, LLC