[Bloat] when does the CoDel part of fq_codel help in the real world?

Dave Taht dave at taht.net
Thu Nov 29 02:20:53 EST 2018

Luca Muscariello <luca.muscariello at gmail.com> writes:

> On Wed, Nov 28, 2018 at 11:40 AM Dave Taht <dave.taht at gmail.com>
> wrote:
>     On Wed, Nov 28, 2018 at 1:56 AM Luca Muscariello
>     <luca.muscariello at gmail.com> wrote:
>     >
>     > Dave,
>     >
>     > The single BDP inflight is a rule of thumb that does not account
>     for fluctuations of the RTT.
>     > And I am not talking about random fluctuations and noise. I am
>     talking about fluctuations
>     > from a control theoretic point of view to stabilise the system,
>     e.g. the trajectory of the system variable that
>     > gets to the optimal point no matter the initial conditions
>     (Lyapunov).
>     I have been trying all day to summon the gumption to make this
>     argument:
>     IF you have a good idea of the actual RTT...
>     it is also nearly certain that there will be *at least* one other
>     flow
>     you will be competing with...
>     therefore the fluctuations from every point of view are dominated
>     by
>     the interaction between these flows and
>     the goal is, in general, is not to take up a full BDP for your
>     single flow.
>     And BBR aims for some tiny percentage less than what it thinks it
>     can
>     get, when, well, everybody's seen it battle it out with itself and
>     with cubic. I hand it FQ at the bottleneck link and it works well.
>     single flows exist only in the minds of theorists and labs.
>     There's a relevant passage worth citing in the kleinrock paper, I
>     thought (did he write two recently?) that talked about this
>     problem...
>     I *swear* when I first read it it had a deeper discussion of the
>     second sentence below and had two paragraphs that went into the
>     issues
>     with multiple flows:
>     "ch earlier and led to the Flow Deviation algorithm [28]. 17 The
>     reason that the early work of 40 years ago took so long to make
>     its
>     current impact is because in [31] it was shown that the mechanism
>     presented in [2] and [3] could not be implemented in a
>     decentralized
>     algorithm. This delayed the application of Power until the recent
>     work
>     by the Google team in [1] demonstrated that the key elements of
>     response time and bandwidth could indeed be estimated using a
>     distributed control loop sliding window spanning approximately 10
>     round-trip times."
>     but I can't find it today.
> Here it is
> https://www.lk.cs.ucla.edu/data/files/Kleinrock/Internet%20Congestion%20Control%20Using%20the%20Power%20Metric-Keep%20the%20Pipe%20Just%20Full%2C%20But%20No%20Fuller%20July%202018.pdf

Thank you that is more what I remember reading. That said, I still
remember a really two paragraph thing that went into footnote 17 of the
40+ years of history behind all this, that clicked with me about why
we're still going wrong... and I can't remember what it is. I'll go
deeper into the past and go read more refs off of this.

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