[Cerowrt-devel] [Codel] codel "oversteer"

[Jim Gettys]

On 06/20/2012 06:08 AM, Jonathan Morton wrote:
> Is the cwnd also oscillating wildly or is it just an artefact of the visible part of the queue only being a fraction of the real queue?
>
> Are ACK packets being aggregated by wireless? That would be a good explanation for large bursts that flood the buffer, if the rwnd opens a lot suddenly. This would also be an argument that 2*n is too small for the ECN drop threshold. 

Yeah, I've been worrying about ack compression...  Not sure exactly what
we should be doing about it, as I don't fully understand it.
                    - Jim

>
> The key to knowledge is not to rely on others to teach you it. 
>
> On 20 Jun 2012, at 04:32, Dave Taht <dave.taht at gmail.com> wrote:
>
>> I've been forming a theory regarding codel behavior in some
>> pathological conditions. For the sake of developing the theory I'm
>> going to return to the original car analogy published here, and add a
>> new one - "oversteer".
>>
>> Briefly:
>>
>> If the underlying interface device driver is overbuffered, when the
>> packet backlog finally makes it into the qdisc layer, that bursts up
>> rapidly and codel rapidly ramps up it's drop strategy, which corrects
>> the problem, but we are back in a state where we are, as in the case
>> of an auto on ice, or a very loose connection to the steering wheel,
>> "oversteering" because codel is actually not measuring the entire
>> time-width of the queue and unable to control it well, even if it
>> could.
>>
>> What I observe on wireless now with fq_codel under heavy load is
>> oscillation in the qdisc layer between 0 length queue and 70 or more
>> packets backlogged, a burst of drops when that happens, and far more
>> drops than ecn marks that I expected  (with the new (arbitrary) drop
>> ecn packets if > 2 * target idea I was fiddling with illustrating the
>> point better, now). It's difficult to gain further direct insight
>> without time and packet traces, and maybe exporting more data to
>> userspace, but this kind of explains a report I got privately on x86
>> (no ecn drop enabled), and the behavior of fq_codel on wireless on the
>> present version of cerowrt.
>>
>> (I could always have inserted a bug, too, if it wasn't for the private
>> report and having to get on a plane shortly I wouldn't be posting this
>> now)
>>
>> Further testing ideas (others!) could try would be:
>>
>> Increase BQL's setting to over-large values on a BQL enabled interface
>> and see what happens
>> Test with an overbuffered ethernet interface in the first place
>> Improve the ns3 model to have an emulated network interface with
>> user-settable buffering
>>
>> Assuming I'm right and others can reproduce this, this implies that
>> focusing much harder on BQL and overbuffering related issues on the
>> dozens? hundreds? of non-BQL enabled ethernet drivers is needed at
>> this point. And we already know that much more hard work on fixing
>> wifi is needed.
>>
>> Despite this I'm generally pleased with the fq_codel results over
>> wireless I'm currently getting from today's build of cerowrt, and
>> certainly the BQL-enabled ethernet drivers I've worked with (ar71xx,
>> e1000) don't display this behavior, neither does soft rate limiting
>> using htb - instead achieving a steady state for the packet backlog,
>> accepting bursts, and otherwise being "nice".
>>
>> -- 
>> Dave Täht
>> SKYPE: davetaht
>> http://ronsravings.blogspot.com/
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