[Codel] codel "oversteer"

Kathleen Nichols nichols at pollere.com
Wed Jun 20 16:14:08 EDT 2012

Good traffic mixing seems to be the answer to ack compression and
fqcodel should provide that. I've just started to rerun the reverse
traffic scenarios I ran before we wrote the paper, now using (a slight
variant of Eric's) fqcodel and it looks better at controlling the
queue. It's not that ack compression foils codel; it still works but
not as well. Ack compression foils tcp.


On 6/20/12 8:52 AM, Jim Gettys wrote:
> 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/ 
>>> _______________________________________________ Codel mailing
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