Re: [aqm] CoDel's control law that determines drop frequency
Dave Taht <dave.taht@gmail.com> Wed, 30 September 2015 09:00 UTC
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References: <201311122230.rACMUBmH003536@bagheera.jungle.bt.co.uk> <87wpzfpbd3.fsf@alrua-karlstad.karlstad.toke.dk> <56045CA8.2060103@bobbriscoe.net> <CAPRuP3mmg_-uxmtLUXprCmPyLSUuUA7t2dRZpDs_mwtnTgrSQA@mail.gmail.com> <560BA261.6020206@bobbriscoe.net>
Date: Wed, 30 Sep 2015 02:00:02 -0700
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From: Dave Taht <dave.taht@gmail.com>
To: Bob Briscoe <ietf@bobbriscoe.net>
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Cc: Andrew Mcgregor <andrewmcgr@google.com>, Kathleen Nichols <nichols@pollere.com>, AQM IETF list <aqm@ietf.org>, Van Jacobson <vanj@google.com>
Subject: Re: [aqm] CoDel's control law that determines drop frequency
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On Wed, Sep 30, 2015 at 1:50 AM, Bob Briscoe <ietf@bobbriscoe.net> wrote: > Andrew, > > I am also not so interested in an AQM dealing directly with unresponsive > traffic - I prefer to keep policing and AQM as separately deployable > functions, because AQM should be policy-neutral, whereas policing inherently > involves policy. > > My concern was merely that CoDel's linear increase in drop probability can > take a long time to reach where it intends to get to. I would have thought > some form of exponential increase, or at least super-linear, would have been > more responsive to changing traffic conditions. I.e., rather than have to > answer the question "how quickly should drop probability increase?", make it > increase increasingly quickly. > > Early on, Rong Pan showed that it takes CoDel ages to bring high load under > control. I think this linear increase is the reason. cake uses a better curve for codel, but we still need to do more testing in the lab. http://www.bufferbloat.net/projects/codel/wiki/CakeTechnical > > Bob > > > > On 30/09/15 01:42, Andrew Mcgregor wrote: > > Hmm, that's really interesting. > > Most interesting is that my understanding is that the control law was > intended to deal with aggregates of mostly TCP-like traffic, and that an > overload of unresponsive traffic wasn't much of a goal; this seems like > vaguely reasonable behaviour, I suppose, given that pathological situation. > > But I don't have a way to derive the control law from first principles at > this time (I haven't been working on that for a long time now). > > On 25 September 2015 at 06:27, Bob Briscoe <ietf@bobbriscoe.net> wrote: >> >> Toke, >> >> Having originally whinged that no-one ever responded to my original 2013 >> posting, now it's my turn to be embarrassed for having missed your >> interesting response for over 3 months. >> >> Cool that the analysis proves correct in practice - always nice. >> >> The question is still open whether this was the intention, and if so why >> this particular control law was intended. >> I would rather we started from a statement of what the control law ought >> to do, then derive it. >> >> Andrew McGregor said he would have a go at this question some time ago... >> Andrew? >> >> >> Bob >> >> >> >> On 07/06/15 20:27, Toke Høiland-Jørgensen wrote: >> >> Hi Bob >> >> Apologies for reviving this ancient thread; been meaning to get around >> to it sooner, but well... better late than never I suppose. >> >> (Web link to your original mail, in case Message-ID referencing breaks: >> https://www.ietf.org/mail-archive/web/aqm/current/msg00376.html ). >> >> Having recently had a need to understand CoDel's behaviour in more >> detail, your analysis popped out of wherever it's been hiding in the >> back of my mind and presented itself as maybe a good place to start. :) >> >> So anyhow, I'm going to skip the initial assertions in your email and >> focus on the analysis: >> >> Here's my working (pls check it - I may have made mistakes) >> _________________ >> For brevity, I'll define some briefer variable names: >> interval = I [s] >> next_drop = D [s] >> packet-rate = R [pkt/s] >> count = n [pkt] >> >> >From the CoDel control law code: >> D(n) = I / sqrt(n) >> And the instantaneous drop probability is: >> p(n) = 1/( R * D(n) ) >> >> Then the slope of the rise in drop probability with time is: >> Delta p / Delta t = [p(n+1) - p(n)] / D(n) >> = [1/D(n+1) - 1/D(n)] / [ R * D(n) ] >> = sqrt(n) * [sqrt(n+1) - sqrt(n)] / >> [R*I*I] >> = [ sqrt(n(n+1)) - n ] / R*I^2 >> >> I couldn't find anything wrong with the derivation. I'm not entirely >> sure that I think it makes sense to speak about an "instantaneous drop >> probability" for an algorithm that is not probabilistic in nature. >> However, interpreting p(n) as "the fraction of packets dropped over the >> interval from D(n) to D(n+1)" makes sense, I guess, and for this >> analysis that works. >> >> At count = 1, the numerator starts at sqrt(2)-1 = 0.414. >> Amd as n increases, it rapidly tends to 1/2. >> >> So CoDel's rate of increase of drop probability with time is nearly >> constant (it >> is always between 0.414 and 0.5) and it rapidly approaches 0.5 after a few >> drops, tending towards: >> dp/dt = 1/(2*R*I^2) >> >> This constant increase clearly has very little to do with the square-root >> law of >> TCP Reno. >> >> In the above formula, drop probability increases inversely proportional to >> the >> packet rate. For instance, with I = 100ms and 1500B packets >> at 10Mb/s => R = 833 pkt/s => dp/dt = 6.0% /s >> at 100Mb/s => R = 8333 pkt/s => dp/dt = 0.6% /s >> >> I also tried to test this. I configured CoDel (on a Linux 4.0 box) on >> 1Mbps, 2Mbps and 10Mbps links with interval settings of 1 second and >> 500ms, and a total packet limit of 100k packets. This was to make it >> deliberately slower to react (so the change in drop probability is more >> visible), and to make sure no packets are dropped from queue overflow. >> >> I then sent an unresponsive UDP stream over the link at 110% of the link >> capacity (as passed to Iperf, so approximately), and collected the >> output of `tc -s qdisc` every 0.2 seconds. >> >> The attached plot is of 'pkts dropped / (pkts sent + pkts dropped)' in a >> 2-second sliding window over the duration of the test (the plot is also >> available here: >> https://kau.toke.dk/ietf/codel-drop-rate/codel-drop-rate.svg ). >> >> I've included linear trend lines from the initial time to the point of >> maximum drop probability, and as is apparent from the plot, got quite a >> good fit (r>0.99 for all six data sets). The legend includes the slopes >> of the linear fits for each of the data sets, which are not too far from >> what your analysis predicts (and I'm guessing the difference can be >> attributed to the difference in exact packet rates, but I haven't >> checked). >> >> The Flent data files with the qdisc stats over time (readable by the >> newest git version of Flent), as well as the Python script I used to >> create the graph are available here: >> https://kau.toke.dk/ietf/codel-drop-rate/ >> >> So, in short: It seems that CoDel's "drop rate" does increase linearly >> in the presence of a persistent queue, and that the rate of increase >> depends on both the interval and the link rate. >> >> Now, I'll refrain from commenting on whether or not this is "bad", or >> indeed if it is contrary to design. It was surprising to me at least, so >> I thought I'd share my findings, in the hope that someone would either >> find them useful or tell me how they're wrong (or both!). :) >> >> -Toke >> >> >> >> _______________________________________________ >> aqm mailing list >> aqm@ietf.org >> https://www.ietf.org/mailman/listinfo/aqm >> >> >> -- >> ________________________________________________________________ >> Bob Briscoe http://bobbriscoe.net/ >> >> >> _______________________________________________ >> aqm mailing list >> aqm@ietf.org >> https://www.ietf.org/mailman/listinfo/aqm >> > > > > -- > Andrew McGregor | SRE | andrewmcgr@google.com | +61 4 1071 2221 > > > -- > ________________________________________________________________ > Bob Briscoe http://bobbriscoe.net/ > > > _______________________________________________ > aqm mailing list > aqm@ietf.org > https://www.ietf.org/mailman/listinfo/aqm > -- Dave Täht Do you want faster, better, wifi? https://www.patreon.com/dtaht
- [aqm] CoDel's control law that determines drop fr… Bob Briscoe
- Re: [aqm] CoDel's control law that determines dro… Toke Høiland-Jørgensen
- Re: [aqm] CoDel's control law that determines dro… Bob Briscoe
- Re: [aqm] CoDel's control law that determines dro… Andrew Mcgregor
- Re: [aqm] CoDel's control law that determines dro… Bob Briscoe
- Re: [aqm] CoDel's control law that determines dro… Dave Taht
- Re: [aqm] CoDel's control law that determines dro… Jonathan Morton
- Re: [aqm] CoDel's control law that determines dro… Polina Goltsman
- Re: [aqm] CoDel's control law that determines dro… Toke Høiland-Jørgensen
- Re: [aqm] CoDel's control law that determines dro… Bless, Roland (TM)
- Re: [aqm] CoDel's control law that determines dro… Toke Høiland-Jørgensen
- Re: [aqm] CoDel's control law that determines dro… Bob Briscoe
- Re: [aqm] CoDel's control law that determines dro… Bob Briscoe
- Re: [aqm] CoDel's control law that determines dro… Bless, Roland (TM)
- Re: [aqm] CoDel's control law that determines dro… Bob Briscoe
- Re: [aqm] CoDel's control law that determines dro… Kuhn Nicolas
- Re: [aqm] CoDel's control law that determines dro… Polina Goltsman
- Re: [aqm] CoDel's control law that determines dro… Jeff Weeks
- Re: [aqm] CoDel's control law that determines dro… Polina Goltsman
- Re: [aqm] CoDel's control law that determines dro… Bob Briscoe
- Re: [aqm] CoDel's control law that determines dro… Jeff Weeks
- Re: [aqm] CoDel's control law that determines dro… Dave Dolson
- Re: [aqm] CoDel's control law that determines dro… Andrew Mcgregor
- Re: [aqm] CoDel's control law that determines dro… Jeff Weeks
- Re: [aqm] CoDel's control law that determines dro… Dave Taht
- Re: [aqm] CoDel's control law that determines dro… Jonathan Morton