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McKenney" To: Kathleen Nichols Message-ID: <20121128043838.GX2474@linux.vnet.ibm.com> References: <20121127224915.GM2474@linux.vnet.ibm.com> <20121128002710.GS2474@linux.vnet.ibm.com> <50B5887C.7010605@pollere.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50B5887C.7010605@pollere.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12112804-3620-0000-0000-0000007A0D2A Cc: Paolo Valente , Toke =?iso-8859-1?Q?H=F8iland-J=F8rgensen?= , Eric Raymond , "codel@lists.bufferbloat.net" , "cerowrt-devel@lists.bufferbloat.net" , bloat , Greg White , John Crispin Subject: Re: [Cerowrt-devel] [Codel] FQ_Codel lwn draft article review X-BeenThere: cerowrt-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list Reply-To: paulmck@linux.vnet.ibm.com List-Id: Development issues regarding the cerowrt test router project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 28 Nov 2012 04:38:45 -0000 I guess I just have to be grateful that people mostly agree on the acronym, regardless of the expansion. Thanx, Paul On Tue, Nov 27, 2012 at 07:43:56PM -0800, Kathleen Nichols wrote: > > It would be me that tries to say "stochastic flow queuing with CoDel" > as I like to be accurate. But I think FQ-Codel is Flow queuing with CoDel. > JimG suggests "smart flow queuing" because he is ever mindful of the > big audience. > > On 11/27/12 4:27 PM, Paul E. McKenney wrote: > > On Tue, Nov 27, 2012 at 04:53:34PM -0700, Greg White wrote: > >> BTW, I've heard some use the term "stochastic flow queueing" as a > >> replacement to avoid the term "fair". Seems like a more apt term anyway. > > > > Would that mean that FQ-CoDel is Flow Queue Controlled Delay? ;-) > > > > Thanx, Paul > > > >> -Greg > >> > >> > >> On 11/27/12 3:49 PM, "Paul E. McKenney" wrote: > >> > >>> Thank you for the review and comments, Jim! I will apply them when > >>> I get the pen back from Dave. And yes, that is the thing about > >>> "fairness" -- there are a great many definitions, many of the most > >>> useful of which appear to many to be patently unfair. ;-) > >>> > >>> As you suggest, it might well be best to drop discussion of fairness, > >>> or to at the least supply the corresponding definition. > >>> > >>> Thanx, Paul > >>> > >>> On Tue, Nov 27, 2012 at 05:03:02PM -0500, Jim Gettys wrote: > >>>> Some points worth making: > >>>> > >>>> 1) It is important to point out that (and how) fq_codel avoids > >>>> starvation: > >>>> unpleasant as elephant flows are, it would be very unfriendly to never > >>>> service them at all until they time out. > >>>> > >>>> 2) "fairness" is not necessarily what we ultimately want at all; you'd > >>>> really like to penalize those who induce congestion the most. But we > >>>> don't > >>>> currently have a solution (though Bob Briscoe at BT thinks he does, and > >>>> is > >>>> seeing if he can get it out from under a BT patent), so the current > >>>> fq_codel round robins ultimately until/unless we can do something like > >>>> Bob's idea. This is a local information only subset of the ideas he's > >>>> been > >>>> working on in the congestion exposure (conex) group at the IETF. > >>>> > >>>> 3) "fairness" is always in the eyes of the beholder (and should be left > >>>> to > >>>> the beholder to determine). "fairness" depends on where in the network > >>>> you > >>>> are. While being "fair" among TCP flows is sensible default policy for > >>>> a > >>>> host, else where in the network it may not be/usually isn't. > >>>> > >>>> Two examples: > >>>> o at a home router, you probably want to be "fair" according to transmit > >>>> opportunities. We really don't want a single system remote from the > >>>> router > >>>> to be able to starve the network so that devices near the router get > >>>> much > >>>> less bandwidth than you might hope/expect. > >>>> > >>>> What is more, you probably want to account for a single host using many > >>>> flows, and regulate that they not be able to "hog" bandwidth in the home > >>>> environment, but only use their "fair" share. > >>>> > >>>> o at an ISP, you must to be "fair" between customers; it is best to > >>>> leave > >>>> the judgement of "fairness" at finer granularity (e.g. host and TCP > >>>> flows) > >>>> to the points closer to the customer's systems, so that they can enforce > >>>> whatever definition of "fair" they need to themselves. > >>>> > >>>> > >>>> Algorithms like fq_codel can be/should be adjusted to the circumstances. > >>>> > >>>> And therefore exactly what you choose to hash against to form the > >>>> buckets > >>>> will vary depending on where you are. That at least one step (at the > >>>> user's device) of this be TCP flow "fair" does have the great advantage > >>>> of > >>>> helping the RTT unfairness problem that violates the principle of "least > >>>> surprise", such as that routinely seen in places like New Zealand. > >>>> > >>>> This is why I have so many problems using the word "fair" near this > >>>> algorithm. "fair" is impossible to define, overloaded in people's mind > >>>> with TCP fair queuing, not even desirable much of the time, and by > >>>> definition and design, even today's fq_codel isn't fair to lots of > >>>> things, > >>>> and the same basic algorithm can/should be tweaked in lots of directions > >>>> depending on what we need to do. Calling this "smart" queuing or some > >>>> such > >>>> would be better. > >>>> > >>>> When you've done another round on the document, I'll do a more detailed > >>>> read. > >>>> - Jim > >>>> > >>>> > >>>> > >>>> > >>>> On Fri, Nov 23, 2012 at 5:18 PM, Paul E. McKenney < > >>>> paulmck@linux.vnet.ibm.com> wrote: > >>>> > >>>>> On Fri, Nov 23, 2012 at 09:57:34AM +0100, Dave Taht wrote: > >>>>>> David Woodhouse and I fiddled a lot with adsl and openwrt and a > >>>>>> variety of drivers and network layers in a typical bonded adsl stack > >>>>>> yesterday. The complexity of it all makes my head hurt. I'm happy > >>>> that > >>>>>> a newly BQL'd ethernet driver (for the geos and qemu) emerged from > >>>> it, > >>>>>> which he submitted to netdev... > >>>>> > >>>>> Cool!!! ;-) > >>>>> > >>>>>> I made a recording of us last night discussing the layers, which I > >>>>>> will produce and distribute later... > >>>>>> > >>>>>> Anyway, along the way, we fiddled a lot with trying to analyze where > >>>>>> the 350ms or so of added latency was coming from in the traverse > >>>> geo's > >>>>>> adsl implementation and overlying stack.... > >>>>>> > >>>>>> Plots: http://david.woodhou.se/dwmw2-netperf-plots.tar.gz > >>>>>> > >>>>>> Note: 1: > >>>>>> > >>>>>> The netperf sample rate on the rrul test needs to be higher than > >>>>>> 100ms in order to get a decent result at sub 10Mbit speeds. > >>>>>> > >>>>>> Note 2: > >>>>>> > >>>>>> The two nicest graphs here are nofq.svg vs fq.svg, which were taken > >>>> on > >>>>>> a gigE link from a Mac running Linux to another gigE link. (in other > >>>>>> words, NOT on the friggin adsl link) (firefox can display svg, I > >>>> don't > >>>>>> know what else) I find the T+10 delay before stream start in the > >>>>>> fq.svg graph suspicious and think the "throw out the outlier" code > >>>> in > >>>>>> the netperf-wrapper code is at fault. Prior to that, codel is merely > >>>>>> buffering up things madly, which can also be seen in the pfifo_fast > >>>>>> behavior, with 1000pkts it's default. > >>>>> > >>>>> I am using these two in a new "Effectiveness of FQ-CoDel" section. > >>>>> Chrome can display .svg, and if it becomes a problem, I am sure that > >>>>> they can be converted. Please let me know if some other data would > >>>>> make the point better. > >>>>> > >>>>> I am assuming that the colored throughput spikes are due to occasional > >>>>> packet losses. Please let me know if this interpretation is overly > >>>> naive. > >>>>> > >>>>> Also, I know what ICMP is, but the UDP variants are new to me. Could > >>>>> you please expand the "EF", "BK", "BE", and "CSS" acronyms? > >>>>> > >>>>>> (Arguably, the default queue length in codel can be reduced from 10k > >>>>>> packets to something more reasonable at GigE speeds) > >>>>>> > >>>>>> (the indicator that it's the graph, not the reality, is that the > >>>>>> fq.svg pings and udp start at T+5 and grow minimally, as is usual > >>>> with > >>>>>> fq_codel.) > >>>>> > >>>>> All sessions were started at T+5, then? > >>>>> > >>>>>> As for the *.ps graphs, well, they would take david's network > >>>> topology > >>>>>> to explain, and were conducted over a variety of circumstances, > >>>>>> including wifi, with more variables in play than I care to think > >>>>>> about. > >>>>>> > >>>>>> We didn't really get anywhere on digging deeper. As we got to purer > >>>>>> tests - with a minimal number of boxes, running pure ethernet, > >>>>>> switched over a couple of switches, even in the simplest two box > >>>> case, > >>>>>> my HTB based "ceroshaper" implementation had multiple problems in > >>>>>> cutting median latencies below 100ms, on this very slow ADSL link. > >>>>>> David suspects problems on the path along the carrier backbone as a > >>>>>> potential issue, and the only way to measure that is with two one > >>>> way > >>>>>> trip time measurements (rather than rtt), time synced via ntp... I > >>>>>> keep hoping to find a rtp test, but I'm open to just about any > >>>> option > >>>>>> at this point. anyone? > >>>>>> > >>>>>> We also found a probable bug in mtr in that multiple mtrs on the > >>>> same > >>>>>> box don't co-exist. > >>>>> > >>>>> I must confess that I am not seeing all that clear a difference > >>>> between > >>>>> the behaviors of ceroshaper and FQ-CoDel. Maybe somewhat better > >>>> latencies > >>>>> for FQ-CoDel, but not unambiguously so. > >>>>> > >>>>>> Moving back to more scientific clarity and simpler tests... > >>>>>> > >>>>>> The two graphs, taken a few weeks back, on pages 5 and 6 of this: > >>>>>> > >>>>>> > >>>>> > >>>> http://www.teklibre.com/~d/bloat/Not_every_packet_is_sacred-Battling_Buff > >>>> erbloat_on_wifi.pdf > >>>>>> > >>>>>> appear to show the advantage of fq_codel fq + codel + head drop over > >>>>>> tail drop during the slow start period on a 10Mbit link - (see how > >>>>>> squiggly slow start is on pfifo fast?) as well as the marvelous > >>>>>> interstream latency that can be achieved with BQL=3000 (on a 10 mbit > >>>>>> link.) Even that latency can be halved by reducing BQL to 1500, > >>>> which > >>>>>> is just fine on a 10mbit. Below those rates I'd like to be rid of > >>>> BQL > >>>>>> entirely, and just have a single packet outstanding... in everything > >>>>>> from adsl to cable... > >>>>>> > >>>>>> That said, I'd welcome other explanations of the squiggly slowstart > >>>>>> pfifo_fast behavior before I put that explanation on the slide.... > >>>> ECN > >>>>>> was in play here, too. I can redo this test easily, it's basically > >>>>>> running a netperf TCP_RR for 70 seconds, and starting up a > >>>> TCP_MAERTS > >>>>>> and TCP_STREAM for 60 seconds a T+5, after hammering down on BQL's > >>>>>> limit and the link speeds on two sides of a directly connected > >>>> laptop > >>>>>> connection. > >>>>> > >>>>> I must defer to others on this one. I do note the much lower > >>>> latencies > >>>>> on slide 6 compared to slide 5, though. > >>>>> > >>>>> Please see attached for update including .git directory. > >>>>> > >>>>> Thanx, Paul > >>>>> > >>>>>> ethtool -s eth0 advertise 0x002 # 10 Mbit > >>>>>> > >>>>> > >>>>> _______________________________________________ > >>>>> Cerowrt-devel mailing list > >>>>> Cerowrt-devel@lists.bufferbloat.net > >>>>> https://lists.bufferbloat.net/listinfo/cerowrt-devel > >>>>> > >>>>> > >>> > >>> _______________________________________________ > >>> Codel mailing list > >>> Codel@lists.bufferbloat.net > >>> https://lists.bufferbloat.net/listinfo/codel > >> > > > > _______________________________________________ > > Codel mailing list > > Codel@lists.bufferbloat.net > > https://lists.bufferbloat.net/listinfo/codel > > >