On Tue, Sep 4, 2012 at 8:34 AM, Eric Dumazet <[email protected]> wrote: > On Tue, 2012-09-04 at 08:10 -0700, Nandita Dukkipati wrote: >> The idea of using srtt as interval makes sense to me if alongside we >> also hash flows with similar RTTs into same bucket. But with just the >> change in interval, I am not sure how codel is expected to behave. >> >> My understanding is: the interval (usually set to worst case expected >> RTT) is used to measure the standing queue or the "bad" queue. Suppose >> 1ms and 100ms RTT flows get hashed to same bucket, then the interval >> with this patch will flip flop between 1ms and 100ms. How is this >> expected to measure a standing queue? In fact I think the 1ms flow may >> land up measuring the burstiness or the "good" queue created by the >> long RTT flows, and this isn't desirable.
Experiments would be good. > > Well, how things settle with a pure codel, mixing flows of very > different RTT then ? Elephants are shot statistically more often than mice. > It seems there is a high resistance on SFQ/fq_codel model because of the > probabilities of flows sharing a bucket. I was going to do this in a separate email, because it is a little off-topic. fq_codel has a standing queue problem, based on the fact that when a queue empties, codel.h resets. This made sense for the single FIFO codel but not multi-queued fq_codel. So after we hit X high rate flows, target can never be achieved, even straining mightily, and we end up with a standing queue again. Easily seen with like 150 bidirectional flows at 10 or 100Mbit. (as queues go, it's still pretty good queue. And: I've fiddled with various means of draining multi-queue behavior thus far, and they ended up unstable/unfair) > So what about removing the stochastic thing and switch to a hash with > collision resolution ? Was considered and discarded in the original SFQ paper as being too computationally intensive (in 1993). Worth revisiting. http://www2.rdrop.com/~paulmck/scalability/paper/sfq.2002.06.04.pdf > > -- Dave Täht http://www.bufferbloat.net/projects/cerowrt/wiki - "3.3.8-17 is out with fq_codel!" _______________________________________________ Codel mailing list [email protected] https://lists.bufferbloat.net/listinfo/codel
