Re: [RFC v2] fq_codel : interval servo on hosts

On Tue, Sep 4, 2012 at 8:34 AM, Eric Dumazet [off-list ref] wrote:

On Tue, 2012-09-04 at 08:10 -0700, Nandita Dukkipati wrote:

quoted

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!"