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

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Date:	Tue, 04 Sep 2012 17:34:15 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Nandita Dukkipati <nanditad@...gle.com>
Cc:	Yuchung Cheng <ycheng@...gle.com>, codel@...ts.bufferbloat.net,
	Dave Taht <dave.taht@...il.com>,
	Kathleen Nichols <nichols@...lere.com>,
	netdev <netdev@...r.kernel.org>, Tomas Hruby <thruby@...gle.com>
Subject: Re: [RFC v2] fq_codel : interval servo on hosts

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.
> 

Well, how things settle with a pure codel, mixing flows of very
different RTT then ?

It seems there is a high resistance on SFQ/fq_codel model because of the
probabilities of flows sharing a bucket.

So what about removing the stochastic thing and switch to a hash with
collision resolution ?


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