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

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Message-Id: <1C18E243-42BC-46F0-A336-EAD9BC881C45@gmail.com>
Date:	Tue, 4 Sep 2012 18:25:18 +0300
From:	Jonathan Morton <chromatix99@...il.com>
To:	Nandita Dukkipati <nanditad@...gle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Tomas Hruby <thruby@...gle.com>,
	netdev <netdev@...r.kernel.org>,
	"codel@...ts.bufferbloat.net" <codel@...ts.bufferbloat.net>
Subject: Re: [Codel] [RFC v2] fq_codel : interval servo on hosts

I think that in most cases, a long RTT flow and a short RTT flow on the same interface means that the long RTT flow isn't bottlenecked here, and therefore won't ever build up a significant queue - and that means you would want to track over the shorter interval. Is that a reasonable assumption?

The key to knowledge is not to rely on others to teach you it. 

On 4 Sep 2012, at 18:10, Nandita Dukkipati <nanditad@...gle.com> 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.
> 
> 
> On Sat, Sep 1, 2012 at 5:51 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> On Fri, 2012-08-31 at 18:37 -0700, Yuchung Cheng wrote:
>> 
>>> Just curious: tp->srtt is a very rough estimator, e.g., Delayed-ACks
>>> can easily add 40 - 200ms fuzziness. Will this affect short flows?
>> 
>> Good point
>> 
>> Delayed acks shouldnt matter, because they happen when flow had been
>> idle for a while.
>> 
>> I guess we should clamp the srtt to the default interval
>> 
>> if (srtt)
>>        q->cparams.interval = min(tcp_srtt_to_codel(srtt),
>>                                  q->default_interval);
>> 
>> 
>> 
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