[Codel] Fwd: Re: [tsvwg] new draft on CoDel AQM

Dave Taht dave.taht at gmail.com
Sat Jul 14 17:49:56 EDT 2012

I'm not sure if fred is on the codel list...

On Fri, Jul 13, 2012 at 9:53 PM, Kathleen Nichols <nichols at pollere.com> wrote:

>>> 3.3. About the interval
>>> The interval constant is loosely related to RTT since it is chosen
>>> to give endpoints time to react without being so long that
>>> response times suffer.  A setting of 100ms works well across a
>>> range of RTTs from 10ms to 1 second (excellent performance is
>>> achieved in the range from 10 ms to 300ms). For devices intended
>>> for the normal, terrestrial internet interval SHOULD have the value
>>> of 100ms.
>> For very low RTT environments, such as data centers, or very high
>> delay environments such as multi-hop satcom, is "interval" a
>> configuration constant?

Eric Dumazet has run many of his (targetted at data center) 10GigE
benchmarks with a target delay of 500us and an interval of 20, with
the default packet limit of 10k packets.

5ms is an eternity at those speeds. Overrunning the packet limit
results in "interesting" drop tail behavior, too.

The reference linux implementation allows for setting target,
interval, ecn enablement and packet limits. Additionally the fq_codel
implementation allows for alternate quantums and number of flows in
the hashing algorithm.

Having not read this draft... I do have some real world cautions to make.

IF an ethernet driver (under linux) isn't soft limited (htb or hfsc)
or has does NOT have BQL enabled, the underlying buffering in the
stack can/will defeat codel/fq_codel's management utterly.

On wifi, which features wildly random delays in the .2-20 second
range, I am presently getting poor utilization at 13ms and 27ms
targets in fq_codel.  I frankly didn't expect much at this stage of
the game there (only got one of the core patches last week) - there's
too much work left to be done at the wifi driver layer to draw any
conclusions from the data points I have so far.

I think the ecn implementation of codel in linux needs to be thunk on harder.

> Well, there are words about that in Section 5. Testing and analysis can
> determine
> appropriate settings for those circumstances.
> This seems to just be cross-posted to a Cisco internal list? This was
> not meant for IETF
> discussion?
> _______________________________________________
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> Codel at lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/codel

Dave Täht
http://www.bufferbloat.net/projects/cerowrt/wiki - "3.3.8-6 is out
with fq_codel!"

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