Re: [aqm] Gathering Queue Length Statistics

Dave Taht <dave.taht@gmail.com> Wed, 25 February 2015 18:24 UTC

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Date: Wed, 25 Feb 2015 10:24:45 -0800
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From: Dave Taht <dave.taht@gmail.com>
To: Ryan Doyle <rpdoyle@live.unc.edu>
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Subject: Re: [aqm] Gathering Queue Length Statistics
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On Wed, Feb 25, 2015 at 9:53 AM, Ryan Doyle <rpdoyle@live.unc.edu> wrote:
> Hello,
>
> I am a senior undergraduate student at the University of North Carolina at
> Chapel Hill and am studying the effectiveness of AQMs. I have set up a lab
> network and plan on running different sets of experiments with different
> AQMs. My router machines are running Linux kernel version 3.16.0.
>
> I am using the fq_codel, codel, and pie qdiscs for my research and am
> wondering if there is a way to collect statistics regarding the average
> queue length since a qdisc was enabled? I have looked at tc's "-s" flag for
> statistics, but they show nothing about queue length and I have been unable
> to find anything else that might help me get queue length statistics.

Oh, god. I am getting incredibly sensitive about average queue length,
and I realize that that is not what you meant. But since not enough
people have seemingly read this or any of the related materials, here
it is again.

http://www.pollere.net/Pdfdocs/QrantJul06.pdf

And I of course always recommend van´s talk on the fountain model for
thinking about closed loop servo systems.

http://www.bufferbloat.net/projects/cerowrt/wiki/Bloat-videos

In the bufferbloat project...

We have developed many tools that drive aqms hard, see netperf-wrapper
on github for that, which measures e2e delay without requiring any
tools on the routers inbetween. e2e delay in my mind is way more
important than average queue length. And you can derive the queue
length(s) from tcp timestamps in those netperf-wrapper tests, from
additional packet captures, if you must.

If you absolutely MUST derive average queue length from the box, you
can poll the interface frequently with tc -s qdisc show as well as
with ifconfig- and parse out the number of packets and the number of
bytes. But you can do MUCH more valid statistical analysis than that,
with that sort of data set - and if you poll too frequently you will
heisenbug your tests, as those data collection calls take locks that
interfere with the path. and we have all sorts of advice about traps
for the unwary here:

http://www.bufferbloat.net/projects/codel/wiki/Best_practices_for_benchmarking_Codel_and_FQ_Codel


Please use things like CDFs to see the range of delays, rather than
averages. It is what happens at above 90% of the range that makes
bufferbloat maddening to ordinary users.

I am summarily rejecting any papers that I review that report average
queue length as if it meant anything. And for a few other reasons. You
have been warned. I really lost my temper after the last paper I
reviewed last weekend and the resulting flamage is all over the bloat
list and codel lists, and starts here:

https://lists.bufferbloat.net/pipermail/codel/2015-February/000872.html


> Best,
> Ryan Doyle
>
> _______________________________________________
> aqm mailing list
> aqm@ietf.org
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>



-- 
Dave Täht
Let's make wifi fast, less jittery and reliable again!

https://plus.google.com/u/0/107942175615993706558/posts/TVX3o84jjmb