[Codel] RFC: Realtime Response Under Load (rrul) test specification

Dave Taht dave.taht at gmail.com
Fri Nov 9 05:21:20 EST 2012

On Tue, Nov 6, 2012 at 9:52 PM, David Collier-Brown <davec-b at rogers.com> wrote:
> Dave Taht wrote:
>> I have been working on developing a specification for testing networks
>> more effectively for various side effects of bufferbloat, notably
>> gaming and voip performance, and especially web performance.... as
>> well as a few other things that concerned me, such as IPv6 behaviour,
>> and the effects of packet classification.
>> A key goal is to be able to measure the quality of the user experience
>> while a network is otherwise busy, with complex stuff going on in the
>> background, but with a simple presentation of the results in the end,
>> in under 60 seconds.
> Rick Jones <rick.jones2 at hp.com> replied:
> | Would you like fries with that?
> |
> | Snark aside, I think that being able to capture the state of the user
> | experience in only 60 seconds is daunting at best.  Especially if
> | this testing is going to run over the Big Bad Internet (tm) rather
> | than in a controlled test lab.
>> This portion of the test will take your favourite website as a target
>> and show you how much it will slow down, under load.
> | Under load on the website itself, or under load on one's link.  I
> | ass-u-me the latter, but that should be made clear.  And while the
> | chances of the additional load on a web site via this testing is
> | likely epsilon, there is still the matter of its "optics" if you will
> | - how it looks.  Particularly if there is going to be something
> | distributed with a default website coded into it.
> This, contraintuitive as it might sound, is what will make the exercise
> work: an indication as a ratio (a non-dimensional measure) of how much
> the response-time of a known site is degraded by the network going into
> queue delay.

Exactly! The core comparison of this test is unloaded vs loaded
behavior of a network, which is to a large extent independent of the
underlying raw bandwidth.

I should work harder in bringing this out in the document. I note that
the most core component of the benchmark really is web performance
without and then with load, as exemplified by the short video herein:


with the dozens of dns lookups and short tcp streams that entails.

Regrettably emulating that behavior is hard, so being able to a/b a
random website while under the kinds of loads generated by rrul is a
key intent.

while there are interesting factoids to be gained by the behavior of
the elephantine TCP flows in relation to each other, it's the behavior
of the thinner flows that matters the most.

> We're assuming a queuing centre, the website, that is running at a
> steady speed and load throughout the short test,  and is NOT the
> bottleneck.  When we increase the load on the network, it becomes the
> bottleneck, a queue builds up, and the degradation is directly
> proportional to the network being delayed.
> A traditional measure in capacity planning is quite similar to what you
> describe: the "stretch factor" is the ratio of the sitting-in-a-queue
> delay to the normal service time of the network. When it's above 1,
> you're spending as much time twiddling your thumbs as you are doing
> work, and each additional bit of load will increase the delay and the
> ratio dramatically.

I like the stretch factor concept, a lot.

> I don't know if this will reproduce, but this, drawn as a curve against
> load, the ratio you describe will look like a hockey-stick:
> ............................./
> 3.........................../
> .........................../
> ........................../
> 2......................../
> ......................../
> ......................./
> 1....................-
> ._________----------
> 0....5....10....15....20....25
> Ratio is the Y-axis, load is the X, and the periods are supposed to be
> blank spaces (;-))
> At loads 1-18 or so, the ratio is < 1 and grows quite slowly.
> Above 20, the ratio is >> 1 and grows very rapidly, and without bound
> The results will look like this, and the graphic-equalizer display will
> tell the reader where the big components of the slowness are coming
> from.  Pretty classic capacity planning, by folks like Gunther.
> Of course, if the web site you're measuring gets DDOSed in the middle of
> the test, Your Mileage May Vary!
> --dave
> --
> David Collier-Brown,         | Always do right. This will gratify
> System Programmer and Author | some people and astonish the rest
> davecb at spamcop.net           |                      -- Mark Twain
> (416) 223-8968
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Dave Täht

Fixing bufferbloat with cerowrt: http://www.teklibre.com/cerowrt/subscribe.html

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