[Cake] 5ms target hurting tcp throughput tweakable?

[Sebastian Moeller]

Hi Andy,


> On Feb 27, 2017, at 19:02, Andy Furniss <adf.lists at gmail.com> wrote:
> 
> Sebastian Moeller wrote:
>> Looking at tc-adv, I would recommend to use “rtt 50” (maybe it is
>> “rtt 50ms”) which allows to directly explicitly request a new
>> “interval” (which IIRC is corresponding to the time you allow for
>> the TCP control loop to react to cake’s ecn-marking/dropping)
>> “target’ will be calculated as 5% of the explicit interval, in
>> accordance with the rationale in the codel RFC.
> 
> 50 is certainly better than 10, but still seems to hurt single a bit
> even on a close server.

	Oopps, what I meant to convey is that there is the numeric option “rtt NNN” that allows to select the exact RTT/interval you believe to be valid. I picked 50 just because I wanted to give a concrete example, not because I believe 50 to be correct for your experiments...

> 
> On more distant servers maybe even more - I am getting results that are
> too variable to tell properly at the current time (of day).
> 
> http://www.thinkbroadband.com/speedtest/results.html?id=1488216166262542155
> 
> Almost OK, but with 100ms this test usually shows x1 and x6 the same.
> 
> http://www.thinkbroadband.com/speedtest/results.html?id=1488218291872647755

	Interesting. My mental image is that interval defines the time you give both involved TCPs to get their act together before escalating cake’s/codel’s signaling. The theory as far as I understand it says that the RTT is the lower bound for the time required, so I am not totally amazed that relaxing that interval a bit increases bandwidth utilisation at a vert moderate latency cost (if any). The art is to figure out how to pick the interval (and I believe the codel paper showed, that at least for codel the exact number is not so important but the ballpark/ order of magnitude should match).

> 
> Of course as we all know ingress shaping is a different beast anyway and
> would deserve its own thread.

The cool thing is that ingress shaping with all its “approximateness” (is that a word) works as well as it does ;)

Best Regards