[Cake] Cake throughput levels off during longs tests

[Sebastian Moeller]

Hi Toke,

On Oct 29, 2015, at 16:59 , Toke Høiland-Jørgensen <toke at toke.dk> wrote:

> Toke Høiland-Jørgensen <toke at toke.dk> writes:
> 
>> So I ran a couple of tests of Cake on the Archer-c7 on newest openwrt
>> trunk with sqm-scripts set to 30/30 Mbps. The attached graphs are the
>> result of equivalent fq_codel and cake settings.
>> 
>> As you can see, the cake throughput levels off over long test durations.
>> Anyone has any idea why?
> 
> Right, so did some more experiments, and this is definitely related to
> CPU usage. I did a new build of openwrt with an updated toolchain, and
> now the archer runs fine with Cake at 30 Mbps symmetrical. However, it
> starts to struggle at 50 Mbps; but only after around three minutes of
> continuous load.

	Could this be some sort of thermal throttling? The plot reminds me a bit of the currently popular plots of smartphone performance/CPU-frequency over time under load over time… I do not know whether the archer’s cpu actually can throttle… If it would be pure CPU-ooomph shouldn’t it be there from early on, 200 seconds seem too long to run out of resources like cache?

> 
> See the comparison graphs here:
> https://kau.toke.dk/experiments/cake/archer/cake-30-50.png
> 
> That directory also contains the full Flent data files and packet dumps.
> 
> So the question is now, of course, is there anything that can be done
> about this, or should we simply accept that the box is out of oomph? The
> way it behaves does strike me as weird…

	Maybe cooling it artificially during the test might reveal whether the slow down is temperature dependent? I would test myself but I do not own an archer C7…

Best Regards
	Sebastian


> 
> -Toke
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