[Cake] Testing variants of the MTU latency scaling

[Jonathan Morton]

> Takeaways (see attached plots):
> 
> - The MTU scaling does indeed give a nice benefit in egress mode
>  "tcp-download-totals" plot. From just over 6 Mbps to just over 8 Mbps
>  of goodput on the 10 Mbit link. There is not a large difference
>  between 2MTU and 4MTU, except that 4MTU hurts inter-flow latency
>  somewhat.
> 
> - The effect for upload flows (where Cake is before the bottleneck;
>  10mbit-upload.png) is negligible.
> 
> - The MTU scaling really hurts TCP RTT (intra-flow latency;
>  tcp-upload-tcprtt-10mbit.png and rrul-tcprtt.png).
> 
> - For bidirectional traffic the combined effect is also negligible.
> 
> 
> Based on all this, I propose we change the scaling mechanism so that it
> is only active in egress mode, and change it from 4 MTUs to 2. I'll
> merge Kevin's patch to do this unless someone complains loudly :)
> 
> If you want me to run other tests, let me know.

I'm not actually sure what you've measured here - unless you've somehow managed to swap "ingress" with "egress" mode in a strange manner.  I don't see any systematic measurement of the different MTU scales in ingress mode in your results, which makes your assertion that it should only be active in egress mode rather odd.

 - Jonathan Morton