[Cake] Testing variants of the MTU latency scaling

[Jonas Mårtensson]

One thing that is still not clear to me from these results: if I run cake
on an IFB without ingress mode (i.e. the default?), does the MTU scaling
have any impact on TCP download throughput?

/Jonas

On Sun, Apr 22, 2018 at 11:09 PM, Jonathan Morton <chromatix99 at gmail.com>
wrote:

> > 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
>
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