[Cake] Upstream submission of dual-mode fairness patch

[Jonathan Morton]

> On 3 Mar, 2019, at 1:26 pm, Sebastian Moeller <moeller0 at gmx.de> wrote:
> 
>>> Doesn't this look like ingress magic being applied selectively to the users based on number of flows? I thought that the idea behind the ingress keyword is to effectively shape harder the more bulk flows are coming in. 
>> 
>> No, it simply counts dropped packets against the shaper, as well as those actually transmitted.  
> 
> Sure, but the question is how is the resulting "pressure" to drop/mark distributed over the existing (bulk) flows.
> 
>> There shouldn't be that many packets being dropped to make this much of a difference.
> 
> My intuition (probably wrong) is that is not the few packets dropped, but the fact that the the dropping does seem to be restricted to the flows of the IP with more flows, no?

As long as there is a backoff response to a dropped packet, the extra back-pressure should be contained to the flows experiencing drops, and other flows should see no difference - so you should see a slight reduction in goodput on the 16 flows, but *no increase* on the single flow in parallel.

Even if that were not the case, the single flow should take longer on average to recover from a cwnd reduction (even in CUBIC) to the fair BDP.  That should result in a greater reduction in goodput on the single flow than the many flows - but we see the reverse here.

So I'm not entirely sure what's happening here, but at least the asymmetry isn't too bad; it's achieving significantly better host-fairness than a pure flow-fair system would.

 - Jonathan Morton