[Cake] total download rate with many flows

[Jonathan Morton]

I'm curious as to why you think Cobalt is more aggressive than Codel.  It
does use more accurate approximations to the mathematical ideal than the
"reference" codel does.

It is however very odd that the Diffserv mode has any effect on this at
all.  It could be explained if a lot of the traffic is marked CS1, since
the Bulk tin has looser AQM parameters.  That suggests that selecting
'satellite' might help similarly.

Something worth trying would be to alter the failsafe shaper's rate of
advance.  Currently it has a one-quarter rate, which might be too
restrictive.  Tests at one-half and three-quarters might therefore be
interesting.  Otherwise, I don't think trying to modify the way ingress
mode works will do the right things.

Ultimately, this arises because Cake is having to drop packets in order to
signal congestion, and when there's a lot of flows, a lot of signals must
be sent to get through to them all.  With ECN working, it doesn't need to
waste bandwidth merely to signal.

The only other reasonable approach is to somehow reduce the signalling rate
under heavy flow load.  That requires informing Cobalt of the number of
bulk flows, and using that to scale the signalling frequency somehow.

- Jonathan Morton
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