[Cake] progress? dual-src/dsthost unfairness

Toke Høiland-Jørgensen toke at redhat.com
Thu Feb 14 06:15:37 EST 2019

gamanakis at gmail.com writes:

>> On the contrary, even if a particular flow is sparse, the relevant
>> quantum calculation should involve the number of *bulk* flows
>> attached to the same host. Though there is possibly an argument for
>> making it the *sum* of the sparse and bulk flows, making it just the
>> sparse ones is wrong.
> I was about to point out that my assumption is obviously wrong.
> cake_enqueue() can still assign a packet to a bulk flow. Will try with
> the sum of the flows and report.

>From a fairness perspective it doesn't really matter whether you count
the sparse flows or not. You use this for assigning a single quantum's
worth of initial deficit; any flow that actually needs fairness enforced
is going to be backlogged anyway, and the deficit increase in bulk state
is what matters for enforcing fairness.

What the initial quantum does is that it limits how big packet(s) a
sparse flow can send before it is demoted to bulk. There are certainly
some esoteric cases where this might matter (things like, can a DNS flow
get two small back-to-back packets through in one go); but this is going
to vary with the traffic conditions anyway, so I doubt it matters in

Given this, and given that the state tracking is already plenty
complicated, I'd suggest not counting per-host sparse flows at all, and
just using the bulk count. I'm pretty sure you'll get the same fairness
behaviour in this case.

Care to try that? :)


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