[Cake] A few puzzling Cake results

Toke Høiland-Jørgensen toke at toke.dk
Tue Apr 17 11:10:44 EDT 2018

Luca Muscariello <luca.muscariello at gmail.com> writes:

> I will check that later, still unsure.
> First guess: the quantum component should influence only how close to a
> fluid bit-wise approximation you are.
> So cake gets closer by automatic adjustment.
> The computation of the correction factor should be done by computing the
> probability that a packet
> of a sparse flow loses priority because of the quantum. Bad setting, higher
> probability, ideal setting 0 probability.
> So your formula seems still wrong to me...

The formula expresses the conditions under which a flow is guaranteed to
be treated as sparse by the scheduler, under some quite strong
assumptions (most notably that the sparse flow is sending packets at a
fixed rate).

It's derived from the fact that when a packet from the sparse flow
arrives, it (in the worst case) has to wait for the bulk flow packet
that just started getting service (i.e., wait for L/R), after which the
sparse flow itself will get service (in time L_s/R), then it will have
to wait for its queue to pass through a whole round of scheduling (LN/R)
before it can get service as a sparse flow again. Adding these together,
the inter-arrival time between sparse flow packets has to be greater
then (L(N+1)+L_s)/R, which when converted to a rate gives the formula I

You are right, of course, that in the general case it will be different;
but I was talking about the specific case of the fixed-rate sparse flow
here... :)


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