Re: [Cake] Proposing COBALT

["Luis E. Garcia" <luis@bitamins.net>]

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Jonathan,
I think this would be a great idea to implement and test.
Can COBALT's behavior be easily implemented to test it using the OpenWRT or
LEVE ?

Regards,
Luis

On Fri, May 20, 2016 at 8:36 AM, Jonathan Morton <chromatix99@gmail.com>
wrote:

> >> If the relative load from the flow decreases, BLUE’s action will begin
> to leave the subqueue empty when serviced, causing BLUE’s drop probability
> to fall off gradually, potentially until it reaches zero.  At this point
> the subqueue is naturally reset and will react normally to subsequent
> traffic using it.
> >
> > But if we reach a queue length of codel’s target (for some small amount
> of time), would that not be the best point in time to hand back to codel?
> Otherwise we push the queue to zero only to have codel come in and let it
> grow back to target (well approximately).
>
> No, because at that moment we can only assume that it is the heavy
> pressure of BLUE that is keeping the queue under control.  Again, this is
> an aspect of Codel’s behaviour which should not be duplicated in its
> alternate, because it depends on assumptions which have been demonstrated
> not to hold.
>
> BLUE doesn’t even start backing off until it sees the queue empty, so for
> the simple and common case of an isochronous flow (or a steady flood
> limited by some upstream capacity), BLUE will rapidly increase its drop
> rate until the queue stops being continuously full.  In all likelihood the
> queue will now slowly and steadily drain until it is empty.  But the load
> is still there, so if BLUE stopped dropping entirely at that point, the
> queue would almost instantly be full again and it would have to ramp up
> from scratch.
>
> Instead, BLUE backs off slightly and waits to see if the queue *remains*
> empty during its timeout.  If so, it backs off some more.  As long as the
> queue is still serviced while BLUE’s drop probability is nonzero, it will
> back down all the way to zero *if* the traffic has really gone away or
> become responsive enough for Codel to deal with.
>
> Hence BLUE will hand control back to Codel only when it is sure its extra
> effort is not required.
>
>  - Jonathan Morton
>
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