[j-nsp] CoS buffer size

[Dan Peachey]

Hi Adam,

My understanding was that you might be able to oversubscribe only using PIR
> (need to test).
> And in that case all the queues are in the excess region.
> So only the excess priorities are honoured (HI and LO in strict priority
> fashion) and queues with the same priority are serviced round robin.
>

You can oversubscribe both PIR and CIR (G-rate) so you have to be careful
how much G-rate you allocate (if you are selling a PIR/CIR service that
is). In H-CoS mode with only PIR set all queues are in excess even if the
aggregate of all the shapers does not oversubscribe the interface bandwidth
(or aggregate shaper). In per-unit mode with only PIR set, you have to
oversubscribe the shapers to end up with all queues in excess.


> I also thought that weight with which the queues in excess region are
> served is proportional to the transmit-rate (as percentage of VLAN PIR).

Though looking at the show outputs and reading your test results it looks
> like it's not the case.
>

Well, the weights are determined from the transmit-rates but the queues
aren't proportioned in the way you'd expect them to be relative to the
transmit-rates. For example, you'll find that you can get packet loss in a
queue that is sending less than contracted rate if you oversubscribe
another queue at the same priority level. The weightings don't translate to
an exact percentage of bandwidth in reality.


> But have you tried setting excess-rate for the queues - that should be
> honoured while a given queue is in excess region right?
>

Yep, but if you set excess-rate = transmit-rate then the weights are just
the same as if you hadn't set them and it doesn't affect the behaviour of
the queues.


> I just can't believe that once in excess region the queues (using %) are
> using main interface PIR -is it a bug please?
>

It's more a case that per-queue guaranteed rates are set to zero when you
are in H-CoS or oversubscribed per-unit mode which means that you have to
factor in G-rate for each node when it would be nice not to have to bother
(and you can not bother, but things don't work quite the way you expect
them to).


> With regards to buffers
> May I oversubscribe buffers by configuring say "delay-buffer-rate percent
> 10" for 11 VLANs on a physical interface please?


Not sure on that one, never tried to be honest.

Dan