I don't believe there is a practical way to poll a queue fast enough to 
properly estimate the the queueing delay that an arriving packet will see. 
There was a discussion on the sFlow mailing list around measuring packet 
queueing delay. While implementing the ideas would require ASIC support in a 
physical switch, it's possible that something along the proposed lines could be 
implemented in OVS.

https://groups.google.com/forum/?fromgroups=#!topic/sflow/dz0nsXqBYAw

A flexible way to capture queue depth metrics might be to enhance the 
enqueue:port:queue action to record the queue depth in a register so that it 
could be used to increment a histogram counter or associated with a sampled 
packet?

On Mar 21, 2013, at 12:38 PM, Ben Pfaff <[email protected]> wrote:

> On Wed, Mar 20, 2013 at 03:01:28PM -0700, Geetha S wrote:
>> I have some queues in my Queue table (ovs db). As and when packets flow
>> through the queue interface, I want an event to reach the controller by
>> giving information about the queue stats at that instance of time.
> 
> I don't think we have a way to do that, except for the controller to
> poll the switch.
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