On 30/08/2011, at 3:42 PM, Adrian Chadd wrote:

On 30 August 2011 11:34, Tom Herbert [off-list ref] wrote:

quoted

The generalization of BQL would be to set the queue limit in terms of
a cost function implemented by the driver.  The cost function would
most likely be an estimate of time to transmit a packet.  

That's a great idea.  Best that it be in nanoseconds, we may well have some seriously fast network interfaces to deal with.

quoted

So C(P)
could represent cost of a packet, sum(C(P) for P queued) is aggregate
cost of queue packets, and queue limit is the maximum cost sum.  For
wired Ethernet, number of bytes in packet might be a reasonable
function (although framing cost could be included, but I'm not sure
that would make a material difference).  For wireless, maybe the
function could be more complex possibly taking multicast, previous
history of transmission times, or other arbitrary characteristics of
the packet into account...

I can post a new patch with this generalization if this is interesting.

As I said before, I think this is the kind of thing the rate control
code needs to get its dirty hands into.

With 802.11 you have to care about the PHY side of things too, so your
cost suddenly would include the PER for combinations of {remote node,
antenna setup, TX rate, sub-frame length, aggregate length}, etc. Do
you choose that up front and then match a cost to it, or do you
involve the rate control code in deciding a "good enough" way of
handling what's on the queue by making rate decisions, then implement
random/weighted/etc drop of what's left? Do you do some weighted/etc
drop beforehand in the face of congestion, then pass what's left to
the rate control code, then discard the rest?

Since Minstrel already knows an estimate of the PER to each remote node (expressed in terms of success probability per shot, so there's a bit of math to do), and the stack knows about transmit times, implementing a way to ask the question isn't particularly hard.  Other rate controls could make up their own guesstimates based on whatever factors they want to use.

However, that's going to change rapidly, so I suggest that we want some backlog grooming on a regular basis (like, after each rate control iteration) that might well reevaluate and drop or mark packets that are already in the queues.

C(P) is going to be quite variable - a full frame retransmit of a 4ms
long aggregate frame is SUM(exponential backoff, grab the air,
preamble, header, 4ms, etc. for each pass.)


Adrian

Indeed.

Andrew