Re: [Bloat] lwn.net's tcp small queues vs wifi aggregation solved

[Jonathan Morton <chromatix99@gmail.com>]

>> No, it's in the driver for ath9k. So it would be possible to delay it
>> slightly to try to build a larger one. The timing constraints are too
>> tight to do it reactively when the request is granted, though; so
>> delaying would result in idleness if there are no other flows to queue
>> before then...


There has to be some sort of viable compromise here.  How about initiating the request immediately, then building the aggregate when the request completes transmission?  That should give at least the few microseconds required for the immediately following acks to reach the queue, and be included in the same aggregate.

> ​​​Isn't this sort of delay a natural consequence of a busy channel?
> 
> What matters is not conserving txops *all the time*, but only when the channel is busy and there aren't more txops available....
> 
> So when you are trying to transmit on a busy channel, that contention time will naturally increase, since you won't be able to get a transmit opportunity immediately.  So you should queue up more packets into an aggregate in that case.
> 
> We only care about conserving txops when they are scarce, not when they are abundant.
> 
> This principle is why a window system as crazy as X11 is competitive: it naturally becomes more efficient in the face of load (more and more requests batch up and are handled at maximum efficiency, so the system is at maximum efficiency at full load.
> 
> Or am I missing something here?

The problem is that currently every data aggregate received (one TXOP each from the AP) results in two TXOPs just to acknowledge them, the first one containing only a single ack.  This is clearly wasteful, given the airtime overhead per TXOP relative to the raw data rate of modern wifi.  Relying solely on backpressure would require that the channel was sufficiently busy to prevent the second TXOP from occurring until the following data aggregate is received, and that just seems too delicate to me.

 - Jonathan Morton