[Cake] Fighting bloat in the face of uncertinty

[Jonathan Morton]

> On 8 Sep, 2019, at 2:31 am, Justin Kilpatrick <justin at althea.net> wrote:
> 
> If I set a throughput that's 50% too high should it still help? In my testing it didn't seem to.

In that case you would be relying on backpressure from the network interface to cause queuing to actually occur in Cake rather than in the driver or hardware (which would almost certainly be a dumb FIFO).  If the driver doesn't implement BQL, that would easily explain 300ms of bloat.

In fact I'm unsure as to why changing the AQM parameters would cure it.  You may have benefited from an unintentional second-order effect which we normally try to eliminate, when the 'target' parameter gets too close to the CPU scheduling latency of the kernel.

I generally find it's better to *underestimate* the bandwidth parameter by 50% than the reverse, simply to keep the queue out of the dumb hardware.  But if you want to try implementing BQL in the relevant drivers, go ahead.

 - Jonathan Morton