[Cake] Fighting bloat in the face of uncertinty

[Justin Kilpatrick]

Sadly this isn't a driver, its a point to point wireless device that often seems designed to introduce bloat. I could probably ssh in and configure it to behave properly but that's not very scalable. 

Underestimating link capacity dramatically isn't an option as no matter how buttery smooth the experience people still crave those high speedtest numbers. 

> 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.

So you believe that setting the target RTT closer to the path latency was not the main contributor to reducing bloat? Is there a configuration I could use to demonstrate that one way or the other? 


-- 
  Justin Kilpatrick
  justin at althea.net

On Sat, Sep 7, 2019, at 7:42 PM, Jonathan Morton wrote:
> > 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