[Bloat] AQM and PPP on Linux

[Eric Dumazet]

On Tue, 2015-07-28 at 07:31 -0700, Simon Barber wrote:
> The issue is that Codel tries to keep the delay low, and will start 
> dropping when sojourn time grows above 5ms for 100ms. For longer RTT links 
> more delay is necessary to avoid underutilizing the link. This is due to 
> the multiplicative decrease - it's worst with Reno, where the halving of 
> cWind means that you need to have a full BDP of data in the buffer to avoid 
> the link going idle when cWind is halved. With longer RTTs this means more 
> delay than Codel allows is required to avoid a throughput hit. The worst 
> case happens when a single flow is controlled, but that can be a common 
> situation. My proposal is to sense and have the target value in Codel 
> automatically adjust when this worst case scenario happens - which would 
> mitigate most of the downside.

As I said, I've never seen what you describe.

100ms value is not a go/nogo threshold. It is a hint, based on real
world values. We are speaking of 100 ms sojourn time in the CoDel queue,
not sojourn time in the Internet !

You can still have flows with 500 ms or even 10 sec rtt and codel just
works fine.

Are you sure you understood how this was working ?