[Codel] fq_codel: revenge of the standing queue

[Eric Dumazet]

On Tue, 2012-09-04 at 11:35 -0700, Dave Taht wrote:

> 2) New flows
> 
> The next issue (at these 10 and 100Mbit rates) is the "new flow" idea
> in fq_codel. It is VERY useful pushing sparse flows to the fore of the
> queues, and also provided a boost to long RTT flows. However at high
> levels of occupancy and low bandwidth, flows empty their queue
> rapidly, become "new flows", and then mislead codel for that flow into
> resetting itself again, since we end up with a short delay for the
> re-entrance. This isn't a particularly horrible behavior, but as my
> own hope for this feature was to make voip better primarily, and
> everything else we got from it was a bonus.
> 
> TSQ really made this oddity show up big.  I think on routed traffic it
> will be less of an issue.

This is simply not true, and based on misunderstanding on what does
fq_codel.

A flow never leaves the new flow list before doing a full RR cycle in
old flow list (even without any packet in this flow)

_IF_ we did a full RR cycle, then we accumulated a full quantum of
credit, and lost our rank the 'fq_codel RR queue' (the combination of
the 2 internal queues, new and old)

Therefore, we allow the flow to be queued in 'new flow', to recover a
bit of the lost rank in queue.

Basically you dont interpret correctly the 'new_flow_count' counter.

It means almost nothing at all, since for a given TCP flow, we _can_
increment this counter several time.