[Codel] fq_codel: revenge of the standing queue

[Eric Dumazet]

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

> 1) An empty queue in fq_codel has no meaningful information.
> 
> in: codel_should_drop
> 
>         if (!skb) {
>                 vars->first_above_time = 0;
>                 return false;
>         }



> I believe the codel intent here was to reset codel's state when the
> single FIFO queue was emptied. In fq_codel's case, an empty queue
> contains no information about the state, really, and a
> 
>         if (!skb) {
>                 if(sch->qstats.backlog <= mtu)
>                          vars->first_above_time = 0;
>                 return false;
>         }
> 
> comes closer to the intent. That said, the null skb then bleeds into
> the rest of the algorithm
> 
> in codel_dequeue
> 
> 
>         if (!skb) {
>                  vars->dropping = false;
>                 return skb;
>         }
> 
> later on there's
> 
>         if (vars->dropping) {
>                 if (!drop) {
>                         /* sojourn time below target - leave dropping state */
>                         vars->dropping = false;
> 
> and the same assumption within the while loop and in the else if(drop)...
> 
> I'm pretty sure that leaving of dropping state just because this
> fq_codel queue is (temporarily) empty is not the right thing, and that
> the main reason for exiting the dropping state should be getting under
> the target delay. It might make sense to reschedule the next drop on a
> null skb, perhaps after reducing count...
> 
> While I've fiddled with these ideas, and got some drainage, I do get
> fairly big oscillations in queue depth, and starvation of some flows,
> in various versions of my explorations.  Which led me to looking at
> quantums and...

I have no idea of what you try to say.

Each flow has its own cvars :

skb = codel_dequeue(sch, &q->cparams, &flow->cvars, &q->cstats,
dequeue);

So each flow has its own codel unit.

Try to view fq_codel as a multiplexor, then a Codel unit for each flow.

So if you believe there is a bug in Codel, try to describe the big in
Codel, not in fq_codel, because fq_codel is _not_ a codel variant.

The only thing that fq_codel codel units share are the parameters and
stats.