[Codel] [RFC PATCH] codel: tighten responsiveness and more reliably deal with loads

[Eric Dumazet]

On Sun, 2012-08-26 at 12:02 -0700, Dave Täht wrote:
> From: Dave Taht <dave.taht at bufferbloat.net>
> 
> This updates the codel algorithm to more closely match the current
> experimental ns2 code. Not presently recomended for mainline.
> 
> 1) It shortens the search for the minimum by reducing the window over
>    the intervals and re-running the control law to better schedule
>    the estimate.
> 2) Holds onto the drop schedule harder when re-entering drop state.
> 3) Corrects for newton method running in reverse.
> 
> ---
>  include/net/codel.h |   39 +++++++++++++++++++++++----------------
>  1 file changed, 23 insertions(+), 16 deletions(-)
> 
> diff --git a/include/net/codel.h b/include/net/codel.h
> index 389cf62..57031ad 100644
> --- a/include/net/codel.h
> +++ b/include/net/codel.h

> +		codel_Newton_step(vars);
> +		codel_Newton_step(vars);

This makes no sense for several reasons :

1) If we do the 	vars->count = 1;
        vars->rec_inv_sqrt = ~0U >> REC_INV_SQRT_SHIFT;

Then there is no need of _any_ Newton steps.
The rec_inv_sqrt value is the good one.

2) If we change vars->count to vars->count - 2
  Then a single step is enough.


I can understand that with the current way :

   vars->count =  vars->count - vars->lastcount

then we can have a slight error, but this gave no difference in codel
experimental behavior.

I would say that codel response to bad queue is pretty easy (doing count
++ at each drop), but changes in count to adapt to oscillations between
good and bad queues is yet to be investigated.

Do we have to do 
0) current way (count - lastcount)
1) count = count - 1 
2) count = count - 2 
3) count = count * 88%
4) count = count * 75%
5) count  = count * 50%
6) count = count * 25%
7) count = some clever function using history of previous changes
...

In my prior tests, 1) and 2) were pretty bad, I am sure it is not the
right way.

(the count = 1 is only done when queue was idle for a long time)