Re: [Codel] Fwd: count and rec_inv_sqrt initialization

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2012-07-26 at 16:47 -0700, Anton Mich wrote:
> Hi Eric,
> 
> Thanks for looking into this! 
> May I also ask what is the intuition behind using this delta (which
> is, in my understanding, equal to the number of drops between the
> lastcount and the current count) as the new count value? I tried to
> track down in the codel list when this changed from what's in the
> paper, pseudocode and NS-2 to what is now in Linux but I didn't find
> anything.
> 

Kathleen & Van are still discussing of what should be exactly done here.

The delta idea came from them, at a moment no public discussion was
happening (before publication, we were privately implementing codel)

Before the delta, we had following codes :

Version v1 -> v6 : (05/03/2012 -> 05/05/2012)

+                       if (now.tv64 - q->drop_next.tv64 < 
+                               16 * q->interval.tv64) {
+                               int c = q->count - 1;
+                               q->count = c < 1 ? 1 : c;
+                       } else {
+                               q->count = 1;
+                       }

But count was growing and was not going back to lower values

I suggested to use instead :

if (now - drop_next<  16.*interval) {
        unsigned int c = q->count >> 1;

        count = c < 1 ? 1 : c;
else {
        count = 1;
}

Then we also tried 

c = min(q->count - 1, q->count - (q->count>>4));

(Dave even had a  module param :)

static u32 count_rescale(struct codel_sched_data *q, codel_time_t now) {
        s32 c = 1;

        if (codel_time_after(now - q->drop_next, 16 * q->interval)) {
                switch(decrease_method) {
                        case 3:
                                break;
                        case 2: /* Dumazet 2 */
                                c = q->count >> 1;
                                break;
                        case 1: /* Dumazet 1 */
                                c = min(q->count - 1,
                                        q->count - (q->count >> 4));
                                break;
                        case 0: /* Codel Paper Default */
                        default:
                                c = q->count - 1;
                }
                c = max(1U, c);
        }
        return (u32) c;
}

in v13 ( 05/10/2012) we had the current form

But codel is not finalized ;)