On Fri, 2012-07-06 at 11:20 -0700, Anton Mich wrote:
> I'm looking into the Codel implementation in Linux and the ns-2 and
> ns-3 simulation and found some inconsistency regarding the behavior of
> the count variable.
>
> Specifically, in the ACM paper the count is decreased by 2 in the
> pseudocode (and ns-3 code) when re-entering a dropping state, in Linux
> the count is reduced to the difference of the current count minus the
> last count, and in ns-2 the count is reduced by 1. I assume that the
> most up to date version is that in linux but for me it's not very
> obvious why we set the value of the count like that, departing from
> the ACM paper intuition.
>
> Also, when testing the Linux codel scheme I discovered the following
> behavior:
> Because rec_inv_sqrt is initialized to 0, it remains 0 under some
> specific traffic patterns and that forces the interval/rec_inv_sqrt to
> a very small number, no matter what the actual count is. For this not
> to happen we have to enter the else statement that sets:
>
> else {
> vars->count = 1;
> vars->rec_inv_sqrt = ~0U >> REC_INV_SQRT_SHIFT;
> }
>
> I found that this behavior is fixed when initializing rec_inv_sqrt
> like (reason being that when the newton step is first evaluated, the
> count is equal to 1, so we need a good guess of the rec_inv_sqrt):
>
> @@ -166,6 +166,7 @@
> static void codel_vars_init(struct codel_vars *vars)
> {
> memset(vars, 0, sizeof(*vars));
> + vars->rec_inv_sqrt = ~0U >> REC_INV_SQRT_SHIFT;
> }
>
> Has anybody encountered this behavior? Is this the correct fix?
Hi Anton
Sorry for the delay !
I did some testings and yes, it seems we have an init problem.
But as codel_vars_init() is more often used, I prefer the following
fix :
diff --git a/include/net/codel.h b/include/net/codel.h
index 550debf..c6d92fd 100644
--- a/include/net/codel.h
+++ b/include/net/codel.h
@@ -305,6 +305,8 @@ static struct sk_buff *codel_dequeue(struct Qdisc *sch,
}
}
} else if (drop) {
+ u32 delta;
+
if (params->ecn && INET_ECN_set_ce(skb)) {
stats->ecn_mark++;
} else {
@@ -320,9 +322,10 @@ static struct sk_buff *codel_dequeue(struct Qdisc *sch,
* assume that the drop rate that controlled the queue on the
* last cycle is a good starting point to control it now.
*/
- if (codel_time_before(now - vars->drop_next,
- 16 * params->interval)) {
- vars->count = (vars->count - vars->lastcount) | 1;
+ delta = vars->count - vars->lastcount;
+ if (delta && codel_time_before(now - vars->drop_next,
+ 16 * params->interval)) {
+ vars->count = delta;
/* we dont care if rec_inv_sqrt approximation
* is not very precise :
* Next Newton steps will correct it quadratically.
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