Re: [PATCH] sch_red: fix weighted average calculation
From: Eric Dumazet
Date: Thu Sep 13 2012 - 09:53:51 EST
On Thu, 2012-09-13 at 09:43 -0400, Cyril Chemparathy wrote:
> This patch fixes an apparent bug in the running weighted average calculation
> used in the RED algorithm.
> Going by the described formula:
> qavg = qavg*(1-W) + backlog*W
> => qavg = qavg + (backlog - qavg) * W
> ... with W converted to a pre-calculated shift, this then becomes:
> qavg = qavg + (backlog - qavg) >> logW
> ... giving the modified expression introduced by this patch.
> Signed-off-by: John Dowdal <jdowdal@xxxxxx>
> include/net/red.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/include/net/red.h b/include/net/red.h
> index ef46058..05960a4 100644
> --- a/include/net/red.h
> +++ b/include/net/red.h
> @@ -287,7 +287,7 @@ static inline unsigned long red_calc_qavg_no_idle_time(const struct red_parms *p
> * --ANK (980924)
> - return v->qavg + (backlog - (v->qavg >> p->Wlog));
> + return v->qavg + (backlog - v->qavg) >> p->Wlog;
> static inline unsigned long red_calc_qavg(const struct red_parms *p,
This is going to be a FPP (Frequently Posted Patch)
Current formulae is fine.
Thats because backlog, at start of red_calc_qavg_no_idle_time() is not
yet scaled by p->Wlog. v->avg is scaled, but not backlog.
Have you tested RED after your patch ?
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