| Message ID | 1347543820-27548-1-git-send-email-cyril@ti.com |
|---|---|
| State | Rejected, archived |
| Delegated to: | David Miller |
| Headers | show |
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@ti.com> > --- > 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 ? -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Eric, thank you for reviewing the code. I now see the problem with the patch since backlog is an integer and qavg is a fixed point number at logW. We are considering another patch to update the comments to this code (with the actual C code change reverted) to stop the FPP by showing the derivation of the equation in the comments. Does this sound good? -----Original Message----- From: Eric Dumazet [mailto:eric.dumazet@gmail.com] Sent: Thursday, September 13, 2012 9:54 AM To: Chemparathy, Cyril Cc: linux-kernel@vger.kernel.org; netdev@vger.kernel.org; davem@davemloft.net; david.ward@ll.mit.edu; Dowdal, John; paul.gortmaker@windriver.com Subject: Re: [PATCH] sch_red: fix weighted average calculation 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@ti.com> > --- > 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 ?
On Fri, 2012-09-14 at 13:01 +0000, Dowdal, John wrote: > Eric, thank you for reviewing the code. I now see the problem with > the patch since backlog is an integer and qavg is a fixed point number > at logW. > > We are considering another patch to update the comments to this code > (with the actual C code change reverted) to stop the FPP by showing > the derivation of the equation in the comments. Does this sound good? This would be good, please do so. Thanks -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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 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@ti.com> --- include/net/red.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)