netdev - Re: [PATCH iproute2] Re: HTB accuracy for high speed

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Date:	Fri, 29 May 2009 22:59:49 +0200
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Antonio Almeida <vexwek@...il.com>, netdev@...r.kernel.org,
	kaber@...sh.net, davem@...emloft.net, devik@....cz,
	Eric Dumazet <dada1@...mosbay.com>,
	Vladimir Ivashchenko <hazard@...ncoudi.com>
Subject: Re: [PATCH iproute2] Re: HTB accuracy for high speed

On Fri, May 29, 2009 at 01:49:56PM -0700, Stephen Hemminger wrote:
> On Fri, 29 May 2009 21:46:43 +0200
> Jarek Poplawski <jarkao2@...il.com> wrote:
...
> > > >  /* Avoid doing 64 bit divide by 1000 */
> > > > -#define PSCHED_US2NS(x)                        ((s64)(x) << 10)
> > > > -#define PSCHED_NS2US(x)                        ((x) >> 10)
> > > > +#define PSCHED_US2NS(x)                        ((s64)(x) << 6)
> > > > +#define PSCHED_NS2US(x)                        ((x) >> 6)
> > > >
> > > >  #define PSCHED_TICKS_PER_SEC           PSCHED_NS2US(NSEC_PER_SEC)
> > > >  #define PSCHED_PASTPERFECT             0
> > > 
> > > It's better! This patch gives more accuracy to HTB. Here some values:
> > > Note that these are boundary values, so, e.g., any HTB configuration
> > > between 377000Kbit and 400000Kbit would fall in the same step - close
> > > to 397977Kbit.
> > 
> > Good news! So it seems there are no other reasons of this inaccuracy
> > than too coarse granularity, but I have to check this yet. Alas there
> > is needed something more than this patch, because it probably breaks
> > other things like hfsc.
> > 
> > Thanks,
> > Jarek P.
> > 
> 
> Why would it break hfsc, if it isn't already broken.

I might be wrong but e.g. these usecs could be one reason:

/* convert d (us) into dx (psched us) */
static u64
d2dx(u32 d)
{
        u64 dx;

        dx = ((u64)d * PSCHED_TICKS_PER_SEC);
        dx += USEC_PER_SEC - 1;
        do_div(dx, USEC_PER_SEC);
        return dx;
}

And maybe these shifts need some adjustment:
m = (sm * PSCHED_TICKS_PER_SEC) >> SM_SHIFT;

Jarek P.
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