Re: [RESEND PATCH 2/2] sched/fair: Optimize __update_sched_avg()

[Peter Zijlstra]

On Fri, Mar 31, 2017 at 03:55:40AM -0700, Paul Turner wrote:

> > I'm thinking that we can compute the middle segment, by taking the max
> > value and chopping off the ends, like:
> >
> 
> >
> >              p
> >  c2 = 1024 \Sum y^n
> >             n=1
> >
> >               inf        inf
> >     = 1024 ( \Sum y^n - \Sum y^n - y^0 )
> >               n=0        n=p
> >
> 
> Very nice!
> Minor nit: Second sum needs to be from n=p+1

Correct.

> > +static u32 __accumulate_pelt_segments(u64 periods, u32 d1, u32 d3)
> >  {
> > +       u32 c1, c2, c3 = d3; /* y^0 == 1 */
> >
> >         /*
> >          * c1 = d1 y^(p+1)
> >          */
> > +       c1 = decay_load((u64)d1, periods);
> >
> >         /*
> > +        *             p
> > +        * c2 = 1024 \Sum y^n
> > +        *            n=1
> >          *
> > +        *              inf        inf
> > +        *    = 1024 ( \Sum y^n - \Sum y^n - y^0 )
> > +        *              n=0        n=p+1
> >          */
> > +       c2 = LOAD_AVG_MAX - decay_load(LOAD_AVG_MAX, periods) - 1024;
> 
> decay_load(LOAD_AVG_MAX, periods + 1)

So here, @periods == p+1, see also c1. Yes, this is confusing [*].

In particular, I think the decay terms for c1 and this should be the
same. We cut off this tail end of the series to replace it with c1 after
all.

[*] hysterically p used to be off by 1, which is where the p+1 came
from, but now periods includes it. I was thinking of doing a patch
correcting all the comments to fully eradicate the whole +1 business.


> I computed all the values vs true value that the old/new computations
> result in, and it's very close.  Absolutely it's approximately 2x off
> the previous computation, e.g. if the old value was -15 (relative to
> true value) than the new computation is -30.
> 
> This is definitely more than good enough.  If we want more precision,
> then the correction factor of:
>   +clamp(periods, 0, 45)

Can you do a patch with coherent comment explaining where that
correction term comes from?