linux-kernel - Re: [PATCH 1/2] lib: Proportions with flexible period

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Date:	Fri, 18 May 2012 16:45:04 +0200
From:	Jan Kara <jack@...e.cz>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jan Kara <jack@...e.cz>, Wu Fengguang <fengguang.wu@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 1/2] lib: Proportions with flexible period

On Fri 18-05-12 12:43:44, Peter Zijlstra wrote:
> On Tue, 2012-05-15 at 17:43 +0200, Jan Kara wrote:
> > +void __fprop_inc_percpu_max(struct fprop_global *p,
> > +                           struct fprop_local_percpu *pl, int max_frac)
> > +{
> > +       if (unlikely(max_frac < 100)) {
> > +               unsigned long numerator, denominator;
> > +
> > +               fprop_fraction_percpu(p, pl, &numerator, &denominator);
> > +               if (numerator > ((long long)denominator) * max_frac / 100)
> > +                       return;
> 
> Another thing, your fprop_fraction_percpu() can he horribly expensive
> due to using _sum() (and to a lesser degree the retry), remember that
> this function is called for _every_ page written out.
  The retry happens only when new period is declared while
fprop_fraction_percpu() is running. So that should be rather exceptional.
Regarding the _sum I agree, luckily that's easy enough to fix.

> Esp. on the mega fast storage (multi-spindle or SSD) they're pushing cpu
> limits as it is with iops, we should be very careful not to make it more
> expensive than absolutely needed.
  Yup.

> > +       } else
> > +               fprop_reflect_period_percpu(p, pl);
> > +       __percpu_counter_add(&pl->events, 1, PROP_BATCH);
> > +       percpu_counter_add(&p->events, 1);
> > +} 

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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