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

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Date:	Fri, 18 May 2012 12:43:44 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jan Kara <jack@...e.cz>
Cc:	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 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.

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.

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