linux-kernel - Re: [PATCH 2/2] block: Convert BDI proportion calculations to flexible proportions

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Date:	Fri, 18 May 2012 16:34:37 +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 2/2] block: Convert BDI proportion calculations to
 flexible proportions

On Fri, 2012-05-18 at 16:24 +0200, Jan Kara wrote:
>   Yeah, that should be easy enough so I'll try it that way since I presume
> it's nicer to power usage to use deferred timers if it's reasonably
> possible. 

Btw, your current scheme also drifts. Since you do jiffes + 3*HZ you
period might actually be longer if the timer got delayed.

If you keep an external jiffies count like:

unsigned long period_jiffies = jiffies;

void my_timer_func()
{
	unsigned long delta = jiffies - period_jiffies;
	unsigned long periods = delta / 3*HZ;

	age(periods);

	period_jiffies += 3*HZ * periods;
	mod_timer(&my_timer, period_jiffies);
}


it all works without drift (+- bugs of course).
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