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

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Date:	Fri, 18 May 2012 00:04:33 +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 Tue, 2012-05-15 at 17:43 +0200, Jan Kara wrote:
> +static struct timer_list writeout_period_timer =
> +               TIMER_DEFERRED_INITIALIZER(writeout_period, 0, 0); 

So the problem with using a deferred timer is that it 'ignores' idle
time. So if a very busy period is followed by a real quiet period you'd
expect all the proportions to have aged to 0, but they won't have.

One way to solve that is to track a jiffies count of the last time the
timer triggered and compute the missed periods from that and extend
fprop_new_period() to deal with period increments of more than 1.

The other is of course to not use deferred timers.
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