linux-kernel - Re: [RFC] Imprecise timers.

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Message-ID: <4885E8F8.1050207@keyaccess.nl>
Date:	Tue, 22 Jul 2008 16:04:40 +0200
From:	Rene Herman <rene.herman@...access.nl>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	David Woodhouse <dwmw2@...radead.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: [RFC] Imprecise timers.

On 22-07-08 14:54, Arjan van de Ven wrote:

> On Tue, 22 Jul 2008 09:19:02 +0200
> Rene Herman <rene.herman@...access.nl> wrote:
> 
>> On 22-07-08 05:02, David Woodhouse wrote:
>>
>>> Many users of timers don't really care too much about exactly when
>>> their timer fires -- and waking a CPU to satisfy such a timer is a
>>> waste of power. This patch implements a 'range' timer which will
>>> fire at a 'convenient' moment within given constraints.
>>>
>>> It's implemented by a deferrable timer at the beginning of the
>>> range, which will run some time later when the CPU happens to be
>>> awake. And a non-deferrable timer at the hard deadline, to ensure
>>> it really does happen by then.
>> Are there actually users for this (not just in theory)? The
>> deferrable timer sort of sounds like all I'd ever want if I, as you
>> say, wouldn't really care...
> 
> there's a few; mostly around hardware timeout..For example Stephen want
> it for his drivers.

Hardware I've dealt with is (almost? can't remember anything else) 
exlusively minimal delays and as such this thing seemed like perhaps a 
bit over-apisized...

> EXT3 journal flushing is another one where we can easily say 
> "between 4 and 7 seconds" rather than "exactly at 5"

This a nice-ish example though. It might be considered necessary to make 
the current commit delay when set explicitly be the non-deferrable upper 
bound but almost none do I guess.

Rene.
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