linux-kernel - Re: [PATCH 09/12] irq: implement IRQ expecting

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Message-ID: <20100617232649.6db5cc55@infradead.org>
Date:	Thu, 17 Jun 2010 23:26:49 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>, mingo@...e.hu,
	bphilips@...e.de, yinghai@...nel.org, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	jeff@...zik.org, linux-ide@...r.kernel.org,
	stern@...land.harvard.edu, gregkh@...e.de, khali@...ux-fr.org
Subject: Re: [PATCH 09/12] irq: implement IRQ expecting

On Thu, 17 Jun 2010 18:47:19 +0200
Tejun Heo <tj@...nel.org> wrote:

> 
> Hmmm... the thing is that there will be many cases which won't fit
> irq_expect() model (why irq_watch() exists in the first place) and for
> the time being libata is the only one providing that data.  Would the
> data still be useful to determine which c-state to use?

yes absolutely. One of the hard cases right now that the C state code
has is that it needs to predict the future. While it has a ton of
heuristics, including some is there IO oustanding" ones, libata is a
really good case: libata will know generally that within one seek time
(5 msec on rotating rust, much less on floating electrons) there'll be
an interrupt (give or take, but this is what we can do heuristics for
on a per irq level).
So it's a good suggestion of what the future will be like, MUCH better
than any hint we have right now... all we have right now is some
history, and when the next timer is.... 

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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