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Subject: Re: dynamic-hz
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Russell King <rmk+lkml@arm.linux.org.uk>
Cc: Stefan Seyfried <seife@suse.de>, Con Kolivas <kernel@kolivas.org>,
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       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Andrea Arcangeli <andrea@suse.de>
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Date: Mon, 13 Dec 2004 14:52:46 +0000
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On Llu, 2004-12-13 at 13:58, Russell King wrote:
> Lets take an example.  Lets say that:
> * a CPU runs at about 245mA when active
> * 90mA when inactive
> * the timer interrupt takes 2us to execute 1000 times a second
> * no other processing is occuring

Now take a real laptop and the numbers are in the 20W (15A) range.

> to eliminate the timer tick to save some power.  However, I've
> never been able to justify the extra code complexity against the
> power savings.  It really only makes sense if you can essentially
> _power off_ your system until the next timer interrupt (thereby,
> in the above example, reducing the power consumption by some 174mA)

On a PC it makes huge sense, the deeply embedded folks who do turn the
thing off for 30secs at a time (Eg cellphone) also want it as do
virtualisation people where it trashes your scaling. API wise it isn't
too hard, its just a matter of time to convert the jiffies users away
and to do relative versions of add_timer with accuracy info included.