Subject: Re: [PATCH] x86-64 highres/dyntick support
From: Thomas Gleixner <tglx@linutronix.de>
Reply-To: tglx@linutronix.de
To: Mats Johannesson <spamcan@bredband.net>
Cc: linux-kernel@vger.kernel.org
In-Reply-To: <20070507140747.2c68d7c9@loke.fish.not>
References: <20070507140747.2c68d7c9@loke.fish.not>
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Date: Mon, 07 May 2007 17:13:21 +0200
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On Mon, 2007-05-07 at 14:07 +0200, Mats Johannesson wrote:
> This AMD 64 Mobile processor only has a C1 level which isn't used:
> 
> root@sleipner:~# cat /proc/acpi/processor/CPU0/power
> active state:            C1
> max_cstate:              C8
> bus master activity:     00000000
> maximum allowed latency: 2000 usec
> states:
>    *C1:                  type[C1] promotion[--] demotion[--]
> latency[000] usage[00000000] duration[00000000000000000000]
> 
> But shouldn't the the kernel 'hlt' routine, or whatever it's called,
> work in conjunction with dyntick to achieve... something...?

To make real power savings from dynticks you need deeper power states in
the CPU. Dyntick can give the idle state code an idea how long the sleep
is going to be, so this code can decide to go into deeper power states
in one go rather than stepping down over time. On a CPU which has no
deeper C states the power saving of dynticks is probably not even
measurable,

	tglx


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