linux-kernel - Re: CPU offline but power consumption increased?

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Message-Id: <200704100413.51637.lenb@kernel.org>
Date:	Tue, 10 Apr 2007 04:13:51 -0400
From:	Len Brown <lenb@...nel.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"Andika Triwidada" <andika@...il.com>,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org
Subject: Re: CPU offline but power consumption increased?

On Saturday 07 April 2007 19:38, Andi Kleen wrote:
> "Andika Triwidada" <andika@...il.com> writes:
> 
> [cc linux-acpi]
>  
> > Question: is that normal? I thought power consumption will be
> > automatically reduced if one core offlined.

Known?  Yes.
What people would expect? No.

http://bugzilla.kernel.org/show_bug.cgi?id=5471

> The current cpu offline essentially just runs a special idle loop.
> The standard idle loop is even a bit more aggressive on some systems
> because it knows about the deeper ACPI sleep modi.
> 
> There are also dependencies between cores because current CPUs
> have shared power planes between cores.
> 
> I suppose in the future when a whole socket goes off line one could
> implement special code to turn off the CPU further. But it likely
> won't work on older hardware.

Speaking for all Intel hardware implemented from pre-history until now,
deep C-states is the best you can do, and there is no special offline
mode to save more power.

If you really want to not use a core and the above bug isn't fixed in linux,
you can use maxcpus=1 to never bring the other core on-line in the first place,
and if the BIOS is implemented properly, the core will be spinning in the
deepest available C-state.  Of course, it would probably be more interesting
to simply leave the core on-line and let it go idle...

-Len
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