Re: CPU offline but power consumption increased?

Re: CPU offline but power consumption increased?

[Andi Kleen]

"Andika Triwidada" <andika@gmail.com> writes:

[cc linux-acpi]
 
> Question: is that normal? I thought power consumption will be
> automatically reduced if one core offlined.

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.

-Andi

Re: CPU offline but power consumption increased?

[Len Brown]

On Saturday 07 April 2007 19:38, Andi Kleen wrote:
> "Andika Triwidada" <andika@gmail.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

Re: CPU offline but power consumption increased?

[Andi Kleen]

> 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.

We don't use deep c states currently; just HLT.

Right now it doesn't make much difference because no multi socket
servers do deep C states and secondary cores can normally not go
deep on their own; but if any of this changes this code would
need to be fixed.

But cpu off line is really a special case -- maybe it is even 
possible to do something better on current hardware.

> 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...

I don't see any BIOS call in play_dead()

-Andi