Multicore laptops and turning off CPU cores

[Richard Hughes]

On Thu, 2006-08-17 at 19:28 +0200, Holger Macht wrote:
> On Thu 17. Aug - 18:05:45, Richard Hughes wrote:
> > Now we are playing with cpu frequency scaling, it would make sense in my
> > opinion to expose a OS neutral API to offline and online CPU's.
> > 
> > I think it was Daniel Drake (post beers) who showed me how to do this at
> > GUADEC by just echoing a value somewhere in sysfs for his new shiny
> > dual-core laptop.
> > He also showed me this *dramatically* reduced the power usage of the
> > laptop.
> 
> Well, offlining one CPU just puts it into C4 state. This is what the
> kernel should select anyway if the system is idle.

Gotcha.

> > 
> > I think it went something like this:
> > echo 0 > /sys/devices/system/cpu/cpuX/online
> > 
> > This is Linux specific, but we can add Solaris and *BSD support easily.
> > Artem, Joe, what are the commands?
> > 
> > I'm guessing that a dual core laptop appears as two processor objects in
> > HAL, in which case it makes it trivial to add methods:
> > 
> > void org.freedesktop.Hal.Device.ProcessorHotplug.SetOnline(bool)
> > bool org.freedesktop.Hal.Device.ProcessorHotplug.GetOnline(void)
> > 
> > This scheme wouldn't be possible if the processor object disappears from
> > the HAL device tree which I don't think it does. Daniel can you confirm
> > please?
> > 
> > Note: we can see if a CPU is "offlineable" by the presence of the
> > file /sys/devices/system/cpu/cpuX/online - if it does not exist then you
> > can't offline it, and we shouldn't add the methods or the capability
> > 'processor_hotplug'.
> > 
> > Better interface, capability and method names welcome. :-)
> > 
> > Now, I'm imagining this being used on dual core laptops, but the
> > interface shouldn't rule out other machines, such as insanely big SMP
> > servers with x CPU's.
> > 
> > Sound sane / insane? Imagine a checkbox in a typical policy agent
> > preferences program (heh):
> > 
> > When on battery power:
> > 
> > [x] Turn off additional processors
> > 
> > Now, that would ROCK.
> > 
> > Opinions and comments please.
> 
> I think it doesn't rock that much ;-)
> 
> With offlining a CPU, you just force it into C4. That's the same problem
> like with the powersave governor which forces the CPU to lowest possible
> freuquency. One could argue that you always want to have a job done as
> fast as possible and afterwards go to C4 or lower the frequency.

Gotcha, thanks.

Richard.