Re: 2.6.15-ck1

[Tony Lindgren <tony@atomide.com>]

* Con Kolivas <kernel@kolivas.org> [060104 15:23]:
> On Thursday 05 January 2006 06:57, Dave Jones wrote:
> > On Wed, Jan 04, 2006 at 02:05:54PM -0500, Dave Jones wrote:
> >  > On Wed, Jan 04, 2006 at 12:00:00PM +1100, Con Kolivas wrote:
> >  >  >  +2.6.15-dynticks-060101.patch
> >  >  >  +dynticks-disable_smp_config.patch
> >  >  > Latest version of the dynticks patch. This is proving stable and
> >  >  > effective on virtually all uniprocessor machines and will benefit
> >  >  > systems that desire power savings. SMP kernels (even on UP machines)
> >  >  > still misbehave so this config option is not available by default for
> >  >  > this stable kernel.
> >  >
> >  > I've been curious for some time if this would actually show any
> >  > measurable power savings. So I hooked up my laptop to a gizmo[1] that
> >  > shows how much power is being sucked.
> >  >
> >  > both before, and after, it shows my laptop when idle is pulling 21W.
> >  > So either the savings here are <1W (My device can't measure more
> >  > accurately than a single watt), or this isn't actually buying us
> >  > anything at all, or something needs tuning.
> >
> > Ah interesting. It needs to be totally idle for a period of time before
> > anything starts to happen at all. After about a minute of doing nothing,
> > it started to fluctuate once a second 20,21,19,20,19,20,18,21,19,20,22
> > etc..
> >
> > Goes no lower than 18W, and only occasionally peaks above the old idle
> > power usage. Not bad at all.
> >
> > Causing any activity at all puts it back to the 'have to wait a while
> > for things to start happening' state again.
> 
> Thanks for testing it. Indeed skipping the ticks alone does not really save 
> any significant amount of power. The real chance for power savings comes from 
> using this period for smarter C state programming. The other thing as you've 
> noticed is that timers need to be curbed or minimised to get the maximum 
> benefit and the ondemand governor alone, which unfortunately shows up as 
> something not obvious in timertop, polls at 140HZ itself - fiddling with 
> ondemand/ settings in sys can drop this but slows the rate at which it 
> adapts.

Device specific power states will help with getting power savings too.

Tony