Heads up: cpuspeed removed from f16+

[Jaroslav Skarvada]

----- Original Message -----
> On 07/19/2011 10:23 AM, Przemek Klosowski wrote:
> > On 07/19/2011 11:07 AM, Eric Sandeen wrote:
> >> On 07/19/2011 09:59 AM, Jaroslav Skarvada wrote:
> >
> >>> Sad that the daemon gone. It was able to dynamically switch speed
> >>> (and save power) on systems that have CPUs with high transition
> >>> latency (e.g. old P4, some Atoms, etc.). On such systems the
> >>
> >> Actually, no...
> >>
> >> http://codemonkey.org.uk/2009/01/18/forthcoming-p4clockmod/
> >>
> >>> So the 1.00GHz ‘frequency’ is actually “run at 2GHz, but only do
> >>> work 50% of the time”.
> >>>
> >>> On the surface, this sounds like a good idea. The other 50%, the
> >>> CPU is idle, so you’re saving power, right?
> >>> Not so much. In fact, you could be burning more power. The reason
> >>> for this is that when the processor is sitting there doing
> >>> nothing, it isn’t lower frequency, and more importantly, it very
> >>> likely isn’t entering C states. So you’re burning the same amount
> >>> of power, but now you’re only doing work for 50% of the time. As a
> >>> result of this, your workload takes twice as long to complete.
> >>
> >> I've measured it, and Dave is right. You might get something saying
> >> "1.0Ghz" but you're not saving anything at all.
> >
> > There are second-order effects---the processor probably doesn't use
> > significantly less power but the graphic card and chipset do, for
> > some
> > overall system effects---people quoted numbers like 20% battery
> > savings
> > for 50% slowdown (if p4_clockmod really stopped the CPU 50% of the
> > time,
> > it'd double the battery life, so this really is a very inefficient
> > and
> > crude method).
> 
> I would suggest getting a wattmeter and measuring it... probably the
> simplest way to know for sure.
> 
> I'm pretty sure I measured it directly with a kill-a-watt meter, but I
> no longer have a P4, so can't retest.
> 
> -Eric
> --

Well, I can do it (just for curiosity). But this was not only about P4.
Anything with transition latency over 10ms is rejected by ondemand
governor (and it should be so)

Jaroslav