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 -- devel mailing list devel@lists.fedoraproject.org https://admin.fedoraproject.org/mailman/listinfo/devel
