Heads up: cpuspeed removed from f16+

[Eric Sandeen]

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