cvs commit: src/usr.sbin/powerd powerd.c

[Nate Lawson]

Bruno Ducrot wrote:
> On Mon, Aug 29, 2005 at 10:08:25PM -0700, Nate Lawson wrote:
> 
>>Bruno Ducrot wrote:
>>>I'm pretty sure it's incorrect to add p4tcc and acpi_throttle for power
>>>saving purpose.  I plan to add some flags in order to use only relevant
>>>frequencies to this end, but IMHO that should be done at low-level
>>>drivers.  On the other hand, it is usefull to keep the existing sysctl
>>>freqs, but for cooling purpose only.
>>
>>I think throttling, whether via p4tcc or acpi_throttle, is a useful 
>>addition to absolute frequency control (i.e. est or powernow).  With 
>>appropriate tuning, as I hope the patch I committed provides, the 
>>additional levels should be helpful.
> 
> Apart on older processors, I don't see the usefullness for power saving
> purpose.  The problem is that when the processor is in stop grant state
> in the duty cycle, it will consume more power than when it is in sleep
> or deep sleep states (or deeper sleep state for some).
> If the processor is idle, you will have nearly like 100% of time spend
> in sleep state (for laptops) or stop grant state (for desktop), or even
> better if the system support C3 etc.
> 
> But if you have a duty cycle of (say) 87.5% due to the idleness of the
> system (and the result of powerd), then the processor will be put
> for 87.5% of time in stop grant state which consume more power
> than sleep state.

We do use C3+ if supported, and I agree it gives more power savings when 
active.  But even an idle system gets periodic timer interrupts, battery 
polling, and other events that require it to exit C3.  So running at a 
low CPU rate via throttling may conserve power when not in C3.

-- 
Nate