cpufreqd as standard install?

Phillip Susi psusi at ubuntu.com
Tue Mar 6 02:39:34 UTC 2012


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On 03/05/2012 09:19 PM, Christopher James Halse Rogers wrote:
> Less power per instruction, or less power per instruction amortized over
> the run-time?  My understanding was that hitting the low C-states was
> such a huge power win that the increased power per instruction was
> offset by the longer C-state residency┬╣.
> 
> ┬╣: http://www.codon.org.uk/~mjg59/power/good_practices.html

That's the article that I was thinking of that is mostly about the p4clockmod driver.  Using it means you have the same power per instruction, but also are spending less time in the deeper C states, and so is bad.  With correct frequency management, the lower power per instruction of the lower frequencies outweighs the reduced time in the lower C states.  There probably still is an optimal point somewhere between the min and max frequency where you get the benefit of both lower power per instruction when executing instructions, without giving up too much time in the lower C states, but finding that balance is tricky and highly hardware and load specific.  My current CPU operates from 1600 to 3301 MHz ( where the last 1 MHz just enables turbo boost ).  Disabling that turbo boost state would probably provide the most savings in power power per instruction with minimal loss of deep C6 time.  At 1600 MHz with a load that keeps that frequency mostly busy very well may be more efficient at 
one of the intermediate frequencies, but figuring out which is tricky.  One thing is almost certain: it isn't 3301 MHz.  Of course, a load that keeps 1600 MHz rather busy would trigger the ondemand governor to shift to one of the intermediate frequencies anyway, so the default situation is probably quite near optimal.  Load spikes that would cause the ondemand governor to shift to 3301 ( turbo boost ) would be best disabled when on battery power, but I believe that many laptops have proper ACPI bios that does disable turbo boost when on battery power, so we're good there too.

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