cpufreqd as standard install?

Christopher James Halse Rogers raof at ubuntu.com
Tue Mar 6 02:19:21 UTC 2012


On Mon, 2012-03-05 at 21:10 -0500, Phillip Susi wrote:
> On 03/05/2012 08:10 PM, Christopher James Halse Rogers wrote:
> > To restrict the maximum frequency when on battery / low battery?  The
> > last analysis I've seen, by Matthew Garrett, was that the most
> > power-efficient way to run modern CPUs is to have them run as fast as
> > possible - in order to do the pending work in the shortest possible time
> > - then drop down to a low-power C-state.
> 
> This is incorrect.  Lower frequency ( coupled with lower voltage )
> provides less power per instruction.  You may be confusing some of his
> writing about the p4clockmod driver, which doesn't actually lower the
> cpu voltage or frequency, but rather just forces the cpu to HLT as if
> it were idle for part of the time.  This does not give better
> efficiency, which is why he patched that driver to refuse to bind to
> the ondemand governor.
> 

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
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