cpufreqd as standard install?

Matthew Garrett mjg59 at srcf.ucam.org
Thu Mar 8 17:11:04 UTC 2012

On Thu, Mar 08, 2012 at 11:22:04AM -0500, Phillip Susi wrote:
> On 3/8/2012 11:10 AM, Matthew Garrett wrote:
> >Yes, if those are the actual power figures. But they're typically not
> >going to be.
> Can you be a little less vague and hand wavy?

My i7 draws about 7W when fully loaded at 800MHz, and about 27W when 
fully loaded at 2.7GHz. That's a 3.4x performance improvement at a 
3.9x power increase. So, naively, that does result in a fixed amount 
of work being carried out in a smaller amount of energy, although not 
anywhere near the extent that you're describing.

But this is a very strange workload to be optimising for. First, it's 
entirely CPU-bound. If it involves IO then you're going to be keeping IO 
devices in a higher power state for longer, which wipes out the 
advantage. Second, it makes the assumption that the user doesn't care 
how much time it takes. That's basically never true.

The only reason not to use race-to-idle is because you have an amazingly 
specific workload, one that's CPU bound and not user-interactive. That 
discounts pretty much every desktop, mobile and server use case. It's 
really not worth worrying about.

Matthew Garrett | mjg59 at srcf.ucam.org

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