cpufreqd as standard install?

Phillip Susi psusi at ubuntu.com
Thu Mar 8 16:03:42 UTC 2012

On 3/8/2012 9:47 AM, Matthew Garrett wrote:
> This is (broadly speaking) untrue. There's a bunch of fixed costs that a
> naive P=IVĀ² doesn't take into account. Assuming a fixed amount of work,
> race to idle is almost always the most power efficient strategy.

What fixed costs?  If you spend 5 seconds working at full throttle and 
consuming 100 watts, and then the next 25 seconds in deep C6 consuming 0 
watts, you've spent 500 joules of energy.  If you instead spend 10 
seconds working at half frequency, consuming only 30 watts, then the 
next 20 seconds in deep C6, you've only spent 300 joules for the same 
work.  When you factor in the typical increased execution efficiency you 
get at the lower frequencies, you probably could finish that work in 
only 9 seconds, cutting the energy expenditure down to 270 joules.

Things are of course, quite different if your load involves waking up 
and doing some work every 10ms instead of the simple batch scenario. 
Here the time to enter and exit C6 becomes more significant so if you 
use the lowest frequency, you can end up having little to no time in C6, 
so using a higher frequency to get back into C6 might make sense, but 
using the highest, and most power hungry frequency ( power consumption 
tends to grow exponentially ) is highly unlikely to prove optimal unless 
using the next lower frequency would still leave the cpu very busy.  If 
you can go down one step and the CPU can still spend the majority of its 
time in deep C6, then you should come out ahead.

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