Power saving tips & tricks

[Binky]

Why is the option in cpusettings called "default maximum MHz" ?

 

[WizardStan]

I definitely seem to recall that it was discovered the power usage of an idle CPU at 125mhz was exactly the same as an idle CPU at 500mhz. Originally "low power" mode meant dropping to 14mhz (the lowest it could go) but in experimentation it was found not to make any difference at all; in fact, it could increase power consumption. 125mhz was subsequently used but again it didn't seem to make a difference. Are you sure your data is correct in finding that a lower speed has a noticeable impact on battery life?

 

[Kumaki]

Well, if anyone could or would take me step-by-step through the clocking down and turning off LED when lid closed...and then auto clock-back-up and LED back on when lid up...this would be great.


Of course I ALWAYS want the red LED for Charging to work when the unit is charging. Don't wanna mess with that one.

 

[_wb_]

I definitely seem to recall that it was discovered the power usage of an idle CPU at 125mhz was exactly the same as an idle CPU at 500mhz. Originally "low power" mode meant dropping to 14mhz (the lowest it could go) but in experimentation it was found not to make any difference at all; in fact, it could increase power consumption. 125mhz was subsequently used but again it didn't seem to make a difference. Are you sure your data is correct in finding that a lower speed has a noticeable impact on battery life?

You have to make sure the cpu is really idle: don't leave things like top or system info running, because they keep waking up the cpu. If there still is more than just the kernel to use the cpu, lower clock speed will not help as much because what you gain by clocking lower you lose again by having to use the slower cpu longer. I _think_ it still consumes slightly less power to e.g. use 50% cpu at 200MHz instead of 12.5% at 800MHz, but the difference is not as big as the difference between 0% at 200MHz and 0% at 800MHz.


I measured the long-term power consumption of my idle lid-closed Pandora by modifying the lid-close and lid-open script to output the battery energy left (in mW) and a time stamp to some file in /tmp, making sure that I'm only measuring the lid-closed idle period (if you measure as much as 1 second of lid-open non-idle time, it disturbs the measurement because power consumption is an order of magnitude higher with the lid open). I made sure that it was in lid-closed mode for quite some time (at least an hour). I don't remember the exact numbers, but it was something like this: 109 mW at 700MHz with the power indicator LED on, 91 mW at 700MHz with the power LED off, 78 mW at 125MHz with the power LED off.

 

[notaz]

Instead of underclocking, there is also an alternative to switch to ondemand governor:

sudo su

modprobe cpufreq_ondemand

echo ondemend > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

It's off by default because it causes noriceable slowdowns in games/emulators when it does it's scaling thing.


Maybe switch to it when lid is closed instead?


Can somebody test how well this works with things like audio when lid is closed?