Linux CPU energy saving hacks

Question

I have a quad core CPU (Phenom II 945), because sometimes I need performance, but most of the time, 4×800 MHz (the minimal frequency) is still overkill for most usages.

So, three questions regarding CPU manipulation under Linux. I'd like to know for each

A) how much of effectively reduced energy consumption is possible
B) if allows stable working
C) if it's not damaging my processor.

1. Linux allows to shutdown cores at runtime. However, I read somewhere that disabled cores don't actually use less energy, but as if run at full frequency. Cant find that link again, nor know if this is correct or if this still applies for my CPU. Where can I find that kind of information? Has anybody tested that? Is there an easy way to measure actual energy consumption of the whole CPU and/or single cores? Related but unanswered question: Dynamically disabling cores in a power-efficient way?

2. AFAIK, both

   /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_min_freq
   

   and

   /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
   

   could be set to values lower than the official spec. How can I find the lowest possible value that fit a,b,c? Anybody has experiences with that?

3. That's rather BIOS than Linux, but I read that you can undervolt some CPUs while keeping frequency.

Answer 1

Undervolting will have a proportionately greater impact on energy consumption than frequency, but you can likely change frequency by a much greater percentage. If you can figure a way to alter voltage as a function of frequency you'll get the best results. Several models of CPU in the server market do this.

To test the effectiveness of your attempts you will have to have objective empirical data. You should be able to find a load meter at most electronics vendors and electrical supply shops for under $30. You don't need a fancy EM phase meter for $150. A simple single socket model like this P3 Kill A Watt Electricity Load Meter and Monitor will do. I'm not recommending that one, I haven't used it, just giving an example.

If you concern is reducing energy use directly, you'll find some other components of the system to be much greater energy wasters than your CPU. The worst culprits are likely your: Power Supply, Video Card, CPU, Hard Drive, Fans. Probably in that order. A load meter is the only way to know for sure the actual impact of conservation measures on energy use.