How to set the CPU voltage

Hey guys,

here I am, I bet that soon or late you were waiting for this piece of knowledge.... I guess a little late, but here it comes. In the following guide you'll learn how to save energy, lower CPU temperature and improve your battery duration (at least on laptops) without (or at least minimizing) bugs and freezing, by setting the voltages of your CPU. If you don't have phctool installed take a look here:

http://linuxsolver.blogspot.com/2011/02/undervolting-cpu-in-ubuntu.html

First of all remember that doing those stuff your system might crash without saying "Hey dude, kinda confused here, I'm gonna make a mess soon". So no precious file copying, no disc burning, no thesis writing... I hope you got it, better nothing at all.

Tools

We need few stuff:

• Patience: of course this is what does the job.
• Stress tool: we need something to set the clock to be stressed, and something to put under heavy load the cpu, to see if it fails...  so:
  

  sudo apt-get install cpuburn cpufrequtils lm-sensors

Ok, what have we installed? Cpuburn it's a set of commands, each one built for a specific CPU, that make one CPU core the busiest possible. Cpufrequtils instead gives us the possibility to set the CPU frequency step as we like. At last lm-sensors is a suite to get hardware infos form your box. To install it (from ubuntu HowTo):

1. Run
   

   sudo sensors-detect

   and answer YES to all YES/no questions.
2. At the end of sensors-detect, a list of modules that needs to be loaded will displayed. Type "yes" to have sensors-detect insert those modules into /etc/modules.
3. Next, run "sudo /etc/init.d/module-init-tools restart". This will read the changes you made to /etc/modules in step 2, and insert the new modules into the kernel.

Once you have a working installation of lm-sensors, we have a good way to keep temperatures under control: open a new terminal and launch

while [ true ]; do sensors; sleep 1; done 

Keep it running in another window and go on with our mission!

How to stress

To stress a CPU you have to run one of the cpuburn commands. They are a big family, but can be split in two: burnP* are for intel CPUs, burnK* for AMD ones. So, if you have already opened sensors to keep temperatures under control (last command), the thing to do is just launch from terminal:

burnP6 &

the "&" is needed as we have to stress all the cores, so we have to repeat that command for each core. Be sure to write immediately after the last "burn"

killall burnP6

so you can be ready to stop the test if a too high temperature is reached. If the computer does not freeze and the temperature does not blow your box up at least for a couple of minutes, you have 95% of probability to be on an actually stable setting. To be 99% sure you should run the burn test for 10-15 minutes.

Tuning Voltages

1. The first step is of course to open phctool and take a look at the voltages. In that tab you can see both the frequency steps and the voltages. If you have more than one core you should see on your right several CPUs: you can set and stress all the cores at the same time.
2. Now open another terminal: we will use this to change the CPU frequencies and stress them. To get some extra info start with:

sudo cpufreq-info

And take note of your CPU governor.

3. Repeat the following operations for all the cores you have(-c 0; -c 1 ... etc):

sudo cpufreq-set -c 0 -g userspace

Like this we are able to set our CPU voltage as we like. 

4. From phctool set the lowest freq @ the lowest voltage for all the cores.
5. Launch:

   sudo cpufreq-set -c 0 -f **the freq value**

   **the freq value** has to be written in the human readable format, just without space (e.g. 800MHz or 1.2GHz ect)
6. Now we are not sure to be actually using the lowest voltage, to check that go to the analysis tab of phctool and refresh. If you get a wrong [target voltage]/frequency double check what have you done... if it doesn't work for no reason just leave a comment.
7. If the current voltage does not match the target VID then you have some limitation on the motherboard... so don't struggle on how to workaround it, it depends on your mobo/cpu hardware capablities. So take it as the "absolute zero", set it so that current and target VIDs match, and start stress it. 
8. If the stress test is ok we can move on. The following procedure has to be cycled for all the freq available, for all the cores:
   
   
   • Ø_ Set the frequency (of all the cores) with cpufreq-set:
     

     sudo cpufreq-set -c 0 -f **the freq value**

     Your Voltage Quantum Q is now 0.1V
     
   • A_   Lower the VID (of all the cores) @ that freq by Q
   • B_   Stress test the value @ that freq
   • C_  if the test failed halve Q and set the VID (of all the cores) to the last stable value. If Q is smaller than the VID step then go to the next point, otherwise return to A_
   • D_ If you are here you have configured almost at the limit that freq VID. If you want to be safer increase it of a couple of steps (for all the cores). Now move to the next frequency and restart from Ø_.
     
9. If you completed these operations for all the cores and all the frequencies now you have tuned your box! Remember to set "restore VIDs on load" before you can call it a day ^_^

I think we are done. If you need any help (but even if you don't need it) feel free to leave a comment!



Cheers!