Step 1 :
apt-get install linux-cpupower cpufrequtils thermald
Step 2 :
cpupower frequency-set -g powersave
Step 3 :
nano /etc/rc.local
Place the following line right before "exit 0".
cpupower frequency-set -g powersave
Remarks :
(1) To check the result :
watch -n 1 -d sensors
(2) To reset to "performance" or "ondemand", you just replace "powersave" to "performance" or "ondemand".
That's all! See you.