I just installed Powertop 1.98 on my laptop (T420 + FC15).
There is a section called "Tunables". Half of my devices show as Bad. I see I can toggle from Bad to Good? What this section is for? What should/can I toggle?
Also, is there any documention available for that app? man and google were unhelpful.
It's probably not bad if they're "Bad", when you run on the AC power. When running on battery, the "Good/Bad" settings may or may not help. You can toggle them and observe the effects, but should you toggle them if you want to achieve the highest power-saving? I agree with mavit's answer that the powertop is primarily a diagnostic tool. There are other tools for turning on the power-saving mode.
An Ubuntu guy run exhaustive tests on two laptops and found, that generally it helps switching to the "Good" (low-power) mode, but not always. You can read his answer on askubuntu with the summary results linked from there.
After playing with powertop, I was looking for an automated solution. I use the laptop-mode-tools package to automatically turn on power-saving features when the computer is working on batteries; enable the Kernel Laptop Mode, for example. I run sudo pm-powersave true from the pm-utils package to make my Thinkpad x200s consume minimum power, when I'm travelling. I installed the cpufrequtils package to tweak the CPU frequency governor according to my immediate needs. I observed how the tunables look like when the automated tools configured the system.
Running on AC power after reboot:
Running on AC power after executing sudo pm-powersave true:
Running on battery after executing sudo pm-powersave true:
You see that most of the "Good" are probably really good to turn on, because the utilities turned them on too :-) I can say that the effect is very noticeable and it means for me going from 4 to 6 hous of the battery life, when I'm coding while travelling, for example.
UPDATE: If you want to turn all tunable flags to "Good" in some startup script, you can use the --auto-tune parameter. Together with the pm-utils mentioned above it would be:
sudo pm-powersave true
sudo powertop --auto-tune
UPDATE II: Nowadays I do not use laptop-mode-tools and pm-utils any more. I use TLP from the latest Ubuntu distribution. It includes a daemon to take care of optimising the power consumption, when running on AC or in the battery mode. I still use PowerTOP as a diagnostic tool to check, that the system is configured correctly, when in doubt :-)
Powertop is intended to be primarily a diagnostic tool, and the Tunables tab is there to support that.
The idea seems to be that these are hints to distributors about potential settings they could be change in their defaults, rather than hints to end users about to configure their system.
A discussion on the project's mailing list suggests that these are not (yet) documented, but that changing at least some of them is likely to have undesired results.
