Modern processors implement runtime power saving modes called "C states". Each of these states is denoted by the letter C followed by a number. The larger the number, the deeper the C state and the more power that state saves. In a deep C state the majority of the processor will be unclocked and powered down, reducing the CPU power consumption to a very low level.

C states offer significant power savings, but cannot be entered when the CPU is executing instructions. The best power savings can be obtained by running the CPU as fast as possible until any outstanding work is completed and then allowing the CPU to go completely idle. The powersave governor will extend the time taken to complete the work and reduce the amount of time spent idle. On any modern CPU the benefit of carrying out the work at a lower clock and voltage will be outweighed by the loss of the idle time. In almost any workload, powersave will consume more energy than any other option.

The ondemand governor jumps the processor to full performance once demand reaches a certain threshold. It then drops back to lower performance states when the work is exhausted. This allows a return to idle in as short a timeframe as possible. The reason to prefer ondemand over performance is that very short bursty workloads will prevent the CPU from entering deep C states anyway, but do not generally benefit from being run at the higher CPU speeds. Not raising the processor clock for this workload can save some energy.

The conservative governor behaves like ondemand but takes longer to switch states. This will almost always result in it taking longer for the processor to become idle, thus increasing power consumption.

Summary: Use ondemand. Conservative is a valid option for processors that take a sufficiently long time to switch performance states that ondemand will not work.

Note: There are some workloads which currently interact poorly with the default ondemand settings. If you find one of these workloads, please contact me.