Novell LINUX ENTERPRISE SERVER 10 - INSTALLATION AND ADMINISTRATION 04-08-2006 Installation Manual page 529

hardware or in regard to specific processors or drivers, the userspace implemen-
tation is still the only working solution.
ondemand governor
This is the kernel implementation of a dynamic CPU frequency policy and
should work on most systems. As soon as there is a high system load, the CPU
frequency is immediately increased. It is lowered on a low system load.
conservative governor
This governor is similar to the ondemand implementation, except that a more
conservative policy is used. The load of the system must be high for a specific
amount of time before the CPU frequency is increased.
powersave governor
The cpu frequency is statically set to the lowest possible.
performance governor
The cpu frequency is statically set to the highest possible.
Throttling the Clock Frequency
This technology omits a certain percentage of the clock signal impulses for the
CPU. At 25% throttling, every fourth impulse is omitted. At 87.5%, only every
eighth impulse reaches the processor. However, the energy savings are a little less
than linear. Normally, throttling is only used if frequency scaling is not available
or to maximize power savings. This technology, too, must be controlled by a special
process. The system interface is /proc/acpi/processor/*/throttling.
Putting the Processor to Sleep
The operating system puts the processor to sleep whenever there is nothing to do.
In this case, the operating system sends the CPU a halt command. There are three
states: C1, C2, and C3. In the most economic state, C3, even the synchronization
of the processor cache with the main memory is halted. Therefore, this state can
only be applied if no other device modifies the contents of the main memory via
bus master activity. Some drivers prevent the use of C3. The current state is dis-
played in /proc/acpi/processor/*/power.
Frequency scaling and throttling are only relevant if the processor is busy, because the
most economic C state is applied anyway when the processor is idle. If the CPU is busy,
frequency scaling is the recommended power saving method. Often the processor only
works with a partial load. In this case, it can be run with a lower frequency. Usually,
dynamic frequency scaling controlled by the kernel ondemand governor or a daemon,
Power Management 529