Novell LINUX ENTERPRISE SERVER 11 - ADMINISTRATION Administration Manual page 192

performance governor
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 must be controlled by a special
process, as well. The system interface is /proc/acpi/processor/*/
throttling.
Putting the Processor to Sleep
The operating system puts the processor to sleep whenever there is no activity. 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 on demand governor or a daemon,
such as powersaved, is the best approach. A static setting to a low frequency is useful
for battery operation or if you want the computer to run cool or be quiet.
Throttling should be used as the last resort, for example, to extend the battery operation
time despite a high system load. However, some systems do not run smoothly when
they are throttled too much. Moreover, CPU throttling does not make sense if the CPU
has little to do.
15.2.2 ACPI Tools
The range of more or less comprehensive ACPI utilities includes tools that merely display
information, like the battery charge level and the temperature (acpi, klaptopdaemon,
etc.), tools that facilitate the access to the structures in /proc/acpi or that assist in
178 Administration Guide
The cpu frequency is statically set to the highest possible.