.msg. See Section 23.2.2, “Troubleshooting” (page 423) for more information about
troubleshooting ACPI problems.
23.2.1 Controlling the CPU Performance
The CPU can save energy in three ways:
• Frequency and Voltage Scaling
• Throttling the Clock Frequency (T-states)
• Putting the Processor to Sleep (C-states)
Depending on the operating mode of the computer, these methods can be combined.
Saving energy also means that the system heats up less and the fans are activated less
frequently.
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. Usu-
ally, dynamic frequency scaling controlled by the kernel on-demand governor is the
best approach.
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.
For in-depth information, refer to Chapter 11, Power Management (↑System Analysis
and Tuning Guide).
23.2.2 Troubleshooting
There are two different types of problems. On one hand, the ACPI code of the kernel
may contain bugs that were not detected in time. In this case, a solution will be made
available for download. More often, the problems are caused by the BIOS. Sometimes,
deviations from the ACPI specication are purposely integrated in the BIOS to circum-
vent errors in the ACPI implementation of other widespread operating systems. Hardware
Power Management 423
