.msg. See Section 19.2.3, “Troubleshooting” (page 354) for more information about
troubleshooting ACPI problems.
19.2.1 Controlling the CPU Performance
The CPU can save energy in three ways:
• Frequency and Voltage Scaling (page 351)
• Throttling the Clock Frequency (T-states) (page 353)
• Putting the Processor to Sleep (C-states) (page 353)
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.
19.2.1.1 Frequency and Voltage Scaling
PowerNow! and Speedstep are the designations AMD and Intel use for this technology.
However, this technology is also applied in processors of other manufacturers. The
clock frequency of the CPU and its core voltage are reduced at the same time, resulting
in more than linear energy savings. This means that when the frequency is halved (half
performance), far less than half of the energy is consumed. This technology is indepen-
dent from ACPI.
Power Management 351
