Throttling the Clock Frequency (T-states)
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
for Processor Throttling States (T-states) is /proc/acpi/processor/*/
throttling.
Putting the Processor to Sleep (C-states)
Modern processors have several power saving modes called C-states. They reect
the capability of an idle processor to turn off unused components in order to save
power. 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
idle 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 modies the contents of the main memory via bus master
activity. Some drivers prevent the use of C3. The current state is displayed in /proc/
acpi/processor/*/power.
31.2.2 Tools
To view or adjust the current settings of the CPUfreq subsystem use the tools provided
by cpufrequtils for that. After you have installed the cpufrequtils package,
use the cpufreq-info to retrieve CPUfreq kernel information. The cpufreq-set
command can be used to modify CPUfreq settings. For example, run the following
command as root to activate the on-demand governor at runtime:
cpufreq-set -g ondemand
For more details and the available options, refer to the cpufreq-info and the
cpufreq-set man pages or run cpufreq-info --help or
cpufreq-set --help, respectively.
A useful tool for monitoring system power consumption is powerTOP, available after
installation of the powertop package. It helps you to identify the reasons for unnec-
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