Power management/Processor

Resources

Dynamic frequency scaling

This article describes the setup of power management for processors.

CPU frequency scaling

CPU frequency scaling is a technique whereby the frequency of a processor can be automatically adjusted "on the fly" to conserve power and to reduce the amount of heat generated by the chip.

Installation

BIOS

Some functions can be enabled or disabled in the BIOS. Check that the following are enabled:

• "Processor C1E support"
• "Enhanced Speedstep (EIST)"

Kernel

Activate the following kernel options:

KERNEL Enabling CPU power management options

Power management and ACPI options  --->
     [*] ACPI (Advanced Configuration and Power Interface) Support  --->
         <*>   Processor
     CPU Frequency scaling  --->
         [*] CPU Frequency scaling
             Default CPUFreq governor (ondemand)  --->
                 Select a default governor; see below table
                 Default is ondemand
             *** CPU frequency scaling drivers ***
                 Select a driver; see below table

A governor is needed, which is responsible for changing the frequency based on its profile:

Default CPUFreq governor

Option	Module	Supported Processors	Note
'performance' governor	cpufreq_performance	Sets the frequency statically to the highest available CPU frequency.	For Intel core processors, this should be selected as default.
'powersave' governor	cpufreq_powersave	Sets the frequency statically to the lowest available CPU frequency.	Can't be set as default.
'userspace' governor for userspace frequency scaling	cpufreq_userspace	To set the CPU frequency manually or when a userspace program shall be able to set the CPU dynamically.
'ondemand' cpufreq policy governor	cpufreq_ondemand	Does a periodic polling and changes frequency based on the CPU utilization.	For processors other than Intel core, this should be selected as default.
'conservative' cpufreq governor	cpufreq_conservative	Similar to ondemand. The frequency is gracefully increased and decreased rather than jumping to 100% when speed is required.

CPU frequency scaling drivers

Option	Module	Supported Processors	Note
Intel P state control	intel_pstate	Newer (SandyBridge+) Intel Core
Processor Clocking Control interface driver	pcc-cpufreq
ACPI Processor P-States driver	acpi-cpufreq	Older Intel Core, Intel Atom, Intel Pentium M
AMD Opteron/Athlon64 PowerNow!	powernow-k8	AMD Opteron, AMD Athlon 64, AMD Turion 64
Intel Enhanced SpeedStep (deprecated)	speedstep-centrino	Intel Pentium M (Centrino), Intel Xeon	Deprecated, use ACPI Processor P-States driver instead
Intel Pentium 4 clock modulation	p4-clockmod	Intel Pentium 4, Intel XEON	Causes severe slowdowns and noticeable latencies

Ticks

The processor saves the most energy when the processor stays longer in power savings mode, so reduce the amount of ticks that wakes the processor up.

Installation

BIOS

Some functions can be enabled or disabled in the BIOS. Check that the following are enabled:

• "HPET"
• "Multimedia timer"

Kernel

Activate the following kernel options:

KERNEL Enabling tick optimizing functions in the kernel

Processor type and features  --->
   [*] Tickless System (Dynamic Ticks)
   [*] High Resolution Timer Support
   [*] HPET Timer Support

Note
These options now appear to be under "General setup" -> "Timers subsystem".

PowerTOP

PowerTOP is a utility designed to measure, explain and minimize a computer's electrical power consumption.

When it is run, it sorts the running processes in order of how often they cause the processor to wake up. For details on installation, configuration and usage see the separate PowerTOP article.

Hprofile

It is possible to change the active CPU governor using a simple command:

root #for c in $(ls -d /sys/devices/system/cpu/cpu[0-9]*); do echo ondemand >$c/cpufreq/scaling_governor; done

Read /etc/local.d/README to learn how to execute this command on startup.

Applications such as sys-power/cpupower do little more than the above but with some interface on top of it. However, having to run certain commands as root depending on the system usage is not efficient for most users. It is better to automate some of the decisions of governing CPU frequency. For instance, when not wired to AC power, most users would like to have the system in a power saving mode.

This is where Hprofile comes into play. Please refer to its article for more information and configuration.

See also

• ACPI - Advanced Configuration and Power Management Interface.

External resources

• What exactly is a P-state? (Pt. 1) - An Intel article (kind of) explaining P-states.
• Linux's "Ondemand" Governor Is No Longer Fit - Explains why ondemand should not be used for newer Intel core processors.