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, if available, are enabled:

• "Processor C1E support"
• "Enhanced Intel SpeedStep (EIST)"
• "AMD Cool'n'Quiet (C&Q)"
• "AMD PowerNow!"

Kernel

Activate the following kernel options:

KERNEL Enabling CPU power management options (CONFIG_ACPI_PROCESSOR, CONFIG_CPU_FREQ_STAT)

Power management and ACPI options  --->
     [*] ACPI (Advanced Configuration and Power Interface) Support  --->
         <*>   Processor
     CPU Frequency scaling  --->
         -*- CPU Frequency scaling
             [*]   CPU frequency transition statistics
             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 CPUFreq governor is needed, which is responsible for choosing the target 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 immediately 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.
'schedutil' cpufreq policy governor	cpufreq_schedutil	Aimed at driving the frequency changes by kernel scheduler

Tip
Name of the active CPUFreq governor is available in: /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

CPU frequency scaling drivers

Option	Module / Kernel symbol	Supported Processors	Note
Intel P state control	intel_pstate (CONFIG_X86_INTEL_PSTATE)	Newer (Sandy Bridge+) Intel Core	'intel_cpufreq' mode of the 'intel_pstate' driver enabled for Intel Core/Xeon processors lacking HWP (hwp CPU flag) support.
ACPI Processor P-States driver	acpi-cpufreq (CONFIG_X86_ACPI_CPUFREQ)	AMD EPYC/Ryzen, older Intel Core (pre-Sandy Bridge)/Xeon, AMD Opteron/Phenom, Intel Atom, Intel Pentium M	Acts as a generic CPUFreq driver
AMD Processor P-State driver (kernel v5.17 and above)	amd-pstate (X86_AMD_PSTATE).	AMD Zen 2 and Zen 3 CPU/APUs	Supports the standard CPUFreq governors outlined in the section above (schedutil, ondemand, etc.)
AMD Opteron/Athlon64 PowerNow!	powernow-k8 (CONFIG_X86_POWERNOW_K8)	AMD Opteron/Athlon 64, AMD Turion 64	Supports older AMD K8-based processors
Intel Enhanced SpeedStep (deprecated)	speedstep-centrino (CONFIG_X86_SPEEDSTEP_CENTRINO)	Intel Pentium M (Centrino)/Xeon	Deprecated, use ACPI Processor P-States driver instead
Intel Pentium 4 clock modulation	p4-clockmod (CONFIG_X86_P4_CLOCKMOD)	Intel Pentium 4/Xeon	Causes severe slowdowns and noticeable latency
Processor Clocking Control interface driver	pcc-cpufreq (CONFIG_X86_PCC_CPUFREQ)	Adds support for the Processor Clocking Control (PCC) interface.

Tip
Name of the active CPUFreq driver is available in: /sys/devices/system/cpu/cpu*/cpufreq/scaling_driver

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. Details about the available tickless modes can be found in the kernel documentation.

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 for power saving features:

KERNEL Enabling tick optimizing functions in the kernel (CONFIG_NO_HZ_IDLE, CONFIG_HIGH_RES_TIMERS, CONFIG_HPET)

General setup  --->
   Timers subsystem  --->
      [*] Idle dynticks system (tickless idle)
      [*] High Resolution Timer Support
Device Drivers  --->
   Character devices  --->
      [*] HPET Timer Support

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.

Manual governor change

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

root #echo ondemand | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

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

Hprofile

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 — a power management system that is part of the BIOS.

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.