CPU frequency scaling

From ArchWiki
Revision as of 15:43, 2 August 2013 by Lahwaacz (Talk | contribs) (style)

Jump to: navigation, search

zh-CN:CPU Frequency Scaling Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary end

CPU frequency scaling enables the operating system to scale the CPU frequency up or down in order to save power. CPU frequencies can be scaled automatically depending on the system load, in response to ACPI events, or manually by userspace programs.

CPU frequency scaling is implemented in Linux kernel, the infrastructure is called cpufreq. Since kernel 3.4 the necessary modules are loaded automatically and the recommended ondemand governor is enabled by default. However, userspace tools like cpupower, acpid, Laptop Mode Tools, or GUI tools provided for your desktop environment, may still be used for advanced configuration.

Userspace tools (cpupower)

cpupower is a set of userspace utilities designed to assist with CPU frequency scaling. The package is not required to use scaling, but is highly recommended because it provides useful command-line utilities and a systemd service to change the governor at boot.

The configuration file for cpupower is located in /etc/default/cpupower. This configuration file is read by a bash script in /usr/lib/systemd/scripts/cpupower which is activated by systemd with cpupower.service. You may want to enable cpupower.service to start at boot.

Tip: A frontend for GNOME Shell is available: [1]

CPU frequency driver

  • As of kernel 3.4, the native cpu module is loaded automatically.
  • Starting with kernel 3.9, the new pstate power scaling driver is used automatically for modern Intel CPUs instead of the other drivers below. This driver takes priority over other drivers and in fact is inbuilt as opposed to being a module. This driver is currently automatically used for Sandy Bridge and Ivy Bridge type cpus. If you encounter a problem while using this driver, add intel_pstate=disable to your kernel line. You can use the same user space utilities with this driver but cannot control it.
  • Even P State behavior mentioned above can be influenced with /sys/devices/system/cpu/intel_pstate, e.g. Intel Turbo Boost can be deactivated with # echo 1 > /sys/devices/system/cpu/intel_pstate/no_turbo for keeping CPU-Temperatures low.

cpupower requires modules (see table below) to know the limits of the native cpu. To see a full list of available modules, run:

$ ls /lib/modules/$(uname -r)/kernel/drivers/cpufreq/
Tip: To load the module at boot, run:
# echo module > /etc/modules-load.d/module.conf
Note: Loading the wrong module will result in an error "No such device"

Load the appropriate module with:

# modprobe module
Module Description
acpi-cpufreq CPUFreq driver which utilizes the ACPI Processor Performance States. This driver also supports Intel Enhanced SpeedStep (previously supported by the deprecated speedstep-centrino module).
speedstep-lib CPUFreq drive for Intel speedstep enabled processors (mostly atoms and older pentiums (< 3))
powernow-k8 CPUFreq driver for K8/K10 Athlon64/Opteron/Phenom processors. Deprecated since linux 3.7 - Use acpi_cpufreq.
pcc-cpufreq This driver supports Processor Clocking Control interface by Hewlett-Packard and Microsoft Corporation which is useful on some Proliant servers.
p4_clockmod CPUFreq driver for Intel Pentium 4 / Xeon / Celeron processors. When enabled it will lower CPU temperature by skipping clocks.
You probably want to use a Speedstep driver instead.

Once the appropriate cpufreq driver is loaded, detailed information about the CPU(s) can be displayed by running

$ cpupower frequency-info

Scaling governors

Governors (see table below) are power schemes for the cpu. Only one may be active at a time. For details, see the official documentation in the kernel source.

Note: The kernel loads on_demand by default.
Module Description
cpufreq_ondemand Dynamically switch between CPU(s) available if at 95% cpu load
cpufreq_performance Run the cpu at max frequency
cpufreq_conservative Dynamically switch between CPU(s) available if at 75% load
cpufreq_powersave Run the cpu at the minimum frequency
cpufreq_userspace Run the cpu at user specified frequencies
Tip: To monitor cpu speed in real time, run:
$ watch grep \"cpu MHz\" /proc/cpuinfo

With cpupower

To load and activate a particular governor, one should run:

# cpupower frequency-set -g governor_without cpufreq

Without cpupower

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: The following does not activate the governor. To activate it on every available CPU, run for cpu in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor; do echo governor > $cpu; done. (Discuss in Talk:CPU frequency scaling#)
Tip: To load a governor at boot, run:
# echo module > /etc/modules-load.d/module

To load a particular governor, one should run:

# modprobe governor

Tuning governors

To set the threshold for stepping up to another frequency:

# echo -n percent > /sys/devices/system/cpu/cpufreq/<governor>/up_threshold

To set the threshold for stepping down to another frequency:

# echo -n percent > /sys/devices/system/cpu/cpufreq/<governor>/down_threshold
Sampling rate

The sampling rate determines how frequently the governor checks to tune the cpu Setting sampling_down_factor greater than 1 improves performance by reducing the overhead of load evaluation and keeping the CPU at its highest clock frequency due to high load. This tunable has no effect on behavior at lower CPU frequencies/loads.

To read the value (default = 1), run:

$ cat /sys/devices/system/cpu/cpufreq/ondemand/sampling_down_factor

To set the value, run:

# echo -n value > /sys/devices/system/cpu/cpufreq/ondemand/sampling_down_factor

Setting maximum and minimum frequencies

Note: The governor, maximum, and minimum frequencies can be set in /etc/default/cpupower. To adjust for only a single cpu core: -c core #. clock_freq is a clock frequency with units : GHz,MHz

To set the maximum clock frequency:

# cpupower frequency-set -u clock_freq

To set the minimum clock frequency:

# cpupower frequency-set -d clock_freq

To set the cpu to run at a specified frequency:

# cpupower frequency-set -f clock_freq

Interaction with ACPI events

Users may configure scaling governors to switch automatically based on different ACPI events such as connecting the AC adapter or closing a laptop lid. A quick example is given below, however it may be worth reading full article on acpid.

Events are defined in /etc/acpi/handler.sh. If the acpid package is installed, the file should already exist and be executable. For example, to change the scaling governor from performance to conservative when the AC adapter is disconnected and change it back if reconnected:


    case "$2" in
            case "$4" in
                    echo "conservative" >/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor    
                    echo -n $minspeed >$setspeed
                    #/etc/laptop-mode/laptop-mode start
                    echo "performance" >/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
                    echo -n $maxspeed >$setspeed
                    #/etc/laptop-mode/laptop-mode stop
        *) logger "ACPI action undefined: $2" ;;


Privilege granting under GNOME

Note: systemd introduced logind which handles consolekit and policykit actions. The following code below does not work. With logind, simply edit in the file /usr/share/polkit-1/actions/org.gnome.cpufreqselector.policy the <defaults> elements according to your needs and the polkit manual [2].

GNOME has a nice applet to change the governor on the fly. To use it without the need to enter the root password, simply create following file:


Where the word user is replaced with the username of interest.

The desktop-privilegesAUR package in the AUR contains a similar .pkla file for authorizing all users of the power group to change the governor.


GranolaAUR is a daemon that monitors the cpu usage and uses the cpufreq-userspace module to lessen power usage without any noticeable difference in performance. To use it, first install the package from AUR, the default settings will work for most setups. You will need to load the cpufreq_userspace module, as well as the cpufreq scaling governor for your CPU at startup. Create two files like the following ones to start modules at boot:

/etc/modules-load.d/choosen governor.conf
choosen governor

and reboot.

To test if it worked, run:

$ cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq

or, if you have cpufreq-utils installed:


and check that the cpu frequency is below maximum.


Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: please use the first argument of the template to provide a brief explanation. (Discuss in Talk:CPU frequency scaling#)
  • Some applications, like ntop, do not respond well to automatic frequency scaling. In the case of ntop it can result in segmentation faults and lots of lost information as even the on-demand governor cannot change the frequency quickly enough when a lot of packets suddenly arrive at the monitored network interface that cannot be handled by the current processor speed.
  • Some CPU's may suffer from poor performance with the default settings of the on-demand governor (e.g. flash videos not playing smoothly or stuttering window animations). Instead of completely disabling frequency scaling to resolve these issues, the aggressiveness of frequency scaling can be increased by lowering the up_threshold sysctl variable for each CPU. See how to change the on-demand governor's threshold.
  • Sometimes the on-demand governor may not throttle to the maximum frequency but one step below. This can be solved by setting max_freq value slightly higher than the real maximum. For example, if frequency range of the CPU is from 2.00 GHz to 3.00 GHz, setting max_freq to 3.01 GHz can be a good idea.
  • Some combinations of ALSA drivers and sound chips may cause audio skipping as the governor changes between frequencies, switching back to a non-changing governor seems to stop the audio skipping.

BIOS frequency limitation

Some CPU/BIOS configurations may have difficulties to scale to the maximum frequency or scale to higher frequencies at all. This is most likely caused by BIOS events telling the OS to limit the frequency resulting in /sys/devices/system/cpu/cpu0/cpufreq/bios_limit set to a lower value.

Either you just made a specific Setting in the BIOS Setup Utility, (Frequency, Thermal Management, etc.) you can blame a buggy/outdated BIOS or the BIOS might have a serious reason for throttling the CPU on it's own.

Reasons like that can be (assuming your machine's a notebook) that the battery is removed (or near death) so you're on AC-power only. In this case a weak AC-source might not supply enough electricity to fulfill extreme peak demands by the overall system and as there is no battery to assist this could lead to data loss, data corruption or in worst case even hardware damage!

Not all BIOS'es limit the CPU-Frequency in this case, but for example most IBM/Lenove Thinkpads do. Refer to thinkwiki for more thinkpad related info on this topic.

If you checked there's not just an odd BIOS setting and you know what you're doing you can make the Kernel ignore these BIOS-limitations.

Warning: Make sure you read and understood the section above. CPU frequency limitation is a safety feature of your BIOS and you should not need to work around it.

A special parameter has to be passed to the processor module.

For trying this temporarily change the value in /sys/module/processor/parameters/ignore_ppc from 0 to 1.

For setting it permanent refer to Kernel modules or just read on. Add processor.ignore_ppc=1 to your kernel boot line or create

# If the frequency of your machine gets wrongly limited by BIOS, this should help
options processor ignore_ppc=1

See also