CPU frequency scaling

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Revision as of 05:35, 23 April 2013 by Chadversary (talk | contribs) (→‎Scaling governors: Link to official docs in the kernel source.)
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Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: This article needs cleanup in sections: acpid, laptop-mode-tools, and bios limitations. (Discuss in Talk:CPU frequency scaling#)

cpufreq refers to the kernel infrastructure that implements CPU frequency scaling. This technology enables the operating system to scale the CPU speed 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.

Since kernel 3.4 the necessary modules are loaded automatically and the recommended ondemand governor is enabled by default. However, userspace applications 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 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. To enable cpupower on boot with systemd, run

# systemctl enable cpupower.service

A frontend for gnome-shell can be found at CPU Freq.

CPU frequency driver

Note: As of kernel 3.4; the native cpu module is loaded automatically

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

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 (with cpupower)

Tip: Add the following commands below to /etc/default/cpupower. <percent> is the percentage of cpu load; <governor> is the cpupower governor

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

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 /var/lib/polkit-1/localauthority/50-local.d/org.gnome.cpufreqselector.pkla and populate it with the following:



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.

Laptop Mode Tools

If you are already using or plan to use Laptop Mode Tools for other power saving solutions, then you may want to let it also manage CPU frequency scaling. To do so, you just have to insert the appropriate frequency driver to the /etc/modules.d/ directory. (see #CPU frequency driver above) and then go through the /etc/laptop-mode/conf.d/cpufreq.conf file to define governors, frequencies and policies. You will not need to load other modules and daemons or to set up scaling governors and interaction with ACPI events. Please refer to Laptop Mode Tools for more details.


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 #Changing 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