CPU frequency scaling
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 the 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.
AUR is a Linux daemon used to prevent the overheating of platforms. This daemon monitors temperature and applies compensation using available cooling methods.
By default, it monitors CPU temperature using available CPU digital temperature sensors and maintains CPU temperature under control, before HW takes aggressive correction action. If there is a skin temperature sensor in thermal sysfs, then it tries to keep skin temperature under 45C.
is an i7 (and now i3, i5) CPU reporting tool for Linux. It can be launched from a Terminal with the command
i7z or as GUI with
systemd service to change the governor at boot.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
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.
CPU frequency driver
cpupower requires modules to know the limits of the native CPU:
|intel_pstate||This driver implements a scaling driver with an internal governor for Intel Core (Sandy Bridge and newer) processors.|
|acpi-cpufreq||CPUFreq driver which utilizes the ACPI Processor Performance States. This driver also supports the Intel Enhanced SpeedStep (previously supported by the deprecated speedstep-centrino module).|
|speedstep-lib||CPUFreq driver for Intel SpeedStep-enabled processors (mostly Atoms and older Pentiums (< 3))|
|powernow-k8||CPUFreq driver for K8/K10 Athlon 64/Opteron/Phenom processors. Since Linux 3.7 'acpi-cpufreq' will automatically be used for more modern CPUs from this family.|
|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 which lowers the CPU temperature by skipping clocks. (You probably want to use a SpeedStep driver instead.)|
To see a full list of available modules, run:
$ ls /usr/lib/modules/$(uname -r)/kernel/drivers/cpufreq/
Load the appropriate module (see Kernel modules for details). Once the appropriate cpufreq driver is loaded, detailed information about the CPU(s) can be displayed by running
$ cpupower frequency-info
Setting maximum and minimum frequencies
In rare cases, it may be necessary to manually set maximum and minimum frequencies.
To set the maximum clock frequency (clock_freq is a clock frequency with units: GHz, MHz):
# 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
Governors (see table below) are power schemes for the CPU. Only one may be active at a time. For details, see the kernel documentation in the kernel source.
|performance||Run the CPU at the maximum frequency.|
|powersave||Run the CPU at the minimum frequency.|
|userspace||Run the CPU at user specified frequencies.|
|ondemand||Scales the frequency dynamically according to current load. Jumps to the highest frequency and then possibly back off as the idle time increases.|
|conservative||Scales the frequency dynamically according to current load. Scales the frequency more gradually than ondemand.|
|schedutil||Scheduler-driven CPU frequency selection , .|
Depending on the scaling driver, one of these governors will be loaded by default:
ondemandfor AMD and older Intel CPU.
powersavefor Intel CPUs using the
intel_pstatedriver (Sandy Bridge and newer).
To activate a particular governor, run:
# cpupower frequency-set -g governor
Alternatively, you can activate a governor on every available CPU manually:
# echo governor | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor >/dev/null
Tuning the ondemand governor
See the kernel documentation for details.
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
The sampling rate determines how frequently the governor checks to tune the CPU.
sampling_down_factor is a tunable that multiplies the sampling rate when the CPU is at its highest clock frequency thereby delaying load evaluation and improving performance. Allowed values for
sampling_down_factor are 1 to 100000. 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
Make changes permanent
Even though kernel modules#Using file in /etc/modprobe.d/ or systemd might work to make the changes permanent, in some cases there might be race conditions. This is noted at systemd#Temporary files. udev is doing better.
For example, to set the scaling governor of the CPU core
0 to performance while the scaling driver is
acpi_cpufreq, create the following udev rule:
SUBSYSTEM=="module", ACTION=="add", KERNEL=="acpi_cpufreq", RUN+=" /bin/sh -c ' echo performance > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor ' "
To have the chages persist after boot, when an initramfs is used, follow the example at udev#Debug output.
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 package is installed, the file should already exist and be executable. For example, to change the scaling governor from
conservative when the AC adapter is disconnected and change it back if reconnected:
[...] ac_adapter) case "$2" in AC*) case "$4" in 00000000) echo "conservative" >/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor echo -n $minspeed >$setspeed #/etc/laptop-mode/laptop-mode start ;; 00000001) echo "performance" >/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor echo -n $maxspeed >$setspeed #/etc/laptop-mode/laptop-mode stop ;; esac ;; *) logger "ACPI action undefined: $2" ;; esac ;; [...]
Privilege granting under GNOME
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:
[org.gnome.cpufreqselector] Identity=unix-user:user Action=org.gnome.cpufreqselector ResultAny=no ResultInactive=no ResultActive=yes
Where the word user is replaced with the username of interest.
- 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-demandgovernor 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-demandgovernor (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/Lenovo 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.
A special parameter has to be passed to the processor module.
For trying this temporarily change the value in
# If the frequency of your machine gets wrongly limited by BIOS, this should help options processor ignore_ppc=1