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.
At a minimum, the needed kernel modules have to be loaded and then a governor has to be set. More advanced configurations might include some combination of #cpufrequtils, acpid, laptop-mode-tools, and GUI tools provided for your desktop environment.
- 1 cpufrequtils
- 2 Configuration
- 2.1 CPU frequency driver
- 2.2 Scaling governors
- 2.3 Daemon
- 2.4 Privilege Granting Under GNOME
- 2.5 Laptop Mode Tools
- 3 Troubleshooting
#Daemon below).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 daemon script to set the governor at boot (see
Configuring CPU scaling is a 3-part process:
- Load appropriate CPU frequency driver
- Load desired scaling governor(s)
- Select a method to manage switching and tuning governor(s):
CPU frequency driver
In order for frequency scaling to work properly, the operating system must first know the limits of the CPU(s). To accomplish this, a kernel module is loaded that can read and manage the specifications of the CPU(s). Note that these modules may need related features enabled in BIOS which may be labeled as: Speedstep, Cool and Quiet, PowerNow!, or ACPI.
If you have a 64-bit processor, you will very likely want either
acpi-cpufreq for Intel processors or
powernow-k8 for AMD K8/K10 processors (Athlon 64, Opteron, and Phenom). These modules are built for both 32 and 64-bit kernels so even if you run a 32-bit kernel on your 64-bit hardware they are probably still the ones you want.
To see a full list of available drivers, run the following:
$ ls /lib/modules/$(uname -r)/kernel/drivers/cpufreq/
The following table is a partial list of available frequency drivers (Adapted from kernel source file
|acpi-cpufreq||CPUFreq driver which utilizes the ACPI Processor Performance States. This driver also supports Intel Enhanced Speedstep (previously supported by the depreciated speedstep-centrino module).|
|speedstep-ich||CPUFreq driver for certain mobile Intel Pentium III (Coppermine), all mobile Intel Pentium III-M (Tualatin) and all mobile Intel Pentium 4 P4-M on systems which have an Intel ICH2, ICH3 or ICH4 southbridge.|
|speedstep-smi||CPUFreq driver for certain mobile Intel Pentium III (Coppermine), all mobile Intel Pentium III-M (Tualatin) on systems which have an Intel 440BX/ZX/MX southbridge.|
|powernow-k8||CPUFreq driver for K8/K10 Athlon64/Opteron/Phenom processors.|
|powernow-k7||CPUFreq driver for mobile AMD K7 mobile processors.|
|cpufreq-nforce2||CPUFreq driver for FSB changing on nVidia nForce2 platforms. (AMD K7, Socket A)|
|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 processors. When enabled it will lower CPU temperature by skipping clocks.|
You probably want to use a Speedstep driver instead.
To load the CPU frequency driver manually:
# modprobe acpi-cpufreq
Note that if you attempt to load the wrong module you will get get an error such as
FATAL: Error inserting acpi_cpufreq ([...]/acpi-cpufreq.ko): No such device
Once the appropriate cpufreq driver is loaded, detailed information about the CPU(s) can be displayed by running:
analyzing CPU 0: driver: acpi-cpufreq CPUs which need to switch frequency at the same time: 0 1 hardware limits: 1000 MHz - 2.00 GHz available frequency steps: 2.00 GHz, 1.67 GHz, 1.33 GHz, 1000 MHz available cpufreq governors: ondemand, performance current policy: frequency should be within 1000 MHz and 2.00 GHz. The governor "performance" may decide which speed to use within this range. current CPU frequency is 2.00 GHz. analyzing CPU 1: driver: acpi-cpufreq CPUs which need to switch frequency at the same time: 0 1 hardware limits: 1000 MHz - 2.00 GHz available frequency steps: 2.00 GHz, 1.67 GHz, 1.33 GHz, 1000 MHz available cpufreq governors: ondemand, performance current policy: frequency should be within 1000 MHz and 2.00 GHz. The governor "performance" may decide which speed to use within this range. current CPU frequency is 2.00 GHz.
To load the driver automatically at startup, add the appropriate driver to the
MODULES array in
/etc/rc.conf. For example:
Governors can be thought of as pre-configured power schemes for the CPU. These governors must be loaded as kernel modules in order to be seen by user space programs. One may load as many governors as desired (only one will be active on a CPU at any given time).
- The performance governor is built into the kernel and runs the CPU(s) at maximum clock speed
- Dynamically switches between the CPU(s) available clock speeds based on system load
- Similar to
ondemand, but the CPU(s) clock speed switches gradually through all its available frequencies based on system load
- Runs the CPU(s) at minimum speed
- Manually configured clock speeds by user
For Desktops and most systems, the ondemand governor can provide the best compromise between heat emission, power consumption, performance, and manageability. For Laptops or other mobile systems, the conservative governor can possibly provide significant savings in power consumption.
Load the desired governor(s) with
modprobe. For example:
# modprobe cpufreq_ondemand # modprobe cpufreq_userspace
Or, add the desired governor(s) to the MODULES array in
/etc/rc.conf and reboot.
MODULES=(acpi-cpufreq cpufreq_ondemand cpufreq_powersave)
To see which governors have been loaded:
Manually set the governor by running the
cpufreq-set command (as root). For example:
# cpufreq-set -c 0 -g ondemand
Note that the previous command only sets the governor for the first processor! Multicore or multiprocessor systems require the
-c flag to set the governor for a specific core and to repeat for each remaining. Numbering starts from 0 not 1!
For a quad core chip:
# for i in 0 1 2 3; do cpufreq-set -c $i -g ondemand; done
For a dual core chip:
# for i in 0 1; do cpufreq-set -c $i -g ondemand; done
Additional options such as upper and lower frequency bounds used by the governor can optionally be set by using the
-d options. A processors valid frequencies can be seen by calling cpufreq-info. For example, to set the second core's upper frequency bound as 2.00GHz and its lower bound as 1.00 GHz:
# cpufreq-set -c 1 -g ondemand -u 2.00Ghz -d 1.00Ghz
To manually set a processor to a specific frequency the userspace governor is used. For example, to set core 0 to 2.50Ghz and core 1 to 800Mhz:
# cpufreq-set -c 0 -g userspace -f 2.50Ghz # cpufreq-set -c 1 -g userspace -f 800Mhz
cpufreq-set --help or
man cpufreq-set for more information.
For those who would like a GUI for setting governors or frequency there is trayfreq, a GTK+ application that sits in the system tray.
The monitoring of CPU clock in real-time is accomplished by running:
$ watch grep \"cpu MHz\" /proc/cpuinfo
Improving on-demand performance
With the out-of-the-box configuration, the ondemand governor will result in a slight but measurable and noticeable loss of performance. It will not clock up the CPU when it is at lower than 95% usage, and will sample the usage at the fastest possible frequency when at full clock speeds in order to clock back down as soon as possible.
Tuning the governor for your needs can reduce the performance loss to the point where it is negligible/non-existent if you are willing to lose a lot of the power savings. If you just want to save power while idle, lowering the up_threshold to 11% and raising the sample_down_factor by an order of magnitude can accomplish this.
Tunables are available in
/sys/devices/system/cpu/cpufreq/ondemand/ once the governor is loaded and selected and can be preserved at reboot using
Starting the cpufreq daemon in the background (i.e. with an
/etc/rc.conf) will likely cause
/etc/rc.local to be processed before the cpufreq daemon has a chance to change the governor. Make sure that the cpufreq daemon script will have finished running before the tunables are set. Example:
(sleep 10 && echo -n 25 > /sys/devices/system/cpu/cpufreq/ondemand/up_threshold) &
Changing the on-demand governor's threshold
To change when the
ondemand governor switches to a higher multiplier, one can manipulate
/sys/devices/system/cpu/cpufreq/ondemand/up_threshold. Determine the current setting by issuing the following command as root:
# cat /sys/devices/system/cpu/cpufreq/ondemand/up_threshold
The value returned should be
95, the default setting as of kernel version 3.0. This means that the
ondemand governor currently increases the clock rate if a core reaches 95% utilization. This can be changed, for example:
# echo -n 15 > /sys/devices/system/cpu/cpufreq/ondemand/up_threshold
On-demand governor sampling-down-factor
sampling_down_factor is another global ondemand global tunable.
Set to 1 it makes no changes from existing behavior, but set to greater than 1 (e.g. 100) it acts as a multiplier for the scheduling interval for reevaluating load when the CPU is at its top speed due to high load. This improves performance by reducing the overhead of load evaluation and helping the CPU stay at its top speed when truly busy, rather than shifting back and forth in speed. This tunable has no effect on behavior at lower speeds/lower CPU loads.
Read the value (default: 1):
# cat /sys/devices/system/cpu/cpufreq/ondemand/sampling_down_factor
Set the value:
# echo -n 10 > /sys/devices/system/cpu/cpufreq/ondemand/sampling_down_factor
Tuning conservative governor
While the conservative governor switches smoothly and utilizes all of the available frequencies, users may want to tune it further. Out-of-the-box it will clock the CPU up when load reaches 75% and clock down when load drops to 20%. As with the on-demand governor, decreasing the
up_threshold may improve performance and responsiveness.
Tunables are available in
/sys/devices/system/cpu/cpufreq/conservative/. Refer to the #Improving ondemand performance section for important advice regarding making this changes permanent, and potential daemon loading issues.
Changing the conservative governor's thresholds
Determine the current
up_threshold setting by issuing the following command as root:
# cat /sys/devices/system/cpu/cpufreq/conservative/up_threshold
The value returned should be
75, the default setting as of kernel version 3.0. This means that the
conservative governor currently increases the clock rate to the next highest speed if a core reaches 75% utilization. The can be changed, for example:
# echo -n 40 > /sys/devices/system/cpu/cpufreq/conservative/up_threshold
down_threshold; entering a value under that results in the error, "bash: echo: write error: Invalid argument"
down_threshold value can be read and modified via
/sys/devices/system/cpu/cpufreq/conservative/down_threshold. The default value should be
20 as of kernel version 3.0. This means that the
conservative governor decreases the clock rate to the next lowest speed if a core falls to 20% utilization, which is a sensible default.
While the down sampling rate can also be adjusted for the
conservative governor, increasing it may only help with occasional low usage CPU spikes during high usage applications, as the
down_threshold is a much more direct control for down scaling which does not exist on the
Be aware that setting
down_threshold too close to
up_threshold may result in constant CPU switching, which might be something desirable for certain users and not for others. Setting
up_threshold too low may result in the CPU being clocked higher than it needs sacrificing power saving for performance, and setting
up_threshold too high may result in decreased performance, but reduced power consumption. Experiment to find the optimal values for your system and your needs.
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 ;; [...]
includes a daemon which allows users to set the desired scaling governor and min/max clock speeds for all processor cores at boot-time.
Before starting the daemon, edit
/etc/conf.d/cpufreq as root, selecting the desired governor and setting the min/max speed for your CPU(s), for example:
#configuration for cpufreq control # valid governors: # ondemand, performance, powersave, # conservative, userspace governor="ondemand" # valid suffixes: Hz, kHz (default), MHz, GHz, THz min_freq="1GHz" max_freq="2GHz"
cpufreq-infoafter loading the CPU driver (e.g.
modprobe acpi-cpufreq). However, these values are optional. Users may omit them entirely by deleting or commenting out the min/max_freq lines; scaling will work automatically.
With the appropriate configuration, start the daemon with the following command:
# rc.d start cpufreq
To start the daemon automatically at startup, add
cpufreq to the
DAEMONS array in
/etc/rc.conf, for example:
DAEMONS=(syslog-ng networkmanager @alsa @crond @cupsd @cpufreq)
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
/var/lib/polkit-1/localauthority/50-local.d/org.gnome.cpufreqselector.pkla and populate it with the following:
[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.
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
MODULES array in
/etc/rc.conf (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.
- 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 #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 CPU/BIOS configurations may have difficulties to scale to the maximum frequencies or scale to higher frequencies at all. Sadly there is only a workaround right now. Add "processor.ignore_ppc=1" to your kernel boot line and/or edit the value in
/sys/module/processor/parameters/ignore_ppcfrom 0 to 1. (It seems to be fixed at least since kernel 3.0, for Toshiba NB-100)
- 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.