Difference between revisions of "How to scale CPU frequency with DVFS framework"

Latest revision as of 18:05, 19 May 2018

How to scale CPU frequency with DVFS framework

Intoduction

CPU frequency scaling enables the operating system to scale the CPU frequency in order to minimize power usage (saving power when the full performance of the CPU is not needed). CPU frequency scaling is implemented in the Linux kernel as a Dynamic Voltage and Frequency Scaling (DVFS) framework. DVFS allows to set min/max CPU frequency and choose a scaling governor that governs it.

Dynamic Frequency Scaling – Linux configuration and sysfs interface

To enable dynamic frequency scaling, we need to have a kernel with the CPUfreq drivers built-in (done by default when available):

CPU Power Management  --->
   CPU Frequency scaling  --->
       [*] CPU Frequency scaling
       <*>   CPU frequency translation statistics
       [ ]     CPU frequency translation statistics details
       *** CPU frequency scaling drivers ***
       < >   Generic DT based cpufreq driver
       <*>   Freescale i.MX6 cpufreq support
       <*>   Freescale i.MX7 cpufreq support

The SoMLabs modules supports DVFS feature by default, there is no need to perform manual kernel recompilation.

DVFS framework exports information to userspace through sysfs interface:

root@somlabs:/# ls -l /sys/devices/system/cpu/cpu0/cpufreq/
total 0
-r--r--r-- 1 root root 4096 Apr 24 06:54 affected_cpus
-r-------- 1 root root 4096 Apr 24 06:54 cpuinfo_cur_freq
-r--r--r-- 1 root root 4096 Apr 24 06:54 cpuinfo_max_freq
-r--r--r-- 1 root root 4096 Apr 24 06:54 cpuinfo_min_freq
-r--r--r-- 1 root root 4096 Apr 24 06:54 cpuinfo_transition_latency
-r--r--r-- 1 root root 4096 Apr 24 06:54 related_cpus
-r--r--r-- 1 root root 4096 Apr 24 06:54 scaling_available_frequencies
-r--r--r-- 1 root root 4096 Apr 24 06:54 scaling_available_governors
-r--r--r-- 1 root root 4096 Apr 24 06:54 scaling_cur_freq
-r--r--r-- 1 root root 4096 Apr 24 06:54 scaling_driver
-rw-r--r-- 1 root root 4096 Apr 24 06:54 scaling_governor
-rw-r--r-- 1 root root 4096 Apr 24 06:54 scaling_max_freq
-rw-r--r-- 1 root root 4096 Apr 24 06:54 scaling_min_freq
-rw-r--r-- 1 root root 4096 Apr 24 06:54 scaling_setspeed
drwxr-xr-x 2 root root    0 Apr 24 06:54 stats

For dynamic frequency scaling, the most important part of interface is:

source: www.kernel.org/doc/Documentation/cpu-freq/user-guide.txt

Frequency Scaling How-To

To get list of supported frequencies for iMX6 ULL SoC:

root@somlabs:/# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies
198000 396000 528000 792000

To ask for the current frequency type:

root@somlabs:/# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq
198000

Now let’s perform simple stress test, to check whether the CPU frequency scaling works corectly:

root@somlabs:/# dd if=/dev/zero of=/dev/null &
root@somlabs:/# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq
792000

To show the current "policy limits" (lower and upper frequency limits):

root@somlabs:/# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
198000

root@somlabs:/# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
792000

By “echoing” new values into scaling_min_freq and scaling_max_freq files, you can change these limits. For example you can decrease the maximum frequency to 528 MHz:

root@somlabs:/# echo 528000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq

Scaling Governors

The strategy of frequency scaling is controlled by the "governor" which continuosly monitors the system perfomance requirements. The “scaling governor” decides, what CPU frequency is used at each time.

Linux offers different CPU scaling governors. Let’s list all available governors with the following command:

root@somlabs:/# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors
interactive conservative userspace powersave ondemand performance

source: www.kernel.org/doc/Documentation/cpu-freq/governors.txt

In case of SoMLabs modules, default governor is ondemand. The default governor (and list of available governors) can be configured through make menuconfig tool:

CPU Power Management  --->
    CPU Frequency scaling  --->
        Default CPUFreq governor (ondemand)  --→
    -*-   'performance' governor
    <*>   'powersave' governor  
    <*>   'userspace' governor for userspace frequency scaling
    -*-   'ondemand' cpufreq policy governor
    <*>   'interactive' cpufreq policy governor
    <*>   'conservative' cpufreq governor 

To switch over to another governor (for example powersave):

root@somlabs:/# echo powersave > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
powersave

In order to ask for the current governor:

root@somlabs:/# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
powersave

To find out more about the governors, please visit: www.kernel.org/doc/Documentation/cpu-freq/governors.txt