RZ/G2 Kernel and Linux FAQ
You can enable and disable CPU cores by writing to a sysfs value.
This is helpful for when you want to experiment with the performance of your application if you were to use a processor with less CPU cores.
For example, this command will disable the 2nd core.
$ echo 0 > /sys/devices/system/cpu/cpu1/online
More detailed information can be found here: https://www.cyberciti.biz/faq/debian-rhel-centos-redhat-suse-hotplug-cpu
- In Linux, this is a mechanism that is generally supported by all kernels.(it may depend on the version)
- The Renesas kernel has support them.
About power consumption in RZ/G2 series, we have some supported features to save power cost in default environment:
- CPUHotplug: Turn on/off CPU in runtime.
- CPUIdle: Support 2 modes to turn off clock or power domain of CPU when CPU is idle (nothing to do).
- Sleep mode: put in sleep state.
- Core standby mode: put in shutdown state. It is described in devicetree of each SoC => It has deeper state than sleep mode so that save more power.
- CPUFreq: there are 6 governors to support "Dynamic Frequency Scaling":
- Performance: The frequency is always set maximum => It is using as default in our current environment.
- Powersave: The frequency is always set minimum.
- Ondemand: If CPU load is bigger than 95%, the frequency is set max. If CPU load is equal to or less than 95%, the frequency is set based on CPU load.
- Conservative: If CPU load is bigger than 80%, the frequency is set one level higher than current frequency. If CPU load is equal to or less than 20%, the frequency is set one level lower than current frequency.
- Userspace: It sets frequency which is defined by user in runtime.
- Schedutil: Schedutil governor is driven by scheduler. It uses scheduler-provided CPU utilization information as input for making its decisions by formula: freq_next= 1.25 * freq_max* util_of_CPU.
- Power Domain: it is supported as default by Linux Power Management Framework. If a module is not use, system will disable its clock and power domain automatically.
Therefore, select proper method will be based on user's purpose. Here are my examples:
- Want to use with best performance: disable CPUIdle + use performance frequency governor.
- Want to use less power: enable CPUIdle + use powersave frequency governor.
- Want to balance performance and power: we can use schedutil.
- Want to modify frequency as user's purpose: use userspance frequency governor.
- If user is running realtime environment, I suggest using performance governor to ensure the minimum latency.
Here are some commands to check frequency value and frequency governor in linux:
- Check available CPU frequency:
- Check available CPU frequency governor:
- Change to other governor:
echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor (performance/userspace/schedutil/...)
- Check current frequency: