Difference between revisions of "RZ-G/RZG2 kernel"

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<big> RZ/G2 Kernel and Linux FAQ </big>
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__TOC__
 
 
 
 
 
= CPU Hotplug =
 
You can enable and disable CPU cores by writing to a sysfs value.
 
<br>
 
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.
 
 
 
<code>$ echo 0 > /sys/devices/system/cpu/cpu1/online</code>
 
 
 
More detailed information can be found here: https://www.cyberciti.biz/faq/debian-rhel-centos-redhat-suse-hotplug-cpu
 
 
 
 
 
= Power Saving =
 
* 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:
 
: <code> cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_available_frequencies </code>
 
* Check available CPU frequency governor:
 
: <code>cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_available_governors </code>
 
* Change to other governor:
 
: <code>echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor (performance/userspace/schedutil/...) </code>
 
* Check current frequency:
 
: <code> cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq</code>
 

Latest revision as of 12:01, 23 July 2021

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