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| − | <big> RZ/G2 Kernel and Linux FAQ </big>
| + | this page can be deleted |
| − | __TOC__
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| − | = CPU Hotplug =
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| − | You can enable and disable CPU cores by writing to a sysfs value.
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| − | <br>
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| − | 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.
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| − | For example, this command will disable the 2nd core.
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| − | <code>$ echo 0 > /sys/devices/system/cpu/cpu1/online</code>
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| − | More detailed information can be found here: https://www.cyberciti.biz/faq/debian-rhel-centos-redhat-suse-hotplug-cpu
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| − | = Power Saving =
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| − | * In Linux, this is a mechanism that is generally supported by all kernels.(it may depend on the version)
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| − | * The Renesas kernel has support them.
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| − | About power consumption in RZ/G2 series, we have some supported features to save power cost in default environment:
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| − | * CPUHotplug: Turn on/off CPU in runtime.
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| − | * CPUIdle: Support 2 modes to turn off clock or power domain of CPU when CPU is idle (nothing to do).
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| − | ** Sleep mode: put in sleep state.
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| − | ** 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.
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| − | * CPUFreq: there are 6 governors to support "Dynamic Frequency Scaling":
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| − | ** '''Performance''': The frequency is always set maximum => It is using as default in our current environment.
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| − | ** '''Powersave''': The frequency is always set minimum.
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| − | ** '''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.
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| − | ** 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.
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| − | ** '''Userspace''': It sets frequency which is defined by user in runtime.
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| − | ** '''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.
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| − | * 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.
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| − | Therefore, select proper method will be based on user's purpose. Here are my examples:
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| − | * Want to use with best performance: disable CPUIdle + use performance frequency governor.
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| − | * Want to use less power: enable CPUIdle + use powersave frequency governor.
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| − | * Want to balance performance and power: we can use schedutil.
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| − | * Want to modify frequency as user's purpose: use userspance frequency governor.
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| − | * If user is running realtime environment, I suggest using performance governor to ensure the minimum latency.
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| − | Here are some commands to check frequency value and frequency governor in linux:
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| − | * Check available CPU frequency:
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| − | : <code> cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_available_frequencies </code>
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| − | * Check available CPU frequency governor:
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| − | : <code>cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_available_governors </code>
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| − | * Change to other governor:
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| − | : <code>echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor (performance/userspace/schedutil/...) </code>
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| − | * Check current frequency:
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| − | : <code> cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq</code>
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