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What role does the Activity Monitor (AMU) play in power and performance control?

Hi all,

It is known that Arm introduced Activity Monitor Extension from v8.4-A. AMU supports system reg and utility bus access.

My wonder is why AMU were introduced when PMU already exists. From a hardware perspective, they just do the same (to count events).

An arm white paper Power and Performance Management using Arm SCMI Specification says AMU is used to provide a performance feedback to the OS kernel.

Why does the OS not just take the PMU results to get the performance information?

Thnaks in advance.

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  • One reason is that you could have several things wanting to use the PMU for different things.  For example, wanting to do performance analysis of a user space application using a tool like perf.  The events the profiling tool might want to count could be different to what you'd want for performance feedback to the kernel.  Having the AMU gives you a fixed resource that you can dedicate to system performance monitoring, freeing up the PMU for the other use cases.

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  • One reason is that you could have several things wanting to use the PMU for different things.  For example, wanting to do performance analysis of a user space application using a tool like perf.  The events the profiling tool might want to count could be different to what you'd want for performance feedback to the kernel.  Having the AMU gives you a fixed resource that you can dedicate to system performance monitoring, freeing up the PMU for the other use cases.

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