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I am curious where gnome-system-monitor and top (the command line tool) get their information about the CPU usage in %. Could you please explain it to me?

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up vote 4 down vote accepted

From /proc.

At /proc/[PID]/stat you have kernel statistics about a given process.

You can find more information on about what to look for on the stat file on this previous SO question.

I'm not sure if it's a reasonable assumption but probably the current CPU usage is (100% - [the cpu usage for the idle process]).

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