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Supratik
Asked: 2012-06-13 05:55:50 +0800 CST

How does top calculate CPU usage on a Linux system with a single core?

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How is CPU usage calculated on a Linux system with a single core?

Cpu(s): 28.1%us,  6.5%sy,  0.0%ni, 43.0%id, 21.6%wa,  0.0%hi,  0.5%si,  0.2%st

Please explain me how the us, sy, ni, id, wa, hi, si and st values are obtained.

central-processing-unit linux

3 Answers

  1. CPU usage is calculated and tracked by the kernel. The exact methods and processes involved in this are far beyond the scope of this site (ServerFault).

    If you're wondering what the values represent, I pulled this nice list from IBM's site, though the full article is a good read:

    • us - Percentage of CPU time spent in user space.
    • sy - Percentage of CPU time spent in kernel space.
    • ni - Percentage of CPU time spent on low priority processes.
    • id - Percentage of CPU time spent idle.
    • wa - Percentage of CPU time spent in wait (on disk).
    • hi - Percentage of CPU time spent handling hardware interrupts.
    • si - Percentage of CPU time spent handling software interrupts.

    (from IBM... why it's only embedded in a Java SDK guide is beyond me)

    top gets its information from the /proc directory, which is a special directory used to query the kernel for, among other things, process statistics. man proc can give you more information on how /proc is set up.

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  2. /proc/[pid]/stat results are produced by this code in the Linux kernel.

    http://www.cs.fsu.edu/~baker/devices/lxr/http/source/linux/fs/proc/stat.c

    You can click around and get more info from in there.

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  3. This article describes this clearly.

    http://www.linuxjournal.com/article/9001

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