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Geneos - How does the process plugin compute the percent CPU in Linux?

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The percentCPU of the process plugin is computed from a series of variables within the code. The values are taken from the "/proc/<pid>/stat" file (in Linux); it is used to get the number of jiffies used since the process started.

It takes the utime and adds it to the stime. It takes the difference between the last sample and this sample, divides by the number of seconds between samples. An adjustment happens for the number of logical CPUs.

 

Additional Notes
  • If less than three jiffies have been used, then the value will be reported as 0.
  • The fields in the /proc/<pid>/stat file can be seen by doing 'man proc'.
  • The data is shared between multiple instances of the sampler on the same probe - running multiple at very fast sample rates will hit the 'more than three jiffies needed' limit.

 

The values used in the calculation can be seen by enabling debug: For GW2 this is set in the probe debug 'Debug' section where the Module is processes, and setting is * 

The debug output looks like this:

DEBUG: *:pcpu Proc: 12864 [210 - 187 = 23] [1217495784 - 1217495764 = 20] 1.150000 /home/example/example -foo bar

The fields are:

<pid> [<jiffies> - <jiffies last sample> = <jiffies used this sample>] [<time> - <last sample time> = <time since last sample>] <cpu usage> <args>

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