Preparation

To calculate CPU usage for a specific process you’ll need the following:

1. /proc/uptime
   • #1 uptime of the system (seconds)
2. /proc/[PID]/stat
   • #14 utime – CPU time spent in user code, measured in clock ticks
   • #15 stime – CPU time spent in kernel code, measured in clock ticks
   • #16 cutime – Waited-for children’s CPU time spent in user code (in clock ticks)
   • #17 cstime – Waited-for children’s CPU time spent in kernel code (in clock ticks)
   • #22 starttime – Time when the process started, measured in clock ticks
3. Hertz (number of clock ticks per second) of your system.
   • In most cases, getconf CLK_TCK can be used to return the number of clock ticks.
   • The sysconf(_SC_CLK_TCK) C function call may also be used to return the hertz value.

Calculation

First we determine the total time spent for the process:

total_time = utime + stime

We also have to decide whether we want to include the time from children processes. If we do, then we add those values to total_time:

total_time = total_time + cutime + cstime

Next we get the total elapsed time in seconds since the process started:

seconds = uptime - (starttime / Hertz)

Finally we calculate the CPU usage percentage:

cpu_usage = 100 * ((total_time / Hertz) / seconds)

See also

Top and ps not showing the same cpu result

How to get total cpu usage in Linux (c++)

Calculating CPU usage of a process in Linux