I need to calculate the overall CPU usage of my Linux device over some time (1-5 seconds) and a list of processes with their respective CPU usage times. The programm should be designed and implemented in C++. My assumption would be that the sum of all process CPU times would be equal to the total value for the whole CPU. For now the CPU I am using is multi-cored (2 cores).
According to How to determine CPU and memory consumption from inside a process? it is possible to calculate all "jiffies" available in the system since startup using the values for "cpu" in /proc/stat. If you now sample the values at two points in time and compare the values for user, nice, system and idle at the two time points, you can calculate the average CPU usage in this interval. The formula would be
totalCPUUsage = ((user_aft - user_bef) + (nice_aft - nice_bef) + (system_aft - system_bef)) /
((user_aft - user_bef) + (nice_aft - nice_bef) + (system_aft - system_bef) + (idle_aft - idle_bef)) * 100 %
According to How to calculate the CPU usage of a process by PID in Linux from C? the used jiffies for a single process can be calculated by adding utime and stime from /proc/${PID}/stat (column 14 and 15 in this file). When I now calculate this sum and divide it by the total amount of jiffies in the analyzed interval, I would assume the formula for one process to be
processCPUUsage = ((process_utime_aft - process_utime_bef) + (process_stime_aft - process_stime_bef)) /
((user_aft - user_bef) + (nice_aft - nice_bef) + (system_aft - system_bef) + (idle_aft - idle_bef)) * 100 %
When I now sum up the values for all processes and compare it to the overall calculated CPU usage, I receive a slightly higher value for the aggregated value most of the time (although the values are quite close for all different CPU loads).
Can anyone explain to me, what's the reason for that? Are there any CPU resources that are used by more than one process and thus accounted twice or more in my accumlation? Or am I simply missing something here? I can not find any further hint in the Linux man page for the proc file system (https://linux.die.net/man/5/proc) as well.
Thanks in advance!
