Hello everyone! (I recently posted about this on linux-kernel, but got no responses. I would be very grateful for any pointers.) I am trying to determine what fraction of the whole system's user and sys time are consumed by a specific process, over a period of time. Hence, I read the respective tick counts from the first lines of /proc/stat and /proc/<pid>/stat at the start and end of that period and compute deltas. However, on an otherwise idle system, I often see the ticks consumed by a threaded process exceed those of the whole system. The difference is not just a few ticks, but can amount to over 10%, regardless of how long the observed time period is. There is no virtualization involved. I have been able to reproduce this with the current openSUSE and Ubuntu releases on different x86_64 machines. How can this be explained? Intuitively, I would expect that a tick accounted to a running task is at the same time accounted to the CPU it is running on, and so a process can never be accounted more ticks than the whole system, i.e. all CPUs. I have attached a simple Python script to reproduce this behavior. Example output:
./ticks.py delta process: user: 2495 sys: 181 delta all: user: 2283 sys: 258
Thanks, Peter