(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.
delta process: user: 2495 sys: 181
delta all: user: 2283 sys: 258