When do jiffies increment? How a process runs in a jiffy?

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http://unix.stackexchange.com – I know that jiffies length is selected at kernel compile time and it is defaulted to 250 (4ms). Source: man 7 time - The Software Clock, HZ, and Jiffies I wonder what happens inside a jiffy. What are the conditions to increase the value of utime or stime in /proc/pid/stat? When does it happens? I have some thoughts but I'm not sure they are true: When a process gets time to run, the jiffy count is incremented instantly. Linux can't tell how many operations have been executed in 1 jiffy. Furthermore Linux can't tell how much time has been used by the current process in 1 jiffy. (Has it used (HowTos)