where does top gets real-time data

Question

Where does top application gets it's data on Linux? I would be interested in real-time CPU load/pid data.(I read allmost all documentation in /proc/pid man page, but the info isn't there).

The pid is a jboss. I need the data lightweight (to be exported easily).

Answer 1

If in doubt, use strace(1)!

open("/proc/2/stat", O_RDONLY)    = 4
open("/proc/2/statm", O_RDONLY)   = 4
open("/proc/3/stat", O_RDONLY)    = 4
open("/proc/3/statm", O_RDONLY)   = 4

Answer 2

As documented in proc(5), in the file /proc/(pid)/stat you have the fields:

utime %lu

Amount of time that this process has been scheduled in user mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK). This includes guest time, guest_time (time spent running a virtual CPU, see below), so that applications that are not aware of the guest time field do not lose that time from their calculations.

stime %lu

Amount of time that this process has been scheduled in kernel mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK).

To get CPU usage for a specific process, use those fields. The toplevel process will aggregate CPU usage over all threads; for a per-thread breakdown, you can find the other threads in /proc/(pid)/task.

If you would prefer to be notified when CPU time exceeds some threshold, you can use clock_getcpuclockid to get a handle to its cpu time clock, then timer_create or timerfd to be notified when it hits a specified level. However, note that cross-process cputime timers are an optional feature in the POSIX specification and may not be supported (I've not tested).