Measuring daemon CPU utilization over a portion of its wall clock run time

Question

I am dealing with a network-related daemon: it takes data in, processes it, and spits it out. I would like to increase the performance of this daemon by profiling it and reducing it's CPU utilization. I can do this easily on Linux with gprof. However, I would also like to use something like "time" to measure it's total CPU utilization over a period of time. If possible, I would like to time it over a period that is less than its total run time: thus, I would like to start the daemon, wait awhile, generate CPU statistics, stop generating them, then stop the daemon at some later time.

The "time" command would work well for me, but it seems to require that I start and stop the daemon as a child of time. Is there a way to measure CPU utilization for only a portion of the daemon's wall clock time?

Answer 1

The /proc/<pid>/stat file contains the necessary information - you're after the utime and stime fields. Those are cumulative counters of the process's user-mode and kernel-mode CPU time used; read them at the start of the measuring interval, then read them again at the end and calculate the difference.

That will give you used CPU time in jiffies. To determine the total elapsed wallclock time in jiffies (so you can convert to an average utilisation), sum the numbers on the cpu0 line in /proc/stat (before and after, just like /proc/<pid>/stat).

This is the layout of the first few fields in /proc/<pid>/stat, from Documentation/filesystems/proc.txt in the Linux source:

Table 1-3: Contents of the stat files (as of 2.6.22-rc3)
..............................................................................
 Field          Content
  pid           process id
  tcomm         filename of the executable
  state         state (R is running, S is sleeping, D is sleeping in an
                uninterruptible wait, Z is zombie, T is traced or stopped)
  ppid          process id of the parent process
  pgrp          pgrp of the process
  sid           session id
  tty_nr        tty the process uses
  tty_pgrp      pgrp of the tty
  flags         task flags
  min_flt       number of minor faults
  cmin_flt      number of minor faults with child's
  maj_flt       number of major faults
  cmaj_flt      number of major faults with child's
  utime         user mode jiffies
  stime         kernel mode jiffies
  cutime        user mode jiffies with child's
  cstime        kernel mode jiffies with child's