Dear all,
Please allow me to post this newbie question that must have been
answered many times before.
I have my doubts about the actual resolution of CPU TIME USAGE
MEASUREMENT for individual tasks, that is using the
clock_gettime()
with the thread clock id.
From one side, it looks as the counter of cpu-time use in each thread
is done with a jiffy resolution:
- utime and stime fields in task_struct seem to be unsigned long,
therefore in 32bit archs they cannot have a very high resolution.
http://lxr.linux.no/#linux+v3.6.3/kernel/posix-cpu-timers.c
http://lxr.linux.no/linux+*/include/linux/sched.h#L1362
http://lxr.linux.no/linux+*/include/asm-generic/cputime.h#L7
- utime and stime fields in /proc/<tid>/stat come out in jiffies
according to proc man page.
since I guess that these utime and stime fields are the basis of the
cpuclock implementation I infer that the effective resolution of
thread or process usage is limited by the jiffy resolution (4msg with
CONFIG_HZ=250) of the utime and stime fields in task_struct.
On the other side, clock_getres() returns 1 nanosec and the well known
hrtimers merge in 2.6.21 announced highly and broadly that the jiffy
barrier has been lifted with real time clocks and timers.
So my questions are:
* What is the effective resolution of two invocations of
clock_gettime() on the same running thread for a long period
involving several CPU preemptions?
* Are there other fields apart from stime and utime with the
sufficient precision to maintain a CPU usage count?
* Does the PREEMPT_RT branch improve this resolution somehow?
Thanks in advance for your time.
Cheers,
Miguel Telleria
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Miguel TELLERIA DE ESTEBAN http://www.mtelleria.com
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