Hi,
On x86_64 machine (with xenomai 3.0) sometimes I observed a huge
variance and inconsistency in latency data when measuring latency
delay using clock_nanosleep.
I observed this inconsistency in both normal kernel, as well as xenomai kernel.
I observed the same, even when using the direct xenomai API such as:
rt_time_read().
So, I wanted to know if anybody observed the similar behavior.
Or I am not sure if I am doing some thing wrong in measurement itself.
I will explain my scenario below.
1) I created a single thread using the pthread API.
2) Inside the thread function, I just print one message and sleep for
100 micro-seconds using the clock_nanosleep() API.
3) Then I calculate the time difference before and after sleep.
4) I repeat this for 10 times, and then calculate the average.
5) I build this application for normal kernel and for xenomai using posix skin.
6) Then I compare the latency between both.
What I observed is as follows:
1) Even if I give sleep for 100 micro-seconds, the timing difference
comes in: 150, 160, and even in 600, 1000. Thus the average difference
shows very high.
2) Sometimes, the xenomai timing difference is much larger than normal kernel.
LIKE: Normal kernel => Average time taken = 232.288 us
Xenomai kernel => Average time taken = 427.478 us
3) If I give sleep in less than 5 mirco-seconds, then xenomai shows
better. But still the time taken is not accurate.
Normal kernel => Average time taken = 42.989 us
Xenomai kernel => Average time taken = 26.264 us
4) If I give sleep in 100 milli-seconds, then only the results are
coming some what proper.
Normal kernel => Average time taken = 100.465 us
Xenomai kernel => Average time taken = 100.409 us
What could be the cause of this inconsistency ?
Did anybody observed the similar issue on some system?
Here, is my code snippet.
------------------------------------------------------------------------------
myfunc(...)
{
printf("Something !\n");
tm.tv_sec = 0;
tm.tv_nsec = 100*1000; /* 100 micro-seconds */
clock_nanosleep(CLOCK_MONOTONIC, 0, &tm, NULL);
}
ThreadFunction(..)
{
clock_gettime(CLOCK_MONOTONIC, &prev);
myfunc();
clock_gettime(CLOCK_MONOTONIC, &now);
diff = ((now.tv_sec - prev.tv_sec) * 1000000000ULL +
(now.tv_nsec - prev.tv_nsec)) / 1000;
printf("time taken: %5.3f us\n", diff);
}
------------------------------------------------------------------------------
I even tried with CLOCK_REALTIME and using the TIMER_ABS. Behavior is same.
Do you see any problem here ?
Note: Earlier, I could see the proper latency but suddenly some
variance are seen now. I did not modify anything in the kernel after
that. I just enabled RTDM, RTNET and related drivers.
Even after removing rtnet driver also its the same.
Regards,
Pintu
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