Hi,
I have some code (simplified example below) which uses
'pthread_wait_np(&Overrun)' within a while loop, using a period of
1,000,000ns (1ms) which has been set using 'pthread_make_periodic_np()'.
This wakes the thread up less than 2,000ns late in general, sometimes
20,000ns late, and at most less than 45,000ns late. To ensure that I perform
the tasks within the thread from a consistent 'millisecond tick' I set an
expected time value 45,000ns later than the earliest wake time each period,
and then use a busy loop until the current time equals this expected time.
This busy loop takes < 600ns per cycle, and so the latency from my expected
time should be a maximum of 600ns. The latencies I am actually seeing are as
high as 13,000ns, and these occur during this busy loop (I have timed it),
implying that this loop is being preempted.
while(1)
{
pthread_wait_np(&Overrun);
WakeTime = GetTime();
while(WakeTime < ExpectedTime)
{ WakeTime = GetTime(); } // Preempted here
DoWork();
ExpectedTime += Period;
}
This is a Xenomai POSIX thread with a priority of 99, and is the only thread
within my program with a priority of 99 or higher. There are no context
switches occuring within this thread (confirmed using pthread_set_mode_np(0,
PTHREAD_WARNSW)).
I have had a brief experimentation with kernel settings, for example
applying the SMI workaround, but this produces no difference to the
preemption that I measure. I have also re-read all the documentation I could
find about Xenomai and preemption and I cannot see any way of fixing this
without trial and error with the kernel settings.
Do you have any other ideas for avenues that I could explore, or could you
recommend documentation about parts of the kernel that are more likely to
have an effect on preemption to speed up this process?
I am running Ubuntu on:
Kernel: 2.6.32.8
Xenomai: 2.5.3
Steve
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