Messages in this thread | | | From | jdavis@accessli ... | Subject | Odd Timer behavior in 2.6 vs 2.4 (1 extra tick) | Date | Fri, 8 Apr 2005 10:39:23 -0700 |
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Hello,
I've created a pretty straight forward timer using setitimer, and noticed some odd differences between 2.4 and 2.6, I wonder if I could get a clarification if this is the way it should work, or if I should continue to try to "fix" it.
I create a RealTime Thread( SCHED_FIFO, maxPriority-1 ) (also tried SCHED_RR) ...
that creates a timer ...setitimer( ITIMER_REAL, SIGALRM) with a 10 milli period. (I've also tried timer_create with CLOCK_REALTIME and SIGRTMIN)
and then the thread does a sem_wait() on a semaphore.
the signal handler does a sem_post() .
on 2.4.X the (idle) worst case latency is ~40 microseconds, on 2.6.X the (idle) latency is about the same plus 1 tick of the scheduler ~1000 micro seconds... Always.. Every time. So to work around this on 2.6 I simply subtract 1 millisecond from my timer as a fudge factor and everything works as expected.
I've tried compiling various kernels (2.6.9, 2.6.11) with kernel pre-empting on, etc..
Is this the correct behavior? If so I'm curious who is using up the extra Tick? Does the asynch signal handler use up the whole tick even though it only has to sem_post()?
I am not subscribed to the list, so I would appreciate a CC. Thanks, -JD - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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