Odd Timer behavior in 2.6 vs 2.4 (1 extra tick)

[jdavis@accessline.com]

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
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