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

From: Nish Aravamudan
Date: Fri Apr 29 2005 - 16:37:04 EST

On 4/8/05, jdavis@xxxxxxxxxxxxxx <jdavis@xxxxxxxxxxxxxx> wrote:
> 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.

There are several problems with the current soft-timer subsystem in
this area. Basically, the actual tick rate is slightly more than once
per millisecond, which requires adding one jiffy to compensate.
Additionally, one can't be sure ``where'' one is within a tick, so
another jiffy has to be added, to make sure the timer does not go off
early. sys_nanosleep() does something similar.

I will be submitting an RFC based on John Stultz's timeofday rework
which tries to resolve these issues.

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