Messages in this thread | | | Date | Fri, 27 Aug 2010 11:30:06 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 00/11] sched: CFS low-latency features |
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* Thomas Gleixner (tglx@linutronix.de) wrote: > On Fri, 27 Aug 2010, Mathieu Desnoyers wrote: > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > > Why couldn't the timer_create() call record the start time, and then > > > compute the sleeps from that time? So if timer_create() executed at > > > time t=100 and the period is 5, upon awakening and completing the first > > > invocation of the function in question, the thread does a sleep calculated > > > to wake at t=110. > > > > Let's focus on the userspace thread execution, right between the samping of the > > current time and the call to sleep: > > > > Thread A > > current_time = read current time(); > > sleep(period_end - current_time); > > > > If the thread is preempted between these two operations, then we end up sleeping > > for longer than what is needed. This kind of imprecision will add up over time, > > so that after e.g. one day, instead of having the expected number of timer > > executions, we'll have less than that. This kind of accumulated drift is an > > unwanted side-effect of using delays in lieue of real periodic timers. > > Nonsense, that's why we provide clock_nanosleep(ABSTIME)
If we're using CLOCK_MONOTONIC, you're right, this could work. I was only thinking of relative delays.
So do you think Paul's ideal would be a good candidate for the timer_create SIGEV_THREAD glibc implementation then ?
Thanks,
Mathieu
> > Thanks, > > tglx
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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