Re: [RFC PATCH 00/11] sched: CFS low-latency features

Thomas Gleixner
2010-08-27T15:21:51Z

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)

Thanks,

tglx