Re: latency error (~2ms) with nanosleep

quade
2005-06-14T08:57:57Z

On Mon, Jun 13, 2005 at 09:54:47AM -0700, Nish Aravamudan wrote:
> On 6/13/05, Chris Friesen <cfriesen@nortel.com> wrote:
> > quade wrote:
> > > Playing around with the (simple) measurement of latency-times
> > > I noticed, that the systemcall "nanosleep" has always a minimal
> > > latency from about ~2ms (haven't run it all night, so...). It
> > > seems to be a systematical error.
> >
> > Known issue. The x86 interrupt usually has a period of slightly less
> > than a ms. It will therefore generally add nearly a whole ms to ensure
> > that it does not ever wait for *less* than specified.
>
> Exactly. And the sys_nanosleep() code adds one more if the parameter
> has any positive value at all:
>
> expire = timespec_to_jiffies(&t) + (t.tv_sec || t.tv_nsec);

> current->state = TASK_INTERRUPTIBLE;
> expire = schedule_timeout(expire);
>
> Thanks,
> Nish
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