Re: latency error (~2ms) with nanosleep

Nish Aravamudan
2005-06-13T16:54:51Z

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