Re: latency error (~2ms) with nanosleep

["Richard B. Johnson" <linux-os@analogic.com>]

nanosleep, according to the documation is supposed to sleep
"at least" the 'struct timespec' time. It can return in
a shorter time as a result of a signal and, if so, the
input time-values will be updated accordingly. The resolution
is limited to the HZ value. This means that it will, unless
interrupted, always sleep at least 1 / HZ seconds (about 1 ms
on current x86 distributions).

FYI, there is no 'fine resolution' timer available on any
Linux-ported platform that could take advantage of the nanosecond
input resolution of the function.

On Mon, 13 Jun 2005, 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.
>
> A short investigation shows, that "sys_nanosleep()" uses
> schedule_timeout(), but schedule_timeout() is working exactly
> as expected. Therefore I think it has something to do with
> the scheduling?
>
> Has someone an explanation for the ~2ms error?
> If it is indeed a systematical error, does it make sense to
> "adjust" (correct) this error in the systemcall "sys_nanosleep()"?
>
> Find attached my small test program.
>
> Juergen.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.11.9 on an i686 machine (5537.79 BogoMips).
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