Re: What is the precision of usleep ?

[george anzinger <george@mvista.com>]

Marcus Ramos wrote:
> 
> Hello,
> 
> I am using usleep in an application under RH7 kernel 2.4.2. However,
> when I bring its argument down to 20 miliseconds (20.000 microseconds)
> or less, this seems to be ignored by the function (or the machine's hw
> timer), which behaves as if 20 ms where its lowest acceptable value. How
> can I measure the precision of usleep in my box ? I am currently using
> an Dell GX110 PIII 866 MHz.
> 
> Thanks in advance.
> Marcus.

Well, first, your issue is resolution, not precision.  Current
resolution on most all timers is 1/HZ.  So this should get a min.
nanosleep of 10 ms.

So, could someone explain this line from sys_nanosleep() (
kernel/timer.c):

	expire = timespec_to_jiffies(&t) + (t.tv_sec || t.tv_nsec);

It seems to me this should just be:

	expire = timespec_to_jiffies(&t)

timespec_to_jiffies(&t) seems to do all the needed resolution round up
and such.  Since || is logical, this seems to always add 1, except if
the requested value is 0.  The net result is you always get 1 extra
jiffie (unless you ask for zero, in which case you get a timer that will
expire next tick (thru the wonders of add_timer).

George