Re: nanosleep with small value

Dag Nygren
2005-11-17T18:47:30Z

> On 11/17/05, Dag Nygren <dag@newtech.fi> wrote:

>
> Which kernel, what value of HZ?

Sorry, the kernel is 2.6.13 and HZ is 250.

> In either case, it's absurd to assume
> that the kernel is going to provide you 1 microsecond resolution in
> 2.6 mainline, as the best HZ value is 1000 (1 millisecond). And we
> don't busy-wait ever in nanosleep().

Not?
The man page for nanosleep saya that times under 2 us are implemented
by a busywait and this is why I expected it to work.

> So the fastest your loop can run
> is 1000 * 1 ms = 1 second. That's assuming the only time-consuming
> thing is sleeping (minimal overhead). But, in sys_nanosleep(), we
> convert nanoseconds to jiffies and add 1 if you requested any sleep
> time. So,
>
> HZ = 100
> 1000 * (10 + 1 ms) = 11 s
> HZ = 250
> 1000 * (4 + 1 ms) = 5 s
> HZ = 1000
> 1000 * (1 + 1 ms) = 2 s (which is what Dick Johnson reported).
>
> Note, that with HZ=250, there might be some extra rounding occurring
> timespec_to_jiffies() that I've forgotten.
>
> So 8 s may not be terribly unreasonable. I don't know, though, what's
> add the 3 seconds if you're using 250.

OK, in that case the manpage should be changed. And an alternative
has to be worked out by me ;-).

Thankyou
Dag