Re: nanosleep with small value

From: Nish Aravamudan
Date: Thu Nov 17 2005 - 14:17:27 EST

On 11/17/05, Dag Nygren <dag@xxxxxxxxxx> wrote:
> > On 11/17/05, Dag Nygren <dag@xxxxxxxxxx> wrote:
> > > 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.
> >
> > Update your manpages. You're depending on 2.4 behavior in a 2.6 kernel.
> You are right. The system is one I have upgraded piece by piece and the
> manpages
> weren't upgraded.

No problem.

> But what is the point of having a nanosleep() in that case when you could do
> just fine with usleep() ?

Check the usleep() manpage:

This function is obsolete. Use nanosleep(2) or setitimer(2) instead.

And in any case, I think usleep() just ends up calling nanosleep()?
It's not a sys-call in an of itself, like sys_nanosleep().

> > > OK, in that case the manpage should be changed. And an alternative
> > > has to be worked out by me ;-).
> >
> > My man-pages are quite clear on what nanosleep() does. Nothing needs
> > to be changed there.
> >
> > Alternative wise, I'm not sure, but you might want to look into the
> > HRT stuff that's going on in Ingo's -RT tree. I don't know if / what
> > changes have been made to sys_nanosleep(), but low-latency is most
> > likely to occur there.
> I will look into that.
> Quite annoying that software that worked just fine in 2.4 doesn't
> work in 2.6.

Well, your general resolution also was improved quite a bit in 2.6
(HZ=1000 vs. HZ=100 is a 10-fold improvement). But I agree, is a big
difference if you depend on that udelay() functionality -- but it was
a delay of up to 2 milliseconds, which is generally frowned upon in
the kernel.

> What does POSIX say about nanosleep()?

Not sure, but I think the only requirement is that we don't return
early (i.e. request 2 milliseconds, return in 1 millisecond).

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