Re: finding out the value of HZ from userspace

[Peter Williams]

Richard B. Johnson wrote:
> On Thu, 1 Apr 2004, Jamie Lokier wrote:
>
>
> > Arjan van de Ven wrote:
> >
> > > HZ doesn't mean nothing, esp when we go to a tickless kernel...
> >
> > As explained several times in this thread, HZ is meaningful because it
> > affects the rounding in select/poll/epoll/setitimer.  A few userspace
> > programs with low jitter soft-RT timing requirements need to
> > compensate for that rounding and/or deliberately synchronise
> > themselves with the tick.
> >
> > Such programs can determine HZ experimentally and lock onto the tick
> > in the manner of a PLL, but it would be nice to simply be able to
> > have the value, to reduce the number of control variables.
> >
> > When we go to a tickless kernel and offer high-resolution timers to
> > userspace, then it will be irrelevant.  Until then, or if the kernel
> > goes tickless but limits the resolution of timers for efficiency, the
> > value of HZ is still relevant.
> >
> > Not to get irritatingly back to the subject of this thread or
> > anything, but...  is the value of HZ reported to userspace anywhere?
> >
> > Thanks :)
> > -- Jamie
>
>
> I may be naive, but what's the matter with:
>
> #include <stdio.h>
> #include <sys/param.h>   // Required to be here!
> int main()
> {
>     printf("HZ=%d\n", HZ);
>     return 0;
> }
> It works for me.

There's no guarantee that the kernel that's running was compiled using 
that header file which (on my system i.e. RedHat 9) comes as part of the 
glibc package.  It also gets the value indirectly via linux/param.h 
which in turn gets it via asm/param.h which makes any such program 
highly non portable.  Not to mention that the HZ obtained this way is 
100 which is actually not the same as HZ in the 2.6.5-rc3 kernel that 
I'm running.

Peter
--
Dr Peter Williams, Chief Scientist                peterw@xxxxxxxxxx
Aurema Pty Limited                                Tel:+61 2 9698 2322
PO Box 305, Strawberry Hills NSW 2012, Australia  Fax:+61 2 9699 9174
79 Myrtle Street, Chippendale NSW 2008, Australia http://www.aurema.com

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