Re: finding out the value of HZ from userspace

[Peter Williams]

Jamie Lokier wrote:
> Peter Williams wrote:
>
> > > 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.
> >
> > The resolution will always be limited.  That's the nature of digital 
> > systems.  Unlimited resolution would require real "real" numbers and 
> > that's not possible.  The nearest you get on a digital system is the 
> > floating point APPROXIMATION to real numbers.
>
>
> Sure, but HZ will still be irrelevant.  There won't be a HZ to report.
>
>
> > IMHO, as I've said several times, USER_HZ should be changed to be equal 
> > to or greater than HZ.  In fact, if having USER_HZ greater than HZ would 
> > still make it unusable for your purposes, I'd change that opinion to say 
> > USER_HZ should be equal to HZ (or, in other words, cease to exist).
>
>
> It's not possible to change USER_HZ.  There are too many programs with
> the number hard-coded into the binary.

This is an argument that the tail should be allowed to wag the dog and 
is not really valid :-)

>  The best we could do is make
> the HZ userspace macro non-constant, so it calls sysconf(_SC_CLK_TCK),
> and wait a few years until practically all programs being used no
> longer contain a hard-coded constant.  Then we could get rid of USER_HZ again.

If USER_HZ is dispensed with the programs will get fixed pretty quick 
but as long as this concession to buggy programs is made they won't get 
fixed (because they don't have to be).

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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