Re: finding out the value of HZ from userspace

[Peter Williams <peterw@aurema.com>]

Jamie Lokier wrote:
> Peter Williams wrote:
> 
>>>Will this be USER_HZ or kernel HZ?
>>>Someone earlier suggested it would be USER_HZ which would make it
>>>pointless.
>>
>>It has to be whatever enables user space to correctly interpret values 
>>sent to user space as "ticks".  That means USER_HZ and it's not useless 
>>as it enables USER_HZ to be different and/or change without breaking 
>>programs that use values expressed in "ticks".
> 
> 
> It is, however, useless for the _other_ reasons userspace needs to
> know kernel HZ, including as I mentioned userspace timer granularity.

Theoretically, which I know can be a pain, user space timer granularity 
should be in USER_HZ as, theoretically, this is the only one user space 
is supposed to know about.  Because of this, in my view, HZ and USER_HZ 
should be the same or USER_HZ should be greater than HZ.

> 
> (Btw, that usage would be better as a period rather than a frequency,
> so that a "tickless" kernel can report zero).

_SC_CLK_TCK is a POSIX.1 definition and can't be changed.  But I don't 
think that there's any impediment to adding new parameters that can be 
reported by sysconf().

> 
> The fundamental problem is that there are two values,  and both values
> have programs which can usefully use them.
> 
> How hard can it be to export both?
> 

Making HZ == USER_HZ would also solve the problem.

Peter
-- 
Dr Peter Williams, Chief Scientist                peterw@aurema.com
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