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SubjectRe: finding out the value of HZ from userspace
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
>>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.

Dr Peter Williams, Chief Scientist
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

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