Re: finding out the value of HZ from userspace

From: Peter Williams
Date: Thu Mar 25 2004 - 18:27:35 EST

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 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

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