Subject: Re: finding out the value of HZ from userspace
From: Albert Cahalan <albert@users.sf.net>
To: Jamie Lokier <jamie@shareable.org>
Cc: Albert Cahalan <albert@users.sourceforge.net>,
       "Randy.Dunlap" <rddunlap@osdl.org>, Peter Williams <peterw@aurema.com>,
       arjanv@redhat.com, ak@muc.de, Richard.Curnow@superh.com, aeb@cwi.nl,
       linux-kernel mailing list <linux-kernel@vger.kernel.org>
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On Thu, 2004-04-01 at 10:54, Jamie Lokier wrote:
> Albert Cahalan wrote:
> > If you rely on sysconf(_SC_CLK_TCK) to work, then
> > your software will support:
> > 
> > * all systems with a 2.6.xx kernel
> > * all systems with a 2.4.xx kernel and recent glibc
> > * all i386 systems running with the default HZ
> > 
> > That's quite a bit I suppose. Maybe you have no
> > interest in supporting a 1200 HZ Alpha with an old
> > kernel or glibc. Maybe you don't care about somebody
> > running a 2.2.xx kernel with modified HZ.
> 
> I'm still unclear.  Does sysconf(_SC_CLK_TCK), when it is reliable,
> return HZ or USER_HZ?

I consider "reliable" to mean it returns whatever is
used by /proc and other kernel interfaces. Prior to the
2.6.xx (and late 2.5.xx) kernels USER_HZ did not exist.

On a 2.6.xx kernel, you get back USER_HZ.

On a 2.4.xx kernel with recent glibc, you get
back HZ, which works OK since there isn't any
HZ to USER_HZ conversion.

On any i386 system with the default HZ, you
will get back 100. On older systems, glibc is
just giving you a constant value -- so it is
correct if your system is an i386 without any
non-Linus modifications. An old glibc can only
do sysconf(_SC_CLK_TCK) this way.


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