linux-kernel - Re: HZ from userspace

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Message-ID: <2c0942db0808061516r730496d6hbc75eb9c5b49923a@mail.gmail.com>
Date:	Wed, 6 Aug 2008 15:16:37 -0700
From:	"Ray Lee" <ray-lk@...rabbit.org>
To:	"Piotr Jaroszyński" <p.jaroszynski@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: HZ from userspace

On Wed, Aug 6, 2008 at 3:07 PM, Piotr Jaroszyński
<p.jaroszynski@...il.com> wrote:
>>> I want to be able to convert starttime from /proc/#/stat to absolute
>>> time and it seems that I need HZ to do that as it is measured in
>>> jiffies. How can I get HZ? I have seen some discussion about this but
>>> haven't found a definite answer. procps is using some hacks do it and
>>> I would prefer to avoid them.
>>> I only need a solution for modern kernels, say 2.6.20+ if that matters.
>>
>> `man sysconf`, in particular you want sysconf(_SC_CLK_TCK).
>
> manpage says it's obsolete and i read in some discussion that it only
> returns the default and not the real HZ used.

I believe the claim that it's out of date is out of date. manpages
3.0.1 has an update to this, according to a message I saw on this list
a month or so ago.

Regardless, the fields you're reading should all be showing USER_HZ
which is fixed at 100 per second regardless of the underlying kernel
HZ parameter.

Try it.