linux-kernel - Re: HZ from userspace

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Date:	Wed, 6 Aug 2008 15:20:10 -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 3:16 PM, Ray Lee <ray-lk@...rabbit.org> wrote:
> 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 I believe the claim that it's out of date is out of date.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.
>

Here's the latest proc(5) man page from June 25, 2008. Look for
_SC_CLK_TCK about a third of the way down.

http://www.kernel.org/doc/man-pages/online/pages/man5/proc.5.html