Subject: Re: finding out the value of HZ from userspace
From: Bernd Petrovitsch <bernd@firmix.at>
To: Peter Williams <peterw@aurema.com>
Cc: Andi Kleen <ak@muc.de>, linux-kernel@vger.kernel.org
In-Reply-To: <40569655.2030802@aurema.com>
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	 <40569655.2030802@aurema.com>
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Organization: Firmix Software GmbH
Message-Id: <1079428604.32739.26.camel@tara.firmix.at>
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Date: Tue, 16 Mar 2004 10:16:45 +0100
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On Die, 2004-03-16 at 06:53, Peter Williams wrote:
> Andi Kleen wrote:
> > Peter Williams <peterw@aurema.com> writes:
[...]
> > Already exists for a long time - AT_CLKTCK. glibc has a nice wrapper
> > for it too (sysconf)
> 
> So it does and POSIX.1 (_SC_CLK_TCK) compliant as well.  Unfortunately, 
> the presence of this functionality makes it VERY difficult to understand 
> why ticks are being converted from HZ==1000 values to HZ=100 values when 
> they are being exported to user space especially as this conversion 
> throws away precision.  Can anyone enlighten me?

1) Because Linux had long time HZ=100 hardcoded (except on Alphas) and
   lots of applications probably use that value today (as HZ in their
   source and not sysconf(...))  - especially since 2.4 (at least most
   of them) has HZ=100 except for 64bit CPUs).
2) There are patches which dynamically change the CPU speed. And it
   probably (IMHO) makes sense to change HZ dynamically too in that
   situations. And a over-time changing HZ value is useless in
   user-space.

	Bernd
-- 
Firmix Software GmbH                   http://www.firmix.at/
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