Date: Mon, 15 Mar 2004 10:13:40 +0000
From: Richard Curnow <Richard.Curnow@superh.com>
To: Horst von Brand <vonbrand@inf.utfsm.cl>
Cc: lkml <linux-kernel@vger.kernel.org>
Subject: Re: finding out the value of HZ from userspace
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References: <20040311141703.GE3053@luna.mooo.com> <200403140245.i2E2jKSx005375@eeyore.valparaiso.cl>
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* Horst von Brand <vonbrand@inf.utfsm.cl> [2004-03-14]:
> Micha Feigin <michf@post.tau.ac.il> said:
> > Is it possible to find out what the kernel's notion of HZ is from user
> > space?
> 
> What for? It should be invisible to userspace...
> 

A related issue that's bugged me for a long time is lack of userspace
access to the quantity that's called 'freq_scale' in 2.4, where it's
(1<<SHIFT_HZ)/HZ for HZ!=100 and 128/128.125 for HZ==100.  (I haven't
started to reverse-engineer the equivalent value in 2.6, I took a quick
look once and concluded things had got a little more hairy.)

My interest is that I maintain (in spare-time) an NTP application called
chrony (http://chrony.sunsite.dk/), originally written to be good for
dial-up, i.e. NTP servers accessible for a short window once or twice a
day.  This app wants to tune the parameters it passes to adjtimex() to
take a best shot at keeping the system clock correct over the
potentially 'long' offline period.  To do this well, it has to
reverse-compensate for the freq_scale multiplier that the kernel will
apply to the frequency value passed to adjtimex().  Getting the right
value for this across different kernels has always been a fragile
exercise.

-- 
Richard \\\ SH-4/SH-5 Core & Debug Architect
Curnow  \\\         SuperH (UK) Ltd, Bristol
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