lkml.org 
[lkml]   [2004]   [Mar]   [15]   [last100]   RSS Feed
Views: [more markup]  [less markup]  [headers]  [forward] 
 
Messages in this thread
/
DateMon, 15 Mar 2004 10:13:40 +0000
FromRichard Curnow <>
SubjectRe: finding out the value of HZ from userspace
* 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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 13:01    [from the cache]
©2003-2008