Re: finding out the value of HZ from userspace

From: Richard Curnow
Date: Mon Mar 15 2004 - 05:18:42 EST

* Horst von Brand <vonbrand@xxxxxxxxxxxx> [2004-03-14]:
> Micha Feigin <michf@xxxxxxxxxxxxxx> 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 (, 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

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