Re: [chrony-users] gpsd, pps and chrony

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On Tue, Apr 05, 2011 at 11:23:34PM +0100, Ed W wrote:
> OK, so I have chrony built in a uclibc chroot with /usr/src/linux holding a 
> 32bit kernel with HZ set to 1024 (I think).  The host is a 64bit kernel with 
> HZ set to 250 - however, I don't think this should be relevant?
> This is then installed on my Alix (32bit only). If I set "linux_hz 1024" in 
> my conf file I get:

> Apr  5 22:05:37 localhost chronyd[3457]: set_config_hz=1 hz=1024 shift_hz=7 basic_freq_scale=0.12500000 nominal_tick=977 slew_delta_tick=81 max_tick_bias=97

> My question is why I see "hz=100" and why the 1024 config causes the fatal error?

Because the userspace and kernel HZ are different. The userspace HZ is
scaled so it's constant on a given architecture to avoid problems with
applications assuming a fixed HZ.

The hz value is needed to determine tick values which are valid in
the adjtimex() call.

If nominal_tick and max_tick_bias are incorrect, you may get the fatal
error when chrony is trying to compensate for a large frequency
offset or fast slewing.

> I'm fairly sure I was still getting hz=100 in the logs when built against a kernel source using 1000hz and 250hz. Also I only looked at the config code quickly, but I can't see where it's trying to read the kernel HZ value at compile time?

The hz value is determined at start from the current tick value, it's
assumed to be 100 or a power of 2.

> Note, seems to be working fine without any config stanzas, I just thought it curious?

Yes, keep it at 100 :).

Miroslav Lichvar

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