[ntp:questions] ntpd and hw clock

David Woolley david at ex.djwhome.demon.co.uk.invalid
Thu Feb 26 23:05:30 UTC 2009

Wayne Liu wrote:
> I just wanted to verify that ntpd is NOT attempting to directly set
> hardware clock. It just uses system calls such as settimeofday to set

I am not aware of any platform on which ntpd directly manipulates the 
hardware clock, although the only real requirements are that it sets the 
same one it reads and that it can read and modify it with fairly high 
resolution (precision, in ntpd jargon).

However, even on machines with settimeofday, it only uses that as a last 
resort, and should never use it after the startup transients.  The 
preferred option for Unix is to tell the kernel how much the time 
differs from its measurement, and rely on the kernel to use this to 
adjust the clock in the way that ntpd expects (which does not mean that 
the kernel will apply anything like that full difference before the next 
update).  The fallback for Unix type systems is to periodically send a 
time delta (in which case, ntpd does the smoothing, so the corrections 
are less than the measured offset; it also interpolates between 
measurements).  For NT, it varies the assumed frequency of the clock 
used to convert ticks to time of day.

> the time to the Kernel and the time has to be sent to rtc, if so
> desired, by the Kernel itself or separately using e.g. hwclock.

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