Re: [chrony-users] makestep in Chrony

[Stephen Satchell <list@xxxxxxxxxxxx>]

On 05/15/2018 04:23 AM, Hei Chan wrote:
> Since the application calling rdtsc+clock_gettime()+rdtsc (to create
> the mapping file) has its own dedicated core, and this application is
> only called after "chronyd -q 'pool [some NTP server/switch which is
> 1 switch away] iburst'" returns, at that time, I believe the clock is
> synchronized (right?).  Then, it is very rare to have other processes
> to update the clock.
Synchronized, but not regulated.  Let me give you an example:  in my 
admin desktop system, chronyc reports that the frequency of the computer 
clock is 12.373 ppm slow.  So that means that the real-time clock will 
lose time after being synchronized but not regulated.  Chrony will turn 
those knobs to compensate for the difference.

There are knobs in most operating systems's time software to compensate 
for the inaccuracies, so that long-term accuracy of the real-time clock 
is improved.  Because the crystals will wander over temperature, one 
would need to periodically re-synchronize, and perhaps tweak the RTC 
clock regulation.  That's what chrony (and NTP) do.

Both NTP and chrony assume that the base clock has an absolute accurate 
of 100 ppm or better, and a relative accuracy of 5 ppm or better. 
Virtually all modern computers (including cell phones) have crystals 
that meet this specification.

Now, in a VM object, the base clock used for timekeeping is almost 
worthless because the error and jitter fall outside of those boundaries. 
 That's why I scream and yell every time one of my clients installs NTP 
into a virtual machine and calls it good.  You are almost guaranteed to 
have a false ticker.  I could tell you stories...

Are you trying to do astronomical observations?  If so, use the search 
engine of your choice to find research papers on this subject.

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