Re: [chrony-users] makestep in Chrony

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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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