Re: [chrony-users] makestep in Chrony

[Hei Chan <structurechart@xxxxxxxxx>]

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.

And let's say it happens occasionally, I can just drop those mappings in the file, and continue my estimation with the mappings before and after.

Hope it makes sense to you.

On Tuesday, May 15, 2018, 3:38:51 PM GMT+8, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:

On Tue, May 15, 2018 at 05:57:04AM +0000, Hei Chan wrote:
>  If I remember correctly that there was a post explaining why it wasn't a bug, the post mentioned the value was written to a shared memory (or some sort), and the writer and reader aren't protected by a lock for performance reason, and so it needs to spin (i.e while loop) to get the value out as soon as the writer finishes.

How do you avoid this issue in your solution? If you update the
estimated frequency offset between real time and tsc, you will need to
prevent the conversion from using partially updated values. If you
don't update the offset at all (estimated only once on start), the
tsc-based clock will drift away.

--
Miroslav Lichvar

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