| Re: [chrony-users] Accurately measuring clock drift |
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- To: chrony-users@xxxxxxxxxxxxxxxxxxxx
- Subject: Re: [chrony-users] Accurately measuring clock drift
- From: Miroslav Lichvar <mlichvar@xxxxxxxxxx>
- Date: Tue, 20 Jun 2023 09:48:22 +0200
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On Mon, Jun 19, 2023 at 03:11:44PM -0700, Thangalin wrote:
> I'm wondering, in no particular order:
>
> - Do the *esterror* values from calling *adjtimex()* yield an
> apples-to-apples comparison between chrony and NTP?
The values are estimated differently. Not an apples-to-apples
comparison.
> - Do we need to combine PPS1 with the NTP server?
> - If so, how?
Your configuration looks good to me.
> - What else would we need to do to achieve sub-30 μs clock drift (or
> sub-10 μs)?
That depends on the hardware. There might be a better way to timestamp
the PPS that doesn't involve interrupts. Is it an x86_64 machine?
See these two examples:
https://chrony.tuxfamily.org/examples.html#_server_using_reference_clock_on_serial_port
https://chrony.tuxfamily.org/examples.html#_server_using_reference_clock_on_nic
> - How can we verify that the clock isn't drifting more than 30 μs,
> programatically (i.e., what API calls return the recent clock drift
> adjustment value)?
I don't think that is possible without a more accurate time source.
> - What API call returns the most recent clock drift adjustment value in
> nanoseconds?
This information is not available in the kernel (adjtimex). You would
need to use chronyc sources.
--
Miroslav Lichvar
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