[ntp:questions] frequency adjusting only

maxime louvel m.louvel at gmail.com
Wed Apr 30 06:43:15 UTC 2008


On Tue, Apr 29, 2008 at 6:27 PM, Unruh <unruh-spam at physics.ubc.ca> wrote:

> m.louvel at gmail.com (maxime louvel) writes:
>
> >Hi,
>
> >I have know run a lot of tests.
> >Just to let you know what I've got so far.
> >I have tried NTP, and NTP + PTP (Precision Time Protocol).
> >I haven't tried Chrony nor TSClock.
> >I have used the software only implementation of PTP (ptpd).
>
> >With NTP only I have got an accuracy around 1ms,
>
> accuracy? Do you mean the variation of the offset between ntp and the
> remote ntp server? YOu need to tell us what you mean by the term.
>

By accuracy I actually meant the offset between my nodes.


>
>
> >with one node synchronising to public NTP server, the others nodes
> >synchronising to it with PTP I got 10 usec (between the nodes).
> >I haven't used any hardware support (not GPS,PPS, for PTP).
>
> Then you do not really know what the accuracy is of the other nodes. For
> example, your system could have a 100us assymmetry in communication which
> would make the accuracy 50us, but with a fluctuation of 10us.
>

You're right on this point, what I  actually  get with PTP is delay of 100
usec, with a variation of 10 usec.
But I'm only interested in the variation as the 100usec can be removed
easily because it's constant

>
>
> >I might try the other solutions, if yeI do I'll send an email to complete
> >the results.
>
> As I said, I am completely at a loss to know why you would only be getting
> 1ms using ntp to synchronize. That is much worse than my experience. (Look
> at flory on www.theory.physics.ubc.ca/chrony/chrony.html which uses ntp vs
> the other machine which all use chrony. flory's fluctutations are
> certainly  not 1ms.)
>

I have to say that I 'm also surprise of the bad performance of NTP, I will
try chrony if it is suitable to my nodes (embedded cards, with the latest
linux kernel).


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Maxime Louvel
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