Re: [chrony-users] Any clock crystal experts here?

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

On Mon, 5 Nov 2012, Ed W wrote:

> Hi I have a pile of Alix boards that we use as part of a commercial 
> use, but I thought I might use a couple for NTP duties.  We already 
> have some real machines in service as NTP internet facing servers, but 
> they have other duties and are now loaded enough that there is some 
> small wobble in accuracy.  I thought I might re-use some of the Alix 
> boards as an inexpensive project to try and get to sub 10uS accuracy on 
> a local LAN?
>
> As part of that I would need a better clock crystal since the Alix part 
> is quite low quality and doesn't seem to correlate particularly well 
> with only temperature (according to my log files...)

Not sure what that evidence is. 
Remember it is the temperature of the clock crystal, not of the CPU. And
remember that the temperature affects the crystal rate not the offsets. Ie,
you should compare the rate  of the clocks with the temperature of the
crystal.

IF you have the machines connected to a gps PPS, then sub 10us should not be
hard at all. If not, the network jitter will be a problem. And if you are
using gigabit, the interrupt coalescing will make that network jitter really
bad. Ie it will be really hard to get to 10us. Remember that chrony collects
only 64 data points at best (assuming tha the rate does not change), which
means only a factor of 8 reduction by statistical averaging in the jitter from
the network. It is usually much worse because the rate changes means that
chrony will not ever full its statistical buffer.


> Anyone got any experience here and could offer some DIY advice?
>
> Thanks
>
> Ed W

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William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
Physics&Astronomy  |     Advanced Research  |     Fax: +1(604)822-5324
UBC, Vancouver,BC  |   Program in Cosmology |     unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1     |      and Gravity       |  www.theory.physics.ubc.ca/

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