[ntp:questions] Which version of Linux works best?

unruh unruh at wormhole.physics.ubc.ca
Thu Mar 11 21:37:12 UTC 2010


On 2010-03-11, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
> David J Taylor wrote:
>> "Miroslav Lichvar" <mlichvar at redhat.com> wrote in message 
>> news:20100311124036.GA22872 at localhost...
>> []
>>> I did a NTP vs chrony comparison last June with GPS 18x LVC in an
>>> office environment, clock drift was moving in about 0.8ppm range. Here
>>> are distributions of PPS samples received from gpsd:
>>>
>>> http://fedorapeople.org/~mlichvar/chrony/chrony_vs_ntp.png
>>>
>>> With recent chrony, NTP and kernel versions the results might be
>>> different though.
>>>
>>> -- 
>>> Miroslav Lichvar
>> 
>> Miroslav,
>> 
>> Thanks for that, a most interesting comparison, and thanks to everyone 
>> for their input.
>> 
>> I've recently switched the old (2005) FreeBSD system back on, to see how 
>> well in performs in my own environment.  From what's been said, I rather 
>> suspect that were I to go for a more modern, faster, Intel Atom system, 
>> any improvement in accuracy I might get could be swamped by the 
>> temperature changes in the room.
>> 
>> There's also, I will admit, a slight doubt about the effort involved for 
>> the benefit to be gained.  With Windows, I am quite happy, and 
>> configuring, using or testing NTP is no problem.  With FreeBSD is seems 
>> that the old PPS atom driver has gone, and I may need to configure yet 
>> another driver - gpsd.  The number of variants of Linux doesn't help - I 
>> only need a command-line or Telnet interface.  And remembering how long 
>> it took to recompile the kernel last time, and the amount of help I 
>> needed to know how to do that, also fills me with doubt.
>> 
>> So I suspect that the performance I'm now seeing from Windows (well 
>> within 100us) may well be "good enough" for me.  Perhaps if I get more 
>> free time, and a little more income this year, I may get a 
>> paperback-sized Intel Atom box and see how it does.  At least some do 
>> have serial ports!  And I would be most interested to hear of anyone who 
>> does configure such a device.
>> 
>> Cheers,
>> David
>
> 100 microseconds is pretty good.  Getting the time *into* a computer 
> takes time and the time taken is not easy to measure.

Considering that chrony can give sub microsecond resolution from say a
GPS source, ( and ntpd 2usec) 100usec is good only only in a certain
defintion of good. Getting the time into the computer from a refclock is
on the 1usec level ( measured), certainly not 100usec.




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