On Mar 10, 2010, at 7:25 AM, David J Taylor wrote:
> Yes, I know it's one of those low long is a piece of string questions, but 
> I'm now considering a dual-core Intel Atom system, which is "Compatible with 
> Linux" according the the very minimal blurb I have right now.  If the system 
> is to be used purely for NTP with Linux as a serial-port GPS/PPS stratum-1 
> server (and, yes, I know dual-core isn't needed for that, but I might want to 
> boot Windows-7 64-bit occasionally), and considering that I know very little 
> about Linux, which version of Linux would the group recommend?  Does it make 
> any difference as far as timekeeping is concerned?

The hardware you get matters much more than which OS or flavor of Linux you 
use; tweaking kernel compiler optimization flags matters even less.  It would 
be good to look into the hardware you get in terms of support for HPET, p-state 
invariant TSC, or how good the ACPI timers are.

> Two secondary questions:
> - how much better might FreeBSD be than Linux?  Any actual measurements?

There are some nice comparisons between different platforms here:

  http://www.dragonflybsd.org/presentations/nanosleep/

PHK, who wrote the FreeBSD timer code, is measuring ~150 ns timekeeping 
precision for a stratum-1 timesource:

  http://phk.freebsd.dk/soekris/pps/
  http://phk.freebsd.dk/pubs/timecounter.pdf

You might find some threads here interesting, also:

  http://unix.derkeiler.com/Mailing-Lists/FreeBSD/current/2005-10/1018.html

> - and one for Bill, how much better might chrony be than official NTP? Does 
> it have a preferred Linux, or even freeBSD?

I gather that crony is intended for machines with infrequent network 
connections.  I can't imagine trying to run it for a permanently networked 
stratum-1 timesource.

> The system would /not/ be in a temperature controlled environment.

That's unfortunate; this effects your time stability more than any other factor 
being considered.  Well, try to make sure you get the PPS timesource working, 
as that will help.

Regards,
-- 
-Chuck

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