On Dec 11, 2010, at 1:32 PM, Warner Losh wrote:
> That's one reason why you're seeing the push back from people on this list:
> many have tried to deploy systems where sub-second synchronization was
> required by the application and have run into many problems...
May I point out that the current squabble has nothing to do with the
astronomers in the room? (And precious little to do with leap seconds.)
Astronomers (or at least astronomical software engineers) tend to love NTP - to
the extent that we're willing to put up with some poor implementations on
bizarre legacy equipment. (The algorithms may drive some to distraction,
however - strongly recommend Dave Mills' book.)
This rather terse conference paper:
http://www.adass.org/adass/proceedings/adass96/reprints/seamanr2.ps.gz
describes a project from the mid-1990's that relied on NTP (not described since
I only had 4 pages), to provide a timescale precise to 0.01s (SunOS Sparc).
I didn't run into any significant problems that I recall.
This was an "appropriate technology" project on both the computer science and
astronomy sides. Pragmatic choices (eg, using a heliocentric correction
instead of barycentric - Earth's motion was transverse and Jupiter's lever arm
was minimal) made the implementation simpler and more robust. NTP served
extremely well - and that host is still in service and has been running NTP
with zero headaches (from NTP) for 15 years since. (The particular camera
relies on an S-bus interface card that emulates an even older DEC standard.)
In the mean time the observatory's NTP stratum architecture has been rearranged
any number of times, for instance to relayer on GPS receivers.
Our telescopes support quite a few cameras built by outside groups. One of the
several requirements is that the data acquisition host run NTP. I'm
responsible for the initial archival data capture. NTP provides a reliable
timestamp from a clock synchronized mountain-wide and across continents. The
host computers are a wide range of vintages and operating systems (though I
admit I only see the data from the PC-driven cameras after it crosses to the
inevitable Linux box). NTP runs fine on various MacOS servers, too.
This debate about the necessary requirements for civil timekeeping has been
artificially cast as a disagreement between astronomers and computer
scientists. Rather, timekeeping is a complex issue for all communities. The
question is how to make the appropriate pragmatic choices. Pretending there's
only one kind of time is not pragmatic.
Rob
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