[ntp:questions] Is 24PPM an Excessive Real-Time Clock Correction?
thomas at Hax.SE
Thu Jul 5 20:46:38 UTC 2007
"David T. Ashley" <dta at e3ft.com> writes:
> "Richard B. Gilbert" <rgilbert88 at comcast.net> wrote in message
> news:468D4A70.9030508 at comcast.net...
>> David T. Ashley wrote:
>>> Any insight into whether 24 PPM is excessive for my server?
>> Consider that a computer manufacturer typically spends something like $2
>> US on the components for the clock! Consider, also, that they provide no
>> way to adjust the clock hardware. Computers are designed to compute, not
>> to keep time. The original IBM PC and PC/XT did not even have a clock; if
>> you wanted a clock it was an add-on at extra cost from a third party.
>> 24 PPM is pretty good. Anything up to a hundred or two will usually work
>> just fine.
> Thanks. Interesting. It seems to be a really steady error, i.e. seems to
> always be between 24 PPM and 24.5 PPM.
That is the static frequency offset of your oscillator.
> I think I'll write a little software to plot it at a sampling frequency of
> once per hour. I'm wondering if it varies by time of day, temperature, etc.
It varies widely with temperature.
I once plotted the drift over a few days on an old Sun ss20 I had on
my desk, just inside a window, and it was easy to see if it had been a
sunny day or not.
I also plotted drift vs cpu temperature on an old Sun UE-450, and from
the plots I cold deduct how the office air conditioning was
programmed, even if the cpu temp only changed by about 1° between day
and night and over the weekends.
I have tuned a kernel variable in my pc running Solaris nevada so the
drift is currently -0.229. I may be able to get it closer to 0, but
I'm alright with that value. It was around -35 when I started, and the
old motherboard was around -17.
ntp does a good job to compensate for this, so only real nerds, like
me, bother with this. :-)
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