[ntp:questions] So what? (Re: 500ppm - is it too small?)
Unruh
unruh-spam at physics.ubc.ca
Thu Aug 20 05:40:21 UTC 2009
Nero Imhard <nim at pipe.nl> writes:
>nemo_outis wrote:
>> A clock with a frequency deviation is NOT, per se, broken. If the
>> deviation, even if it be, say, 5000 ppm, is consistent, stable and
>> regular, then the clock could be a superb timekeeper.
>Yes of course, but you miss the point (or at least my point). Such a
>large deviation can easily be corrected for *before* ntp gets to sync
>the clock. Again, the margin into which you should tune your clock rate
>is a matter of taste and quite arbitrary.
And you miss the point-- ntp itself should be doing that. It is a
program that is supposed to discipline the clock. It should not ask you
to do part of its job.
>But since you are pushing it, here's my personal take: I deem it
>reasonable to require a clock's frequency error to be in the same
>ballpark as (let's say at most one order of magnitude larger than) its
>instability. Anything larger should be (and is in fact easily) tuned out
>through other means than ntp's clock discipline. 500 is *huge* already.
>I'm not versed in the theory enough to make any assertions about how a
>large frequency error affects behaviour and performance of the clock
>disciplining algorithm, but my gut feeling tells me it's better to have
>a small frequency error. If someone who does know these things
>convincingly tells me otherwise, I'll gladly adjust my opinion. For the
>moment, I trust my gut.
>N
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