[ntp:questions] 500ppm - is it too small?

Uwe Klein uwe_klein_habertwedt at t-online.de
Thu Nov 12 18:13:24 UTC 2009


Richard B. Gilbert wrote:
> 
> Do we have any data on just how large a percentage of computer clocks 
> are bad enough to exceed the 500 PPM limit?
enough to kick off this  discussion. repeatedly.
> 
> And is there any reason for NTPD to stand on its head to cope with a 
> clock that is broken that badly?

you are lacking semantic finesse and differentiation ;-)

A. you have crystals that behave eratic ( I would tag that "malfunction" )
B. you have crystals that are off spec frequencywise. ( "off spec" )
C. you have boards that lower apparent frequency due to spread spectrum features
D. you have boards that loose or gain ticks ("hardware")
E. you have OSes   that loose or gain ticks ("OS bug")
F. ....

A is unfixable -> Broken
B is fixable   -> could work if ntp would "yield"
C is fixable   -> could work ""         ...
	( not 100% sure, depends on how spread spectrum is done in each specific case )
D is unfixable -> Broken , get other hardware
E is fixable   -> but not via ntp which is OK


There is an interesting short from Larry Niven's Draco Tavern series that
takes up this semantic issue : "Grammar Lesson"

> If you really believe that it's an arbitrary limit perhaps you should 
> change it to 5,000 or 50,000 in your copy of NTPD and run it for a 
> while.  Please let us know how well, or how badly, it works.
> 
I currently don't have a MOBO that would require such fiddling.

uwe




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