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

Ulrich Windl Ulrich.Windl at RZ.Uni-Regensburg.DE
Thu Nov 12 12:35:36 UTC 2009

"nemo_outis" <abc at xyz.com> writes:

> "Richard B. Gilbert" <rgilbert88 at comcast.net> wrote in
> news:TLSdnQ2E26bBLBnXnZ2dnUVZ_sydnZ2d at giganews.com: 
>> nemo_outis wrote:
> ...
>>> I fail to see the value or relevance of "500ppm satisfies 98% of
>>> computer clocks" if some other number, perhaps 5000 ppm, could
>>> satisfy yet even more than 98% of computer clocks with no downside -
>>> as indeed seems to be the case!  Chrony, whatever its other merits
>>> and demerits, is an "existence proof" for this proposition.
>> I can't follow Dave's math but I'm reasonably sure that there is a
>> good reason for the 500 PPM limit.  Since almost all computer clocks
>> can meet this criterion I'm not going to worry about it.
> Hmm, "faith-based" ntp?  Not for me.  If there is a "good reason" I'd 
> like to hear it - 500 ppm has the smell of arbitrariness about it.

As arbitrary as there are 8 bits in a byte.

>> If you have a computer with a frequency error of more than 500 PPM,
>> you can either get it fixed or hack the ntpd code to allow +/- 600 or
>> 6000 or whatever PPM.
> Of course, one can hack the code.  But one should not have to do this to 
> overcome arbitrarily-imposed unjustified constraints.  That Chrony can 

I see the argument: When about 20 years ago all the computers had clock
within the 500PPM range, it's technical progress that the systems sold
nowadays have much more terrible clocks. Another sign that "technical
progress is moving in the wrong direction".

> manage much more than 500 ppm may not not be absolutely probative in this 
> regard but it sure is strongly indicative.
> Regards,

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