[ntp:questions] stepping and slewing

Richard B. Gilbert rgilbert88 at comcast.net
Tue May 1 11:40:41 UTC 2007


Riccardo Castellani wrote:
> According to www.ntp.org in stanard Linux o.s. (adjtime(2) - http://www.ntp.org/ntpfaq/NTP-s-algo.htm#S-ALGO-BASIC) time adjusting has rate of 0.5ms per second to slew time but because do you speak about "maximum"  rate of 0.5 ms/sec. ? Does ntpd use always the same 0.5 as value or it's a variable parameter ?
> 
> I'm confused because "Rob MacGregor" said about step method (128ms < offset < 1000s) :
> Stepping: Time changes in large units, quickly
> 
> With "Step" method (settimeofday), time is gradually changed with higher rate or time is changes immediately to correct time.
> example for use step method : my local clock is 5:00 pm and real time is 5:05 pm, Ntpd set immediately local clock to 5:05 pm or it corrects time gradually ?

If your local clock is 5:00 PM and real time is 5:05 PM, when you start 
ntpd with "-g", time will be set (stepped) immediately to 5:05 PM.  In 
this case the magnitude of the error means nothing.  The clock is set 
unconditionally.  This is done once only!  Thereafter, ntpd follows the 
rules we have been discussing; if the error is less than 128 
milliseconds, the clock will be slewed (changed gradually) or, if the 
error is greater than 128 milliseconds, ntpd will step the clock.




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