[ntp:questions] Re: NTP and Cron

Danny Mayer mayer at gis.net
Tue Nov 30 22:54:01 UTC 2004


Steven Hajducko <Steven.Hajducko at DigitalInsight.com> wrote in message news:<mailman.36.1101808416.54146.questions at lists.ntp.isc.org>...
> Is it possible for NTP to reset the time back enough that cron will fire off
> jobs twice?
> 
> I realize it's poor programming in whatever jobs we have running that they
> don't check if they're already running, but that is out of my hands.  The
> issue that was given to me was to write a monitoring script around it
> happening again.  I'd instead like to make sure that NTP can't do that.  Do
> you have to specify a threshold for time increments and if so, is it
> possible to set the time increments to be under 1 second ( or is this the
> default already? )
> 
> In our log, we have the following:
> 
> Nov 23 08:55:25 xeon xntpd[10464]: time reset (step) 14.427314 s
> 
> And afterwards, the log goes on to show one of the jobs firing off, again.
> I'm not exactly sure this IS the issue, as I don't see how it could have set
> off another cronjob, being that 14 second step would have simply gone to
> 8:55:11, or if that timestamp is after the step, then 8:55:39 and since cron
> only goes by a minimum of minutes, well...
> 
> But I'd like to clear up the issue so I can present it to them.  So is it
> possible for NTP to reset the time in a big enough increment that it could
> make cron fire off a job twice?  And if so, are there ways to prevent that?

ntp never decrements the time. If it's necessary it will reduce the
adjustments. Remember that time is monotonically increasing.

The only time you could get something like this happening is if
you were to use ntpdate to SET the time. It's another reason to
deprecate ntpdate.

Danny



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