[ntp:questions] Does NTP cause an abrupt/sudden change in an orphan's system time?

unruh unruh at invalid.ca
Fri Aug 10 12:45:32 UTC 2012

On 2012-08-09, E-Mail Sent to this address will be added to the BlackLists <Null at BlackList.Anitech-Systems.invalid> wrote:
> unruh wrote:> David Woolley wrote:
>>> Steve Kostecke wrote:
>>>> What will happen when ntpd is started has been discussed
>>>>  elsewhere in this thread.
>>> If you want to clear a very large error by a pure slew,
>>>  the best way is not to start ntpd until you have cleared
>>>  the error, but, instead, to use whatever utility your OS
>>>  uses to access the system calls that ntpd uses, and set
>>>  a high slew rate, which you only clear when you are
>>>  approaching zero offset.
>> But why should he have to do this by hand?  Why should ntpd
>>  not do it for him?  Taking care of such housekeeping is
>>  what computers are good for.
> It will, that is what the step is, if panic is disabled.

No It is not what the step is. speeding up (or slowind down) the clock by altering the
tick rate is not the same as a step. The suggestion was that before you
start ntp you use that to bring the clock (monotonically) to
approximately the right time ( <128ms) and then start ntp to get rid of
the rest. A step is a discontinuous change in the time, which can be
expecially bad if it is a step backwards.

> I am unclear though if the OP didn't want to step the time
>   when starting ntp, apparently long after system is booted,

No. He was running ntp all the time, only in orphan mode, when suddenly
his network came up and the network servers came on line. At that point,
the computer clock was out ( because orphan mode had drifted) and ntp
stepped the time (or crashed if the time was out more than the panic). 

>    {If so I doubt they would want to manually step either};
>  or just not wanting a step when internet connectivity reoccurs,
>   while the system and ntp had been already running?

On linux, there are two clock adjustments. One is limited to 500PPM, the
other tickadjust is limited to 100000PPM (.1sec/sec). That latter
technique allows you to slew the clock without a step and a lot lot
faster than using the 500PPM of ntpd. (1 second in 10 sec rather than 1
sec in 1/2 hr, or one day in 10 days rather than 6 years)


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