[ntp:questions] stepping and slewing

Steve Kostecke kostecke at ntp.isc.org
Tue May 1 13:10:46 UTC 2007

On 2007-05-01, Riccardo Castellani <castellani.riccardo at tiscali.it>

> 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

That's the _maximum_ slew rate. The actual slew rate depends on a number
of factors.

> to slew time but because do you speak about "maximum" rate of 0.5
> ms/sec. ?

The maximum slew rate is 500ppm; this is the equivalent of half a
millisecond per second or 43 seconds per day.

> Does ntpd use always the same 0.5 as value or it's a variable
> parameter ?

500ppm is the _maximum_ slew rate that most kernels can tolerate. The
actual slew rate depends on a number of factors.

> I'm confused because "Rob MacGregor" said about step method (128ms <
> offset < 1000s) :

1000 seconds == the default panic threshold. ntpd will abort when
it sees an offset greater than the panic threshold

128ms == the default step/slew threshold. ntpd will slew offsets below
this threhold and will step offsets above this threshold

>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.

step == reset the clock to the correct time in _one_ instantaneous step.
A stepped clock can "move backwards".

slew == adjust the clock by speeding it up or slowing it down. A slewed
clock never "moves backwards"

> 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 ?

Slewing the clock to correct a 5 minute offset will take 6.97 days at
the maximum 500ppm slew rate.

5 minutes is greater than the default 128ms step/slew threshold. In this
case ntpd will _step_ the clock.

Could you please explain what problem you are trying to solve and why
you feel you need to solve it with 'ntpd -gq'/ntpdate ?

Steve Kostecke <kostecke at ntp.isc.org>
NTP Public Services Project - http://ntp.isc.org/

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