What is difference among stepping and slewing mode which are used by ntpd ?
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Riccardo Castellani unleashed the infinite monkeys on 29/04/2007 22:34
producing:
> What is difference among stepping and slewing mode which are used by ntpd ?

(I'm sure one of the experts will be along shortly with a highly
detailed explanation, or just pointing you to the right part of the
documentation, however...)

Stepping: Time changes in large units, quickly
Slewing: Time changes slowly

Slewing has the advantage that nothing will get surprised by the sudden
change of time.

-- 
  Rob MacGregor (BOFH)

Rule 37: "There is no 'overkill'. There is only 'open fire'
          and 'I need to reload.'"

Riccardo Castellani wrote:
> What is difference among stepping and slewing mode which are used by ntpd ?
> _______________________________________________
> questions mailing list
> questions@lists.ntp.isc.org
> https://lists.ntp.isc.org/mailman/listinfo/questions
> 

Slewing means that ntpd adjusts the clock at a maximum rate of 500 
microseconds per second.

Stepping means that the clock jumps abruptly from some incorrect time to 
some reasonable approximation of the correct time.  In normal operation 
ntpd will not step the clock; any error less than 128 milliseonds will 
be corrected by slewing the clock.  An error of more that 128 
milliseconds in considered an abnormal condition!

Since correcting large errors by slewing the clock could take days or 
weeks to complete stepping is used but, as mentioned above, this is not 
normal.

>but can I define slew rate and step rate ?
>Pheraps these parameters are defined inside kernel of o.s. ?
>slew rate is always 0.5 ms/sec ?

I don't know what you mean by "step rate".  There is no "rate",
it just changes the clock to the new value.

You can adjust point where it switches from slew to step.
(Some data bases really really don't want time to go backwards.)


You can't change the slew rate.  It is tangled up with
stability considerations.


-- 
These are my opinions, not necessarily my employer's.  I hate spam.

but can I define slew rate and step rate ?
Pheraps these parameters are defined inside kernel of o.s. ?
slew rate is always 0.5 ms/sec ?
  ----- Original Message ----- 
  From: Richard B. Gilbert 
  Newsgroups: comp.protocols.time.ntp
  To: questions@lists.ntp.isc.org 
  Sent: Sunday, April 29, 2007 11:25 PM
  Subject: Re: [ntp:questions] stepping and slewing


  Riccardo Castellani wrote:
  > What is difference among stepping and slewing mode which are used by ntpd ?
  > _______________________________________________
  > questions mailing list
  > questions@lists.ntp.isc.org
  > https://lists.ntp.isc.org/mailman/listinfo/questions
  > 

  Slewing means that ntpd adjusts the clock at a maximum rate of 500 
  microseconds per second.

  Stepping means that the clock jumps abruptly from some incorrect time to 
  some reasonable approximation of the correct time.  In normal operation 
  ntpd will not step the clock; any error less than 128 milliseonds will 
  be corrected by slewing the clock.  An error of more that 128 
  milliseconds in considered an abnormal condition!

  Since correcting large errors by slewing the clock could take days or 
  weeks to complete stepping is used but, as mentioned above, this is not 
  normal.

  _______________________________________________
  questions mailing list
  questions@lists.ntp.isc.org
  https://lists.ntp.isc.org/mailman/listinfo/questions
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but can I define slew rate and step rate ?
Pheraps these parameters are defined inside kernel of o.s. ?
slew rate is always 0.5 ms/sec ?

Where can i find doc. about stepping and slewing ?
  ----- Original Message ----- 
  From: Richard B. Gilbert 
  Newsgroups: comp.protocols.time.ntp
  To: questions@lists.ntp.isc.org 
  Sent: Sunday, April 29, 2007 11:25 PM
  Subject: Re: [ntp:questions] stepping and slewing


  Riccardo Castellani wrote:
  > What is difference among stepping and slewing mode which are used by ntpd ?
  > _______________________________________________
  > questions mailing list
  > questions@lists.ntp.isc.org
  > https://lists.ntp.isc.org/mailman/listinfo/questions
  > 

  Slewing means that ntpd adjusts the clock at a maximum rate of 500 
  microseconds per second.

  Stepping means that the clock jumps abruptly from some incorrect time to 
  some reasonable approximation of the correct time.  In normal operation 
  ntpd will not step the clock; any error less than 128 milliseonds will 
  be corrected by slewing the clock.  An error of more that 128 
  milliseconds in considered an abnormal condition!

  Since correcting large errors by slewing the clock could take days or 
  weeks to complete stepping is used but, as mentioned above, this is not 
  normal.

  _______________________________________________
  questions mailing list
  questions@lists.ntp.isc.org
  https://lists.ntp.isc.org/mailman/listinfo/questions
_______________________________________________
questions mailing list
questions@lists.ntp.isc.org
https://lists.ntp.isc.org/mailman/listinfo/questions

Riccardo Castellani wrote:
> but can I define slew rate and step rate ?
> Pheraps these parameters are defined inside kernel of o.s. ?
> slew rate is always 0.5 ms/sec ?

No!  That is the maximum slew rate.  It's a limit built into most Unix 
and Unix-like operating systems.

What problem are you trying to solve?  ntpd works very well for most of 
us, just as it is.

On 2007-04-30, Riccardo Castellani <castellani.riccardo@tiscali.it> wrote:

> Where can i find doc. about stepping and slewing ?

1. Try you  favorite search engine

or ...

2. Go to www.ntp.org and:

	- Click the documentation link then

	- Click the Official Documentation link or the FAQ link

or ...

3. Visit the Community Supported Documentation at http://ntp.isc.org and
search around.

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

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 ?


  ----- Original Message ----- 
  From: Richard B. Gilbert 
  Newsgroups: comp.protocols.time.ntp
  To: questions@lists.ntp.isc.org 
  Sent: Monday, April 30, 2007 9:02 PM
  Subject: Re: [ntp:questions] stepping and slewing


  Riccardo Castellani wrote:
  > but can I define slew rate and step rate ?
  > Pheraps these parameters are defined inside kernel of o.s. ?
  > slew rate is always 0.5 ms/sec ?

  No!  That is the maximum slew rate.  It's a limit built into most Unix 
  and Unix-like operating systems.

  What problem are you trying to solve?  ntpd works very well for most of 
  us, just as it is.

  _______________________________________________
  questions mailing list
  questions@lists.ntp.isc.org
  https://lists.ntp.isc.org/mailman/listinfo/questions
_______________________________________________
questions mailing list
questions@lists.ntp.isc.org
https://lists.ntp.isc.org/mailman/listinfo/questions

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.

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

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@ntp.isc.org>
NTP Public Services Project - http://ntp.isc.org/

Riccardo Castellani wrote:

> But can I define slew rate and step rate ?

Please explain what "step rate" is in your mind.