[ntp:hackers] First sample after f15 minutes used. Really?

Rob Neal hundoj at comcast.net
Sun Sep 9 23:58:24 UTC 2007


The IBM Mainframe 9037 Sysplex Timer is dead, long live the new regime:

http://www.redbooks.ibm.com/abstracts/sg247280.html?Open

Maybe someday it will even learn to listen to NTP...


Rob

On Sun, 9 Sep 2007, David L. Mills wrote:

> Brian,
>
> You mean you actually read the documentation? Darn, not many people do
> that.
>
> Who do love, the TOY chip or a neighborhood server? There is code in the
> ntp_util.c source, but it is not clear it works with all machines. There
> are different opinions on this. Some set the TOY once an hour when
> disciplined, some set it on shutdown and so forth. All I can say is that
> if the TOY is not within several minutes at startup, somebody should
> know about it. You can of course use the -g option to set the time even
> if outside step or panic; that's what cisco routers do. The question is
> whether to set the TOY on a step as well.
>
> You don't ever want to set the TOY equivalent on an IBM mainframe. The
> operator would need to shut all applications down first and then have a
> tug of war with the 9037 Sysplex Timer. Also for grins, there is in fact
> a SHARE program that operates as an NTP server, but no program available
> that operates as an NTP client. I cut my teeth on the IBM 7090 and later
> the IBM System/360 and so am not surprised at all about that.
>
> Dave
>
> Brian Utterback wrote:
>
> > Yes, that is what I was saying, that the standard mitigation
> > algorithms are still in effect. I
> > thought that the wording in the docs made it sound like this was not
> > the case. I read the doc as
> > saying that the first sample after 15 minutes (i.e. from the packet
> > currently being processed) would
> > be accepted, not the offset calculated after the mitigation and
> > selection algorithms.
> >
> > Another thing, in the html man page for ntpd, it says that if the TOY
> > chip is not working,
> > then ntpd exits. Here is the text:
> >
> > "In case there is no TOY chip or for some reason its time is more than
> > 1000 s"..."exit
> > with a panic message to the system log"
> >
> > Now, of course we are all aware of the panic threshold for the
> > calculation of the offset, but
> > I am not sure what is meant here about the TOY chip.  If there is no
> > TOY chip ntpd exits?
> > That doesn't seem right.
> >
> > David L. Mills wrote:
> >
> >> Brian,
> >>
> >> You might misunderstand the purpose of the scheme. First, the
> >> mitigation is done before the threshold is checked, so presumably we
> >> have the best candidates to wiggle the clock. Although a rogue sample
> >> or two might exceed the step threshold, it's highly likely a good
> >> sample will come along and reset the stepout counter.
> >
> > [snip]
> >
> >>
> >> Dave
> >>
> >> Brian Utterback wrote:
> >>
> >>
> >>
> >>> To answer my own question, the wording isn't exactly right. What
> >>> happens is that any offset greater
> >>> than 128ms is ignored for 15 minutes. If it has been over 15 minutes
> >>> since the last clock update, then
> >>> the offset (Still calculated by the normal method) is allowed. So,
> >>> the standard clustering and combining
> >>> is still in effect.
> >>>
> >>>
> >>> Brian Utterback wrote:
> >>>
> >>>
> >>>
> >>>> (Resending, including Dave this time.)
> >>>>
> >>>>
> >>>> In the documentation for ntpd, it says:
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>> The ntpd  algorithms  discard  sample
> >>>>>      offsets  exceeding  128 ms, unless there is no sample offset
> >>>>>      of less than 128 ms for 15 minutes. The first  sample  after
> >>>>>      that,  no  matter  what  the  offset, steps the clock to the
> >>>>>      indicatedA time.
> >>>>>
> >>>>
> >>>> Is this really true? The first sample after 15 minutes is used,
> >>>> even if
> >>>> it is a lone wolf? What if you have 5 servers and 4 of them give an
> >>>> offset of 2 seconds and one has an offset of 10 seconds. If the 10
> >>>> second guy happens to be the first one after 15 minutes of such
> >>>> samples, he gets used? This seems wrong to me.
> >>>>
> >>>> Brian Utterback
> >>>>
> >>>> _______________________________________________
> >>>> hackers mailing list
> >>>> hackers at lists.ntp.org
> >>>> https://lists.ntp.org/mailman/listinfo/hackers
> >>>> _______________________________________________
> >>>> hackers mailing list
> >>>> hackers at lists.ntp.org
> >>>> https://lists.ntp.org/mailman/listinfo/hackers
> >>>>
> >>>
> >>>
> >>
> >>
> >> _______________________________________________
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> >> hackers at lists.ntp.org
> >> https://lists.ntp.org/mailman/listinfo/hackers
> >>
> >
> >
>
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