[ntp:questions] Google and leap seconds

Harlan Stenn stenn at ntp.org
Thu Sep 22 02:15:20 UTC 2011


Bill wrote:
> On 2011-09-21, Harlan Stenn <stenn at ntp.org> wrote:
> > Bill wrote:
> >> On 2011-09-21, Harlan Stenn <stenn at ntp.org> wrote:
> >> > Bill wrote:
> >> >> Some operating systems (eg Linux) have the ability to do a much faster
> >> >> than 500PPM rate change (100000PPM in the case of Linux) but ntp does
> >> >> not make use of that. 
> >> >
> >> > ... because it would violate the assumptions and rules about the loop
> >> > behavior (I'm painting with a wide brush here).  If you want a system
> >> 
> >> ? But that 500PPM was an arbitrary and random value chosed X years ago. 
> >
> > "Arbitrary and random" sounds like Spin to me.
> >
> > It was several sigmas bigger than the expected worst-case "acceptable"
> > clock drift.
> 
> And it could have been 1000 PPM or 2000 AFAIK, and it would not have
> made any difference.

At some levels, sure.  But a step is an indication that something is
*wrong* and it is a perfectly reasonable and normal design decision to
state that the clock freq is within some given range, and once it is
there one should *only* see a step if there is a problem.

And if it takes 2000 seconds to add in a slewed leap-second at 500ppm,
it would take 1000 seconds at 1000ppm, and 500 seconds at 2000ppm.
Arbitrary numbers again, or are there specific constraints you are
trying to hit by changing that value?

The POSIX folks made a choice about leap seconds, and that choice was
clearly sub-optimal, and now we're dealing with that friction.

>> What are the effects of a different (presumably larger) value going to
>> have on the behavior of the loop?
> 
> Why should it have any effect? steps do not appear to have any effect. 

Statements like that cause me to wonder about you.

I suck at filter/oscillator theory and even I know that one cannot make
arbitrary changes to such a core value and expect the framework to
simply continue to work.

>> What will the effect be on other systems that "associate" with these
>> boxes that have the faster rate?
>>
>>>> that is designed to work with a faster slew rate that's fine, but then
>>>> you also have to consider legacy and interoperability issues.
>>> 
>>> sure. But what legacy issues do you have in mind? 
>>
>> machines that operate at the 500ppm limit that are now talking to
>> machines that operate at this new higher rate.
> 
> Clearly if the machine is at the end of the chain, it should not
> matter.

It could trigger steps where they would not have happened before.  This
could cause time islands and oscillations.

NTP is robust.  What you are currently advocating seems to fly in the
face of robust behavior.

What is your goal?

> If it operates as a server, the 500PPM system will fall behind until
> the rate drops and then will catch up. Or it will have an offset
> greater than .128ms and step.

But you are imposing your policy on others, apparently with no
justification for this change, and with no regard for the effects it
will have on their "systems".

>> Applications that "knew" that if a time correction was going to
>> happen, it would be at the 500ppm rate - this goes to data
>> correlation and timestamp sequencing.
> 
> But ntpd is allowed to step, so it has to assume that an infinte rate is
> possible. 

So what?  Steps SHOULD NOT HAPPEN.  If they do, something is WRONG.

Find the problem and fix it.

And when ntpd steps, it changes its "state" to accomodate that rate change.

>> That's just 2 legacy issues off the top of my head.
>>
>>> > It also goes to timestamp correlation issues.
>>> 
>>> It would seem to me that a step (infinite PPM) would be far far worse.
>>> ntpd does NOT limit the slew rate to 500PPM. It limits it to infinite
>>> PPM. 
>>
>> And when it does it logs the event, so it is still traceable and
>> auditable.  It's also an indication of "something is wrong, as this
>> should not have happened."
> 
> But your questions where what happens to other systems. a drift of
> 500PPM for an hour is surely just as bad. 

Why do you say 1 hour?  Why do you maintain it "is surely just as bad?"

Exactly what problem(s) are you solving?

H



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