[ntp:questions] Leap second to be introduced in June

Martin Burnicki martin.burnicki at meinberg.de
Tue Jan 13 11:56:19 UTC 2015

Marco Marongiu wrote:
> On 12/01/15 11:48, Martin Burnicki wrote:
>> Fortunately Dave Hart had some time to have a closer look at this, and
>> fix it for 4.2.6, so unless something has been broken again in the mean
>> time it should be fixed in 4.2.6 and later, and should work correctly.
> Let me understand: you mean that in 4.2.6 ntpd will slew down the clock
> by two seconds on systems where you can't notify the kernel?

No. This is *only* in the Windows-specific code of ntpd. Summary:

1.) If the OS kernel supports it then ntpd just passes the leap second 
warning down to the kernel, and the kernel handles it depending on how 
it has been implemented in the OS type and version.

2.) If the OS kernel doesn't support this then ...

2a.) non-Windows versions 4.2.6 and newer step the OS time back by 1 
second at the leap second event time (don't know from top of my head if 
at the beginning or end of the leap second, would have to check the 
source code).

Ntpd versions prior to 4.2.6 would simply do nothing to handle the leap 
second. They would just observe a sudden 1 second offset after the leap 
second, and step the time as usual after some minutes.

2b.) Windows versions after our xmas edition from December 2005 slew the 
Windows system time over 2 seconds to account for the leap second. 
Specifically, this is in the official stable NTP code since NTP v4.2.4.

> If so, does it also mean that it would do the same when you disable the
> kernel discipline by adding a disable kernel in ntp.conf?

If I remember correctly then this code depends on a preprocessor symbol, 
which means it's compiled with or without support for the kernel API. 
Not sure what happens if it's compiled for kernel PLL but kernel PLL is 
disabled at runtime.

I'll try to figure this out, though.

>> I'm planning to do some testing soon to verify this.
> If you shared the results of your testing when you're done, that would
> be great ;-)

Yes, of course.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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