Re: [Ntp] Antw: [EXT] Re: CLOCK_TAI (was NTPv5: big picture)

[Martin Burnicki <[e-mail]>]

Doug Arnold wrote:
> When I first heard of leap seconds I thought it was a terrible idea, a clumsy hack.  Since then I've come to understand that database and other application programmers are not timing people and often don't handle the possibility of leap seconds, or don't handle it correctly.  I believe that Google and other large data center companies have figured out that it causes less problems to mess up the time for a while than to have leap seconds.

With the systems (and specifically the *applications*) we have now, you
have 2 different choices when a leap second is inserted:

a) Let the kernel step the time back by 1 s, so applications may observe
duplicate time stamps. This can only be avoided if each application that
is possibly affected is aware of leap seconds, e.g. on Linux systems it
could periodically query the kernel if a leap second has been announced,
and then try to handle it properly.

The advantage of this approach is that the system time can be very
precise *except* during the leap second.


b) The system time can be smeared around a leap second, in which case an
application doesn't even notice there is a leap second, so there is no
problem with time steps back.

The disadvantage of this approach is that the system time is off by up
to 1 s if you smear the time completely before or after the leap second,
or up to +/- 0.5 s if you do symmetric smearing before and after the
leap second.


If you can accept such a time offset (which is huge, compared to the
normal millisecond offset) depends on the application.

In any case this is a dilemma that can't be fixed by simply using TAI
time stamps instead of UTC time stamps in a protocol.


Martin
-- 
Martin Burnicki

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