Paul Eggert <eggert@cs.ucla.edu> wrote:
With leap smearing, the ideal clock is at most 0.5 s off UTC, as opposed to being at most 1.0 s off with strict POSIX time.
It's a bit more complicated than that :-) There are a number of variations of leap smear. The ones that centre the smear interval on the leap second have an up-to-0.5s divergence from UTC. The ones that have the whole smear interval before (or after) the leap second have an up-to-1s divergence from UTC. If I understand the ntpd documentation, they have implemented a configurable smear interval that occurs before the leap second. If you have a traditional ntp setup without kernel leap second support, ntpd will react to a leap second by slewing the clock after it realises it is one second wrong, when it next polls its servers after the leap second. This is effectively a leap smear after the second, implemented as a side-effect of ntp's sync algorithms rather than being deliberately specified. Ops people have often configured ntpd to work this way rather than using kernel leap second support in order to avoid the clock appearing to go backwards, which frequently causes enormous consternation for sensitive software. Explicit leap smear is a more principled way to avoid this kind of upset, rather than old-ntp-style smear-as-a-side-effect. Tony. -- f.anthony.n.finch <dot@dotat.at> http://dotat.at/ - I xn--zr8h punycode East Sole, Lundy, Fastnet, Irish Sea: Southeasterly 4 or 5, veering southerly 5 or 6. Slight or moderate. Occasional drizzle later. Moderate or good, occasionally poor later.