On 2011-09-20, Marco Marongiu <[email protected]> wrote: > Hello > > An official Google blog post[1] drove me to this question: if the OS > implements the kernel discipline, and we have a positive leap second, do > we expect that the time will step back one second when the leap second > is inserted? > > The question arose after mumbling a bit about the blog post. > > In the post, a Google engineer says that: "Computers traditionally > accommodate leap seconds by setting their clock backwards by one second > at the very end of the day."
No they do not. They insert an extra second ( second number 60) into the time stream. Ie, the last second of the month becomes 23:59:60 rather than 23:59:59, as you state below. While ntp eventually steps the clock if the system does not understand leap seconds, chrony will slew the clock to gain the extra second. > > By the way, in the NTP FAQ[2] I read: "If the operating system > implements the kernel discipline [...] the kernel will handle the leap > seconds without further action necessary. If the operating system does > not implement the kernel discipline, [...] the situation will be handled > just like an unexpected change of time: [...] eventually ntpd will step > the time." > > So I expect that OSs implementing the kernel discipline will go through > > 23:59:58 -> 23:59:59 -> 23:59:60 -> 00:00:00 > > in case of a positive leap second, and > > 23:59:58 -> 00:00:00 -> 00:00:01 > > in case of a negative one. This shouldn't have any negative effect on > applications, unless they are designed to always expect second 00 to > follow 59. > >>From Dr.Mills' pages[3] I understand that the kernel discipline is > implemented at least in "Solaris, Tru64, FreeBSD and Linux, and possibly > others". > > Now, I don't know what OSs Google is referring to in that blog post, but > I'd expect it is one of those listed above. And they are saying they had > to do something because in 2005 their computer clocks stepped back one > second and screwed their apps. > > That's why I think that I am missing something, and hence the question > above: are leap seconds supposed to be managed by stepping back the > clock when using NTP? Or wasn't it that the OS at the time of the fault > didn't support the KD, or had a faulty management of the leap seconds? > Or, maybe, it was their apps's fault? Maybe they were running windows? > > Can anyone help me understand? > > Thanks in advance > > Ciao > -- bronto > > [1] > http://googleblog.blogspot.com/2011/09/time-technology-and-leaping-seconds.html > > [2] http://www.ntp.org/ntpfaq/NTP-s-algo-real.htm#AEN2499 > > [3] http://www.eecis.udel.edu/~mills/ntp/html/extern.html _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
