Date: Sat, 7 Dec 2013 05:02:06 +0100 From: Szabolcs Nagy <nsz@...t70.net> To: musl@...ts.openwall.com Subject: Re: [PATCHv2] Add support for leap seconds in zoneinfo files * Rich Felker <dalias@...ifal.cx> [2013-12-06 21:31:14 -0500]: > On Fri, Dec 06, 2013 at 11:29:33PM +0000, Laurent Bercot wrote: > > On 06/12/2013 11:38, Raphael Cohn wrote: > > >Date and time match has been got wrong in every system > > >(...) > > >Personally, I think apps should just use a monotonic source of seconds > > > from an epoch, and use a well-developed third party lib dedicated to > > > the problem if they need date math (eg Joda time in Java). > > > > I absolutely agree with you on the first part. I disagree on the second > > part. Dealing with time shouldn't be a burden on the application - devs > > have other things to think about, and experience shows that most of them > > won't care, they'll just use the primitives provided by the system. So, > > the system should do the right thing, i.e. provide something that works > > no matter how applications are using it. Here, it means providing a > > linear CLOCK_REALTIME, because people use it as if it were CLOCK_MONOTONIC. > > If that's your only goal, it's easily achieved without storing TAI-10 > in the CLOCK_REALTIME clock, assuming by "linear" you just mean > "monotonic, continuous, and accurate within the margin of measurement > error". Discontinuous jumps and non-monotonicity in CLOCK_REALTIME > were a monstrosity invented by the NTPD folks in their implementation > for mapping TAI onto POSIX time, not any fundamental requirement. it's worse than that, the proposed solution does put a burden on userspace (unix time can no longer be used to calculate or represent dates) while with the standard 1 day == 86400 s apps and admins do not even need to know about leapseconds (only the time daemon and kernel) about linearity: if you only smooth out a leapsecond in the month it was inserted, then a unix second would be about 30*86400/(30*86400+1)-1 = -0.386 ppm (= 386ns) shorter than a SI second in that month, by comparision (according to the ntp clock quality faq) the oscillator used by most hw usually have more than 1 ppm frequency error and it can change 1 ppm/C when heated up if no temperature correction is done, so apps already have to deal with larger errors and the smoothing logic is in the kernel/ntpd they just don't apply it to leapseconds this will work for more than 1000 years (earth rotation drift is <12s/year) and somewhere during that time leapseconds will be abolished and it stops being a problem..
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