Date: Wed, 7 Jan 2009 19:31:27 +0000
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: "M. Warner Losh" <imp@bsdimp.com>
Cc: mayer@ntp.isc.org, linasvepstas@gmail.com, david@lang.hm, hancockr@shaw.ca,
       kyle@moffetthome.net, slashdot@jameshallam.info, goodgerster@gmail.com,
       davidn@davidnewall.com, linux-kernel@vger.kernel.org,
       ntpwg@lists.ntp.isc.org, pretzalz@techhouse.org,
       burdell@iruntheinter.net, nick@nick-andrew.net, jeff@kosowsky.org
Subject: Re: [ntpwg] Bug: Status/Summary of slashdot leap-second crash on
 new years 2008-2009
Message-ID: <20090107193127.0bec8ad8@lxorguk.ukuu.org.uk>
In-Reply-To: <20090107.103947.1324582654.imp@bsdimp.com>
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	<3ae3aa420901050942y56f0ecdei39c091a73e49c1fd@mail.gmail.com>
	<49642674.9080703@ntp.isc.org>
	<20090107.103947.1324582654.imp@bsdimp.com>
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> time, causing time to jump backwards by 1s (or violate POSIX time_t's
> invariant that midnight time_t is % 86400 == 0).  This jump backwards
> is a pita in the kernel, and violates the assumption that many
> programs have that time doesn't flow backwards.

They can slew the clock slowly as well. There is a wonderful quote from
one of the summaries of the POSIX committee discussions on time that says
quite simply "the posix clock is not guaranteed to be accurate"

As it currently stands the kernel contains sufficient support that at the
point you know a leap second is coming you can adjust the second length
marginally over the entire period.

The current behaviour is an implementation decision. Jumping on a second
shouldn't be an issue to most people, jumping back is asking for badness
but isn't in fact used in the world today. Slewing the entire day so that
each second is 1/86400 of a second longer or shorter wouldn't be noticed
by anyone.

Alan
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