Cc: alan@lxorguk.ukuu.org.uk, david@lang.hm, hancockr@shaw.ca,
       kyle@moffetthome.net, slashdot@jameshallam.info, goodgerster@gmail.com,
       mayer@ntp.isc.org, davidn@davidnewall.com, linux-kernel@vger.kernel.org,
       ntpwg@lists.ntp.isc.org, pretzalz@techhouse.org,
       burdell@iruntheinter.net, linasvepstas@gmail.com, nick@nick-andrew.net,
       jeff@kosowsky.org
Message-Id: <9981A902-73ED-44B9-87B5-803621BF307B@multicasttech.com>
From: Marshall Eubanks <tme@multicasttech.com>
To: "M. Warner Losh" <imp@bsdimp.com>
In-Reply-To: <20090108.080229.1159156694.imp@bsdimp.com>
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Subject: Re: [ntpwg] Bug: Status/Summary of slashdot leap-second crash on new years 2008-2009
Date: Thu, 8 Jan 2009 13:57:24 -0500
References: <496579C2.5050800@ntp.isc.org> <20090107.214220.-233694826.imp@bsdimp.com> <20090108104854.2dbc41b1@lxorguk.ukuu.org.uk> <20090108.080229.1159156694.imp@bsdimp.com>
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On Jan 8, 2009, at 10:02 AM, M. Warner Losh wrote:

> In message: <20090108104854.2dbc41b1@lxorguk.ukuu.org.uk>
>            Alan Cox <alan@lxorguk.ukuu.org.uk> writes:
> : > On FreeBSD, Solaris and Digital Unix, I'll point out, that jumping
> : > backwards is used, and has been used since at least 1994.  So  
> saying
> : > it isn't used in the world today is flat out wrong.
> :
> : I stand by my comment - when was the last time the IERS used a leap
> : second removal ? The code may exist but it doesn't happen.
>
> Jumping backwards is used for every leap second that IERS has ever
> done, which was your original comment.  There's has never been a case
> where there was a leap second for jump forward though.  The proper
> technical term here is 'negative leap second'.  All leap seconds up
> until now have been positive leap seconds, and it is unlikely there
> ever will be a negative one.

I disagree. In the 1970's, the excess LOD was as much as 3 msec.
After going down some, the mid 1990's it rose to around 2 msec.
Now, it is around 1 msec.

Here is a plot

http://www.iers.org/MainDisp.csl?pid=95-100

Only the long period variations count for leap seconds - the seasonal  
and other high frequency
oscillations tend to average out.

In the early part of the last century (~1905), it decreased by ~ 5  
msec in a year or so.
If that happened right now, it would go to ~ -4 msec negative, and we  
would be seeing
2 negative leap seconds or more per year. Even if the decrease from  
1975 to 1985 happened again, it
would be at -1 msec, and we would have a negative leap second every  
two years or so.

What is a reasonable assumption is that we would likely have a year or  
more warning of the
likelihood of a negative leap second.

Regards
Marshall Eubanks

>
>
> Warner
> _______________________________________________
> ntpwg mailing list
> ntpwg@lists.ntp.org
> https://lists.ntp.org/mailman/listinfo/ntpwg

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