[ntp:questions] Google and leap seconds
"terje.mathisen at tmsw.no" at ntp.org
Tue Sep 20 17:25:33 UTC 2011
Dave Hart wrote:
> As I understand it, all POSIX ntpd will step backward one second to
> accomplish a leap second insertion (we've yet to see a deletion).
> Windows ntpd differs, and is closer to Google's smeared timescale in
> spirit. Leap seconds are inserted by Windows ntpd by slewing the
> clock for 2 seconds, that is, the clock is run at half speed for two
> seconds. The Windows ntpd code doesn't yet accommodate leap second
> deletions. The advantage of this approach is time moves unceasingly
> forward. The disadvantage, particularly with such a short-lived
> smear, is that interval timing that starts or ends during the special
> two seconds will be inaccurate by up to a second.
Google's hack is to use a cos() function to smear the time delta, this
means that as long as they do the smearing over a sufficiently long time
period all client systems would stay in sync from start to end.
The key requirement is to keep the frequency delta well below the 500
ppm which is the maximum ntp slew rate, and since many systems need
50-200 ppm under stable conditions, I'd like to limit the additional
delta to 100 pm or so.
This would require 10000 seconds with linear slew, but lead to frequency
steps at both ends. Since Google used a cos() function they avoid any
step functions, but need to increase the time period by pi/2.
15.7 K seconds is still only 4+ hours, so if Google used the last 12
hours of the day they would keep the slew rate at around 35 ppm.
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"
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