On Wed, Jul 4, 2012 at 6:11 PM, Leonid Isaev <lisaev@umail.iu.edu> wrote:
On Wed, 04 Jul 2012 22:06:27 +0100 Mauro Santos <registo.mailling@gmail.com> wrote:
From data I have access to, taken from machines running ntpd, I can say the following about the drift in PPM stored in ntpd's drift file:
my laptop: -9.699 machine 1: -8.762 machine 2: -443.266 machine 3: -35.417
Machine 1 is the newest and machine 3 is the oldest.
AFAIK comparing RTCs on different machines is meaningless because there seem to be no quality handle for RTC. So, RTC may exist or not, but you can't choose (and therefore guarantee) to have a good RTC, at least on consumer (not server) motherboards. NTPD on the other hand implements a protocol $\equiv$ standard.
FWIW, my driftfile yields +6.982ppm which I guess is good compared to yours.
Re machine 2 -- I bet your CMOS battery is dying.
per NTP FAQ, as basis for estimate: http://www.ntp.org/ntpfaq/NTP-s-sw-clocks-quality.htm#AEN1220 "[...] I'll simply state that 12 PPM correspond to one second per day roughly." ... actual value is (1/86400)*1000000 = 11.574. -9.375 = local server (never off/moved) -22.538 = docked laptop (never off/moved) -65.089 = remote server (linode.com [xen]) so my local clocks run a wee bit hot (and xen clocksource sucks?). since kerberos clockskew defaults to 300 seconds, i would begin to perm fail authentication (local -> remote server) after ... p = ppm, s = sec, d = day s / ( p / ( p / ( s / d ) ) ) s / ( p * ( ( s / d ) / p ) ) s / ( s / d ) s * ( d / s ) d skew-sec-max / ( ABS(skew-ppm-var) / skew-ppm-per-sec-per-day ) 300 / (( 65 - 9 ) / 12 ) ... 62.32 days, or ~2 months [annoying] ... yay for NTP! ... (and also math! if it's correct!) clearly i went a bit overboard, but it seems like the only thing you can depend on is RTC failing you 0-12 times-per-year, depending on your gear and sensitivity to time variance. although, it would be cool if authenticated NTP were more widespread/ubiquitous -- a la Windows time service [signed packets] -- but since i've never cared much about it i'm probably just not aware ... -- C Anthony