[PLUG] automatic running of ntpdate

Denis Heidtmann denis.heidtmann at gmail.com
Thu Dec 3 05:15:45 UTC 2009


On Wed, Dec 2, 2009 at 5:55 PM, Mike Connors <mconnors1 at gmail.com> wrote:
> Denis Heidtmann wrote:
>> This AM the log showed an offset of 0.366 seconds, so it looks like
>> the hardware clock was reset.  Is it possible that the hardware clock
>> is reset (on shutdown) to the nearest second?  That would account for
>> the offsets bouncing around, but staying under 1 second.
> Here's an excellent article on system clock, hardware clock, and NTP.
> https://wiki.ubuntu.com/HardwareClock
>
> What I gleaned from it:
> The OS stores(shutdown) and retrieves(bootup) system time from the hardware clock.
> At bootup after the system clock is set per the hw clock, it's going to query an NTP
> server via ntpdate for a more accurate time setting.
>
> Although the hw clock can keep time, it's not as accurate as any level of stratum (1-4) clock.
> It "free runs" w/o a reference source. So I would expect small stepping like this to occur,
> especially if you're only syncing the time once a day.
>
> * Interesting factoid: A GPS receiver can be used as a Stratum 1 clock source.

Data:  ntpdate runs during boot and logs an offset of something from
.3 to .9 seconds.  A short time after boot runing ntpdate from the
command line reports an offset of .00##--something close to zero.
Shutdown and reboot shortly later, ntpdate runs during boot and logs
an offset of something from .3 to .9 seconds. Repeat as desired.

I take this to mean that 1) the hardware clock was not synchronized
better than 1 second on shutdown, or 2) the hardware clock gets bumped
during the shutdown/reboot cycle, or 3) the hardware clock is unstable
in the extreme, or 4) the system clock gets set to the rtc to within 1
second on boot.  1) and 4) seem to me to be effectively the same
thing.

I vote for 4), because I find in the logs:
Dec  2 17:58:46 R2D4 kernel: [    3.906800] rtc_cmos 00:08: setting
system clock to 2009-12-03 01:58:34 UTC (1259805514)

If it were set more accurately than the nearest second why would it
not show in the log?

-Denis



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