[ntp:questions] Reference clock driver for /dev/rtc

Rob nomail at example.com
Sat Jun 26 14:49:49 UTC 2010

unruh <unruh at wormhole.physics.ubc.ca> wrote:
> On 2010-06-26, David Woolley <david at ex.djwhome.demon.invalid> wrote:
>> unruh wrote:
>>> Linux, depending on the setting of a flag in the adjtimex setup, sets
>>> the rtc from the system time once every  11 min. . This is a disaster if
>> I think you missed Dave Mills' point that ntpd does this every 60 
>> minutes, so will also break mechanisms for compensating for RTC drift 
>> whilst the processor is powered down.
> His point seemed to be that a) He did not know aboutthe Linux 11 min
> mode (a statement that may have been a rheotorical device) and b) that
> ntp did it every 60 min. I do hope that ntp's mechanismcan be switched
> off by some option. That 60 min mode is as silly as the 11 min mode
> (except giving the rtc more time to drift if wha you want is to minimize
> the rtc offset while the computer is running and rtc is never used.)
> Of course this is in the context of VM using the rtc driver to read the
> underlying system clock, a misuse of the rtc concept IMHO, and either
> mode would be a disaster if the rtc driver on the VM actually had
> permission to change the system clock on the underlying real machine.

Of course it hasn't!  The VM monitor will either ignore setting of the
RTC, or it will keep an offset between host time and the VM time and
change that when the guest attempts to change the RTC.
(so that the guest sees the effect of its operation like it would see in a
physical machine)

You seem to be ignorant about the problems of keeping time in a
virtual machine.  Many techniques used in modern operating systems
simply don't virtualize very well, making it very difficult to keep
accurate time in a VM.  You should not downplay an effort to serve
correct time to a VM without understanding all the problems.

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