[ntp:questions] Linux 11-minute mode (RTC update)

Maciej W. Rozycki macro at linux-mips.org
Wed Apr 9 20:19:55 UTC 2008

On Wed, 9 Apr 2008, Serge Bets wrote:

> Granted, the eleven-minutes mode is bad. But not *so* bad. It sets the
> RTC tick in sync with real seconds, with a random error of maximum
> 7 milliseconds.

 The worst problem with the automatic RTC update happens on some Linux 
platforms that use the RTC periodic interrupt as the source of the system 
timer interrupt.  Updating the RTC causes a considerable disruption of the 
interrupt -- the next one after the update is shifted by an amount of time 
corresponding to the offset between the moment the timer interrupt to 
trigger the update happens and the moment the half of a second of calendar 
time falls -- breaking the accuracy of timekeeping badly.

 Of course the same happens on these systems when `hwclock --systohc' is 
called, but that at least can be controlled by the system administrator 
and for example run only on shutdown of the affected systems when the 
rough accuracy is perfectly fine.


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