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

Unruh unruh-spam at physics.ubc.ca
Wed Apr 9 23:53:39 UTC 2008

"Maciej W. Rozycki" <macro at linux-mips.org> writes:

>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.

WHAT? What linux platform uses the "1 interrupt per second" of the rtc
clock as the timer interrupt? (never mind that the latest motherboards do
not have an rtc at all, and use the HPET in poll mode to mimic an rtc).

> 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.

>  Maciej

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