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

Unruh unruh-spam at physics.ubc.ca
Thu Jul 24 18:51:03 UTC 2008

Serge Bets <serge.bets at NOSPAM.laposte.invalid> writes:

> On Thursday, July 24, 2008 at 1:48:40 +0000, Bill Unruh wrote:

>> Apparently the HPET disables the rtc interrupts and takes them over.

>Those HPET emulated RTC interrupts indeed seem to be nothing else than
>unusable rubbish, by design. They introduce an unacceptable level of
>error in the measures. Error 4 orders of magnitude greater than what's
>reasonably expectable from proper RTC interrupts.

Three orders. (avg 8ms vs about 20usec) but yes, that is pretty terrible. 

>> No amount of care in the hwclock program can get around this kind of
>> nonesense.

>Exactly. Disabling HPET seems the only solution to have proper RTC
>interrupts. Alternatively tell hwclock to *not* use interrupts, via
>either the --directisa or --nointerrupt options. This is less elegant,
>and costs many processor cycles. But accuracy should be good.

Reading the HPET spec and the hpet.c code in the kernel it would seem to me
that you could disable the legacy behaviour while keeping the hpet for high
speed timing/cpu clock/tickless operation. 

>Serge point Bets arobase laposte point net

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