[ntp:questions] drift modeling question

Martin Burnicki martin.burnicki at meinberg.de
Mon Jul 21 14:29:09 UTC 2008

Unruh wrote:
> Martin Burnicki <martin.burnicki at meinberg.de> writes:
>>David and Hal,
>>David Woolley wrote:
>>> Hal Murray wrote:
>>>> Most PCs have 2 xtals.  One at 14.xxx MHz (cheap, 4X color burst)
>>>> that drives the CPU and most motherboard logic through a magic clock
>>>> generator (PLL) chip, and another that is a 32 KHz watch crystal for
>>>> keeping time when the CPU is off.  The latter also makes interrupts
>>>> for the scheduler.
>>> Historically interrupts from the 32kHz clock have not been used, except,
>>> possibly, in powered down states to initiate a restart from suspend or
>>> hibernate.  It is possible that has changed very recently, but they
>>> certainly weren't used historically.
>>About which operating system(s) are you talking?
>>The PC's standard RTC chip can certainly generate cyclic interrupts.
>>However, if a cyclic interrupt from the RTC or from another timer chip is
>>used to drive the scheduler depends on the type and eventually on the
>>version of an operating system, isn't it?
>>So one system may be using the RTC's interrupts and another one may not.
> So the question is, do you know of any operating systems which use the RTC
> to drive the scheduler?

No, I just found the statements to which I did respond very general. 

One person claimed the RTC interrupt was used by the scheduler, and the
other one claimed it is not used at all by the scheduler, but none of them
mentioned the scheduler of which OS they were talking.

Since the RTC's interrupt output is wired to the PC's interrupt controller
it's just a matter of the implementation whether the RTC interrupt is used
by a scheduler, or not. E.g. some old Linux version could have done so
whereas recent versions don't.
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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