RE: [PATCH] i386: Selectable Frequency of the Timer Interrupt

Maciej W. Rozycki
2005-07-15T09:39:58Z

On Thu, 14 Jul 2005, Brown, Len wrote:

> >Of course using APIC internal timers is generally the best idea on SMP,
> >but they may have had reasons to avoid them (it's not an ISA interrupt,
> so
> >it could have been simply out of question in the initial design).
>
> Best? No.
>
> Local APIC timers are based on a clock which on many processors will
> STOP when the processor enters power saving idle states, such as C3.
> So the LAPIC timer will not accurately reflect how much time
> has passed across entry/exit from idle.

That's an APIC bug. When Intel originally released the APIC (some
thirteen years ago) they stated it should be used as a source of the timer
interrupt instead of the 8254. There is no excuse for changing the
behaviour after so many years. So if you are on a broken system, you may
want to work around the bug, but it shouldn't impact good systems.

Maciej