On Fri, Jul 15, 2005 at 07:02:24PM +0200, Andi Kleen wrote:
>
> At least on multi processor systems LAPIC has to work anyways (otherwise
> you cannot schedule other CPUs), so it is fine to use there.
>
> AFAIK there are no x86 CPUs right now that do both C3
> and SMP. If they ever do then they will need to keep the
> LAPIC ticking in C3.
>
> This has nothing even to do with advanced power saving,
> but is pretty much a hard requirement for Linux (and I would
> be surprise if it wasn't one for other OS too). Without it
> scheduling and local timers on APs will not work at all.
>
> In theory it could be replaced with HPET if HPET had enough banks (one
> for each CPU - most implementations today usually only have 2 or 4), but
> that would severly limit scalability for lazy tick schemes because
> they would depend on a common resource in the southbridge. Also the
> max number of banks needed on a big system would be huge
> (128? 256?) because you couldn't have more CPUs than that.
>
> With PIC only it's absolutely impossible.
>
I wouldn't say it is totally impossible. There are ways in which Linux can work
without a reliable Local APIC timer. One option being - make one CPU that gets
the external timer interrupt multicast an IPI to all the other CPUs that
wants to get periodic timer interrupt.
Thanks,
Venki
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