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

From: Venkatesh Pallipadi (venkatesh.pallipadi_at_intel.com)
Date: 07/15/05

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    Date:	Fri, 15 Jul 2005 10:23:49 -0700
To: Andi Kleen <ak@suse.de>

    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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