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    SubjectRe: [PATCH] dynamic tick patch
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    On Wed, 2005-01-19 at 15:45 -0800, john stultz wrote:
    > On Thu, 2005-01-20 at 00:26 +0100, Thomas Gleixner wrote:
    > > On Wed, 2005-01-19 at 14:59 -0800, George Anzinger wrote:
    > > > I don't think you will ever get good time if you EVER reprogramm the PIT.
    > >
    > > Why not ? If you have a continous time source, which keeps track of
    > > "ticks" regardless the CPU state, why should PIT reprogramming be evil ?
    >
    > That's a big if. The problem is that while the PIT has its problems
    > (such as lost ticks), it runs at a known frequency and is reasonably
    > accurate. Time sources like the TSC have the problem that it doesn't run
    > at a known frequency, and thus we have to calibrate it (usually using
    > the PIT). Unfortunately this calibration is not extremely accurate
    > (George can go on to the reasons why), which causes the TSC to be a poor
    > stand alone time source.
    >
    > That said, the PIT is a poor time source as well, as it does loose ticks
    > and is very slow to access. ACPI PM and HPET are better as they don't
    > have the lost tick problem, but they are still off chip and slower to
    > access then the TSC.

    And they aren't available on every board - especially not on embedded
    ones.

    > For an example of your ideal continuous timesource, check out the
    > timebase on PPC/PPC64. Other arches also have similar well behaved time
    > hardware.

    Yes, I'm aware of that. Unfortunately we live in the x86 universe.

    tglx


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