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    SubjectRe: [2.4.17/18pre] VM and swap - it's really unusable
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    On Tuesdayen den 8 January 2002 21.13, Alan Cox wrote:
    > > low-latency kernel". Now, IF we can come to this decision, then
    > > internal preemption is the way to do it. But it affects ALL kernel
    >
    > The pre-empt patches just make things much much harder to debug. They
    > remove some of the predictability and the normal call chain following
    > goes out of the window because you end up seeing crashes in a thread with
    > no idea what ran the microsecond before
    >
    > Some of that happens now but this makes it vastly worse.
    >
    > The low latency patches don't change the basic predictability and
    > debuggability but allow you to hit a 1mS pre-empt target for the general
    > case.
    >

    Yes, it does make things much much harder to debug - but:
    * If you get a problem on a preemtive UP kernel, it is likely to be a problem
    on a SMP too - and those are hard to debug aswell. But the positive aspect
    is that you get more people that can do the debugging... :-)
    (One CPU gets delayed with handling a IRQ the other runs into the critical
    section)
    * It is optional at compile time.
    And could even be made run time optional / CPU ! Just set a too big value
    on the counter and it will never reschedule...

    /RogerL

    --
    Roger Larsson
    Skellefteå
    Sweden
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