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    SubjectRe: [PATCH] Load balancing problem in 2.6.2-mm1


    Martin J. Bligh wrote:

    >>>If CPU 8 has 2 tasks, and cpu 1 has 1 task, there's an imbalance of 1.
    >>>*If* that imbalance persists (and it probably won't, given tasks being
    >>>created, destroyed, and blocking for IO), we may want to rotate that
    >>>to 1 vs 2, and then back to 2 vs 1, etc. in the interests of fairness,
    >>>even though it's slower throughput overall.
    >>>
    >>>
    >>Yes, although as long as it's node local and happens a couple of
    >>times a second you should be pretty hard pressed noticing the
    >>difference.
    >>
    >
    >Not sure how true that turns out to be in practice ... probably depends
    >heavily on both the workload (how heavily it's using the cache) and the
    >chip (larger caches have proportionately more to lose).
    >
    >As we go forward in time, cache warmth gets increasingly important, as
    >CPUs accelerate speeds quicker than memory. Cache sizes also get larger.
    >I'd really like us to be conservative here - the unfairness thing is
    >really hard to hit anyway - you need a static number of processes that
    >don't ever block on IO or anything.
    >
    >

    Can we keep current behaviour default, and if arches want to
    override it they can? And if someone one day does testing to
    show it really isn't a good idea, then we can change the default.

    I like to try stick to the fairness first approach.

    We got quite a few complaints about unfairness when the
    scheduler used to keep 2 on one cpu and 1 on another, even in
    development kernels. I suspect that most wouldn't have known
    one way or the other if only top showed 66% each, but still.

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