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    SubjectRe: [PATCH 7/8] sched: Split accounting of NUMA hinting faults that pass two-stage filter
    On Fri, Jun 28, 2013 at 03:29:25PM +0100, Mel Gorman wrote:
    > > Oh duh indeed. I totally missed it did that. Changelog also isn't giving
    > > rationale for this. Mel?
    > >
    >
    > There were a few reasons
    >
    > First, if there are many tasks sharing the page then they'll all move towards
    > the same node. The node will be compute overloaded and then scheduled away
    > later only to bounce back again. Alternatively the shared tasks would
    > just bounce around nodes because the fault information is effectively
    > noise. Either way I felt that accounting for shared faults with private
    > faults would be slower overall.
    >
    > The second reason was based on a hypothetical workload that had a small
    > number of very important, heavily accessed private pages but a large shared
    > array. The shared array would dominate the number of faults and be selected
    > as a preferred node even though it's the wrong decision.
    >
    > The third reason was because multiple threads in a process will race
    > each other to fault the shared page making the information unreliable.
    >
    > It is important that *something* be done with shared faults but I haven't
    > thought of what exactly yet. One possibility would be to give them a
    > different weight, maybe based on the number of active NUMA nodes, but I had
    > not tested anything yet. Peter suggested privately that if shared faults
    > dominate the workload that the shared pages would be migrated based on an
    > interleave policy which has some potential.
    >

    It would be good to put something like this in the Changelog, or even as
    a comment near how we select the preferred node.


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