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    SubjectRe: [RFC] respect the referenced bit of KVM guest pages?
    On Thu, Aug 06, 2009 at 09:16:14PM +0800, Rik van Riel wrote:
    > Wu Fengguang wrote:
    >
    > > I guess both schemes have unacceptable flaws.
    > >
    > > For JVM/BIGMEM workload, most pages would be found referenced _all the time_.
    > > So the KEEP_MOST scheme could increase reclaim overheads by N=250 times;
    > > while the DROP_CONTINUOUS scheme is effectively zero cost.
    >
    > The higher overhead may not be an issue on smaller systems,
    > or inside smaller cgroups inside large systems, when doing
    > cgroup reclaim.

    Right.

    > > However, the DROP_CONTINUOUS scheme does bring more _indeterminacy_.
    > > It can behave vastly different on single active task and multi ones.
    > > It is short sighted and can be cheated by bursty activities.
    >
    > The split LRU VM tries to avoid the bursty page aging as
    > much as possible, by doing background deactivating of
    > anonymous pages whenever we reclaim page cache pages and
    > the number of anonymous pages in the zone (or cgroup) is
    > low.

    Right, but I meant busty page allocations and accesses on them, which
    can make a large continuous segment of referenced pages in LRU list,
    say 50MB. They may or may not be valuable as a whole, however a local
    algorithm may keep the first 4MB and drop the remaining 46MB.

    Thanks,
    Fengguang


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