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    SubjectRe: [PATCH -mm v2] mm: have order > 0 compaction start off where it left
    On Thu, Jun 28, 2012 at 01:55:20PM -0400, Rik van Riel wrote:
    > Order > 0 compaction stops when enough free pages of the correct
    > page order have been coalesced. When doing subsequent higher order
    > allocations, it is possible for compaction to be invoked many times.
    > However, the compaction code always starts out looking for things to
    > compact at the start of the zone, and for free pages to compact things
    > to at the end of the zone.
    > This can cause quadratic behaviour, with isolate_freepages starting
    > at the end of the zone each time, even though previous invocations
    > of the compaction code already filled up all free memory on that end
    > of the zone.
    > This can cause isolate_freepages to take enormous amounts of CPU
    > with certain workloads on larger memory systems.
    > The obvious solution is to have isolate_freepages remember where
    > it left off last time, and continue at that point the next time
    > it gets invoked for an order > 0 compaction. This could cause
    > compaction to fail if cc->free_pfn and cc->migrate_pfn are close
    > together initially, in that case we restart from the end of the
    > zone and try once more.
    > Forced full (order == -1) compactions are left alone.
    > Cc: Andrew Morton <>
    > Cc: Mel Gorman <>
    > Reported-by: Jim Schutt <>
    > Signed-off-by: Rik van Riel <>
    > ---
    > v2: implement Mel's suggestions, handling wrap-around etc

    I have not tested it myself but it looks correct! Thanks very much.

    Acked-by: Mel Gorman <>

    Mel Gorman
    SUSE Labs

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