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    Subject[PATCH] mm/compaction: restore COMPACT_CLUSTER_MAX to 32
    Date
    Until now, COMPACT_CLUSTER_MAX is defined as SWAP_CLUSTER_MAX.
    Commit ("mm: increase SWAP_CLUSTER_MAX to batch TLB flushes")
    changes SWAP_CLUSTER_MAX from 32 to 256 to improve tlb flush performance
    so COMPACT_CLUSTER_MAX is also changed to 256. But, it has
    no justification on compaction-side and I think that loss is more than
    benefit.

    One example is that migration scanner would isolates and migrates
    too many pages unnecessarily with 256 COMPACT_CLUSTER_MAX. It may be
    enough to migrate 4 pages in order to make order-2 page, but, now,
    compaction will migrate 256 pages.

    To reduce this unneeded overhead, this patch restores
    COMPACT_CLUSTER_MAX to 32.

    Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    ---
    include/linux/swap.h | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/include/linux/swap.h b/include/linux/swap.h
    index d08feef..31eb343 100644
    --- a/include/linux/swap.h
    +++ b/include/linux/swap.h
    @@ -155,7 +155,7 @@ enum {
    };

    #define SWAP_CLUSTER_MAX 256UL
    -#define COMPACT_CLUSTER_MAX SWAP_CLUSTER_MAX
    +#define COMPACT_CLUSTER_MAX 32UL

    /*
    * Ratio between zone->managed_pages and the "gap" that above the per-zone
    --
    1.9.1


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