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    SubjectRe: [PATCH 4/8] mm: migration: Allow migration to operate asynchronously and avoid synchronous compaction in the faster path
    On Wed, Nov 17, 2010 at 04:22:45PM +0000, Mel Gorman wrote:
    > @@ -484,6 +486,7 @@ static unsigned long compact_zone_order(struct zone *zone,
    > .order = order,
    > .migratetype = allocflags_to_migratetype(gfp_mask),
    > .zone = zone,
    > + .sync = false,
    > };
    > INIT_LIST_HEAD(&cc.freepages);
    > INIT_LIST_HEAD(&cc.migratepages);

    I like this because I'm very afraid to avoid wait-I/O latencies
    introduced into hugepage allocations that I prefer to fail quickly and
    be handled later by khugepaged ;).

    But I could have khugepaged call this with sync=true... so I'd need a
    __GFP_ flag that only khugepaged would use to notify compaction should
    be synchronous for khugepaged (not for the regular allocations in page
    faults). Can we do this through gfp_mask only?

    > @@ -500,6 +503,7 @@ unsigned long reclaimcompact_zone_order(struct zone *zone,
    > .order = order,
    > .migratetype = allocflags_to_migratetype(gfp_mask),
    > .zone = zone,
    > + .sync = true,
    > };
    > INIT_LIST_HEAD(&cc.freepages);
    > INIT_LIST_HEAD(&cc.migratepages);

    Is this intentional? That inner compaction invocation is
    equivalent to the one one interleaved with the shrinker tried before
    invoking the shrinker. So I don't see why they should differ (one sync
    and one async).

    Anyway I'd prefer the inner invocation to be removed as a whole and to
    keep only going with the interleaving and to keep the two jobs of
    compaction and shrinking memory fully separated and to stick to the
    interleaving. If this reclaimcompact_zone_order helps maybe it means
    compact_zone_order isn't doing the right thing and we're hiding it by
    randomly calling it more frequently...

    I can see a point however in doing:

    compaction async
    shrink (may wait) (scan 500 pages, freed 32 pages)
    compaction sync (may wait)


    compaction async
    shrink (scan 32 pages, freed 0 pages)
    compaction sync (hugepage generated nobody noticed)
    shrink (scan 32 pages, freed 0 pages)
    compaction sync
    shrink (scan 32 pages, freed 0 pages)

     \ /
      Last update: 2010-11-18 19:23    [W:0.021 / U:1.088 seconds]
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