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    SubjectRe: VM balancing issues on 2.6.13: dentry cache not getting shrunk enough
    >> > Second is Sonny Rao's rbtree dentry reclaim patch which is an attempt
    >> > to improve this dcache fragmentation problem.
    >> FYI, in the past I've tried this patch to reduce dcache fragmentation on
    >> an Altix (16k pages, 62 dentries to a slab page) under heavy
    >> fileserver workloads and it had no measurable effect. It appeared
    >> that there was almost always at least one active dentry on each page
    >> in the slab. The story may very well be different on 4k page
    >> machines, however.
    > I always thought dentry freeing would work much better if it
    > was turned upside down.
    > Instead of starting from the high level dcache lists it could
    > be driven by slab: on memory pressure slab tries to return pages with unused
    > cache objects. In that case it should check if there are only
    > a small number of pinned objects on the page set left, and if
    > yes use a new callback to the higher level user (=dcache) and ask them
    > to free the object.
    > The slab datastructures are not completely suited for this right now,
    > but it could be done by using one more of the list_heads in struct page
    > for slab backing pages.
    > It would probably not be very LRU but a simple hack of having slowly
    > increasing dcache generations. Each dentry use updates the generation.
    > First slab memory freeing pass only frees objects with older generations.

    If they're freeable, we should easily be able to move them, and therefore
    compact a fragmented slab. That way we can preserve the LRU'ness of it.
    Stage 1: free the oldest entries. Stage 2: compact the slab into whole
    pages. Stage 3: free whole pages back to teh page allocator.

    > Using slowly increasing generations has the advantage of timestamps
    > that you can avoid dirtying cache lines in the common case when
    > the generation doesn't change on access (= no additional cache line bouncing)
    > and it would easily allow to tune the aging rate under stress by changing the
    > length of the generation.

    LRU algorithm may need general tweaking like this anyway ... strict LRU
    is expensive to keep.

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