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    Subject[patch 00/23] Slab defragmentation V6
    Slab defragmentation is mainly an issue if Linux is used as a fileserver
    and large amounts of dentries, inodes and buffer heads accumulate. In some
    load situations the slabs become very sparsely populated so that a lot of
    memory is wasted by slabs that only contain one or a few objects. In
    extreme cases the performance of a machine will become sluggish since
    we are continually running reclaim. Slab defragmentation adds the
    capability to recover wasted memory.

    With lumpy reclaim slab defragmentation can be used to enhance the
    ability to recover larger contiguous areas of memory. Lumpy reclaim currently
    cannot do anything if a slab page is encountered. With slab defragmentation
    that slab page can be removed and a large contiguous page freed. It may
    be possible to have slab pages also part of ZONE_MOVABLE (Mel's defrag
    scheme in 2.6.23) or the MOVABLE areas (antifrag patches in mm).

    The patchset is also available via git

    git pull git:// defrag

    Currently memory reclaim from the following slab caches is possible:

    1. dentry cache
    2. inode cache (with a generic interface to allow easy setup of more
    filesystems than the currently supported ext2/3/4 reiserfs, XFS
    and proc)
    3. buffer_heads

    One typical mechanism that triggers slab defragmentation on my systems
    is the daily run of


    Updatedb scans all files on the system which causes a high inode and dentry
    use. After updatedb is complete we need to go back to the regular use
    patterns (typical on my machine: kernel compiles). Those need the memory now
    for different purposes. The inodes and dentries used for updatedb will
    gradually be aged by the dentry/inode reclaim algorithm which will free
    up the dentries and inode entries randomly through the slabs that were
    allocated. As a result the slabs will become sparsely populated. If they
    become empty then they can be freed but a lot of them will remain sparsely
    populated. That is where slab defrag comes in: It removes the slabs with
    just a few entries reclaiming more memory for other uses.

    - Rediff against 2.6.24-rc2 + mm slub patches.
    - Add reviewed by lines.
    - Take out the experimental code to make slab pages movable. That
    has to wait until this has been considered by Mel.

    - Support lumpy reclaim for slabs
    - Support reclaim via slab_shrink()
    - Add constructors to insure a consistent object state at all times.

    - Optimize scan for slabs that need defragmentation
    - Add /sys/slab/*/defrag_ratio to allow setting defrag limits
    per slab.
    - Add support for buffer heads.
    - Describe how the cleanup after the daily updatedb can be
    improved by slab defragmentation.

    - Support directory reclaim
    - Add infrastructure to trigger defragmentation after slab shrinking if we
    have slabs with a high degree of fragmentation.

    - Clean up control flow using a state variable. Simplify API. Back to 2
    functions that now take arrays of objects.
    - Inode defrag support for a set of filesystems
    - Fix up dentry defrag support to work on negative dentries by adding
    a new dentry flag that indicates that a dentry is not in the process
    of being freed or allocated.

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