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    Subject[patch 19/19] Reduce the radix tree nodes to 64 slots

    Reduce the radix tree nodes from 128 slots to 64.

    - The main reason for this is that on 64-bit/4k page machines, the
    slab allocator has decided that radix tree nodes will require an
    order-1 allocation. Shrinking the nodes to 64 slots pulls that back
    to an order-0 allocation.

    - On x86 we get fifteen 64-slot nodes per page rather than seven
    129-slot nodes, for a modest memory saving.

    - Halving the node size will approximately halve the memory use in
    the worrisome really-large, really-sparse file case.

    Of course, the downside is longer tree walks. Each level of the tree
    covers six bits of pagecache index rather than seven. As ever, I am
    guided by Anton's profiling on the 12- and 32-way PPC boxes.
    radix_tree_lookup() is currently down in the noise floor.

    Now, there is one special case: one file which is really big and which
    is accessed in a random manner and which is accessed very heavily: the
    blockdev mapping. We _are_ showing some locking cost in
    __find_get_block (used to be __get_hash_table) and in its call to
    find_get_page(). I have a bunch of patches which introduce a generic
    per-cpu buffer LRU, and which remove ext2's private bitmap buffer LRUs.
    I expect these patches to wipe the blockdev mapping lookup lock contention
    off the map, but I'm awaiting test results from Anton before deciding
    whether those patches are worth submitting.

    --- 2.5.22/lib/radix-tree.c~ratshrink Sun Jun 16 23:12:54 2002
    +++ 2.5.22-akpm/lib/radix-tree.c Sun Jun 16 23:12:54 2002
    @@ -29,7 +29,7 @@
    * Radix tree node definition.
    -#define RADIX_TREE_MAP_SHIFT 7
    +#define RADIX_TREE_MAP_SHIFT 6

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