[PATCH] 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.
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