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    SubjectRe: BFS cpu scheduler and skip list implementation
    Hi Con,

    On Sat, Sep 24, 2011 at 06:38:06PM +1000, Con Kolivas wrote:
    > That's great then. I'm sure we'd explode in other weird and wonderful ways
    > before the CPU load ever got to 64k. Plus all that would happen is that it
    > would start degenerating from O(log n) insertion to O(n) as the number way
    > surpassed 64k. The number 16 for levels was simply chosen as the one
    > originally used by William Pugh in his sample code, but seems to be ample for
    > this application.

    If you're interested, during the early CFS benchmarks a few years ago, I
    reworked my old binary tree code to make it kernel-compatible. By this, I
    mean that it never needs to allocate memory, it's used just like rbtrees
    or kernel lists, by having a node in your structure and inserting it from
    the root of a tree. It offers the following features :
    - O(log(N)) insertion/lookup
    - O(1) removal
    - O(1) next/prev walk
    - 20 bytes per node on 32-bit pointers, 36-bytes on 64-bit pointers, plus
    the key
    - supports unique or multiple occurrences of the same key (walked in
    insertion order and classed in trees)
    - supports 32/64 bit signed/unsigned integers, strings and memory blocks
    - supports prefixes (eg. to insert IP addresses with masks)
    - supports lookup of greater than or equal to, less than or equal to.

    I replaced the rbtree that was used in haproxy's scheduler with this new
    code and measured a noticeable performance improvement, since haproxy
    does insert/next/remove a lot of times a second, and not having to balance
    a tree saves a huge number of cycles.

    I remember having conducted some tests on CFS a log time ago with it, but
    the only cases where I observed a gain was when running insane amounts of
    tasks at insane context switching rates, which was biased and irrealistic.
    So I stopped trying to put it into the kernel at that time.

    Maybe for your usage it might bring some value. Take a look at the code
    here if you want, it's not too much documented but still enough to start
    with it. There are a few examples that can help get it right. I know that
    a few people use it for their own projects and they did not ask for help :-);a=summary


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