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    Subject[PATCH RFC] Optimize semtimedop
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    We've been poking around in semtimedop for a while now, mostly because
    it is consistently showing up at the top of the CPU profiles for benchmarking
    runs on big numa systems. The biggest problem seems to be the IPC lock, and
    the fact that we hold it for a long time while we loop over different lists and
    try to do semaphore operations.

    Zach Brown came up with a set of patches a while ago that switched away from
    the global pending list, and semtimedop was recently optimized for the
    single sop case by Nick and Manfred.

    This patch series tries to build on ideas from all of these patches. The
    list of pending semaphore operations is pushed down to the individual
    semaphore and the locking is also pushed down into the semaphore. The
    result is much faster with my micro benchmark:

    http://oss.oracle.com/~mason/sembench.c

    It more than doubles the total number of post/wait cycles the benchmark
    is able to get in 30s. Before this patch, semtimedop scored about the
    same as futexes for the post/wait cycles, and so now it is 2x faster.

    I did run this code through all of the ltp ipc tests, and later this week I
    hope to get a full tpc database benchmark on it.



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