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    SubjectRe: [patch 1/4] x86: FIFO ticket spinlocks
    From
    Nick Piggin wrote:
    > Introduce ticket lock spinlocks for x86 which are FIFO. The implementation
    > is described in the comments. The straight-line lock/unlock instruction
    > sequence is slightly slower than the dec based locks on modern x86 CPUs,
    > however the difference is quite small on Core2 and Opteron when working out of
    > cache, and becomes almost insignificant even on P4 when the lock misses cache.
    > trylock is more significantly slower, but they are relatively rare.
    >
    > On an 8 core (2 socket) Opteron, spinlock unfairness is extremely noticable,
    > with a userspace test having a difference of up to 2x runtime per thread, and
    > some threads are starved or "unfairly" granted the lock up to 1 000 000 (!)
    > times. After this patch, all threads appear to finish at exactly the same
    > time.

    I had observed this phenomenon on some 8-ways here as well, but I didn't
    have the bandwidth to code something up. Thumbs up!

    Regards,
    -Greg
    -
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