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    SubjectRE: [patch] printk subsystems
    Robert wrote:

    >There is both a qualitative difference and quantitative difference in a
    >lockless algorithm as described versus one that uses locking. Most
    >importantly for Linux, these algorithms in practice have better performance
    >characteristics.
    >
    Do you have benchmark numbers that compare "lockless" and locking
    algorithms on large MP systems?

    For example, how much faster is one 'lock;cmpxchg' compared to
    'spin_lock();if (x==var) var = y;spin_unlock();'.

    So far I assumed that for spinlock that are only held for a few cycles,
    the cacheline trashing dominates, and not the spinning.
    I've avoided to replace spin_lock+inc+spin_unlock with atomic_inc().
    (Just look at the needed memory barriers: smp_mb__after_clear_bit & friends)

    RCU uses per-cpu queues that are really lockless and avoid the cache
    trashing, that is a real win.

    --
    Manfred

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