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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
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.


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