RE: [patch] printk subsystems

[Manfred Spraul <manfred@colorfullife.com>]

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