Re: [patch 1/4] x86: FIFO ticket spinlocks

[Chuck Ebbert]

On 11/01/2007 10:03 AM, 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.

There's also a very easy way to get better fairness with our current spinlocks:
use xchg to release the lock instead of mov.

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