Messages in this thread | | | Date | Fri, 02 Nov 2007 12:22:38 -0400 | From | Chuck Ebbert <> | Subject | Re: [patch 1/4] x86: FIFO ticket spinlocks |
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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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