Messages in this thread | | | Date | Tue, 5 Jan 2010 09:55:43 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [RFC][PATCH 6/8] mm: handle_speculative_fault() |
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On Tue, 5 Jan 2010, Andi Kleen wrote: > > > Oh well. Somebody who is bored might look at trying to make the wrapper > > code in arch/x86/lib/semaphore_32.S work on x86-64 too. It should make the > > successful rwsem cases much faster. > > Maybe, maybe not.
If there is actual contention on the lock, but mainly just readers (which is what the profile indicates: since there is no scheduler footprint, the actual writer-vs-reader case is probably very rare), then the xadd is likely to be _much_ faster than the spinlock.
Sure, the cacheline is going to bounce regardless (since it's a shared per-mm data structure), but the spinlock is going to bounce wildly back-and-forth between everybody who _tries_ to get it, while the regular xadd is going to bounce just once per actual successful xadd.
So a spinlock is as cheap as an atomic when there is no contention (which is the common single-thread case - the real cost of both lock and atomic is simply the fact that CPU serialization is expensive), but when there is actual lock contention, I bet the atomic xadd is going to be shown to be superior.
Remember: we commonly claim that 'spin_unlock' is basically free on x86 - and that's true, but it is _only_ true for the uncontended state.
Linus
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