CMPXCHG latency

By: Michael S (, March 31, 2008 1:51 am
Room: Moderated Discussions
nick ( on 3/31/08 wrote:
>Michael S ( on 3/31/08 wrote:
>>Gabriele Svelto ( on 3/31/08 wrote:
>>>Linus Torvalds ( on 3/29/08 wrote:
>>>>If Intel really will be able to pipeline the thing, I'll
>>>>be very happy. Spinlocks will still end up being
>>>>very expensive for the cache-bouncing case, but they'd
>>>>be basically free for the non-contended case.
>>>That would be sooo nice. If I end up using a spinlock it is obviously because I've
>>>gone to great lengths to make it as little contended as possible and it sucks *big*
>>>time to have to pay a tens-of-cycles-long price for what is basically a no-op in 99.99% of the cases.
>>Non-contended spinlock doesn't mean the absence of cache-bouncing. I'd guess that
>>in majority of practical cases you will see a combination of very low contention
>>rate with high (or, when the number of threads exceeds two, very high) cache-bouncing rates.
>Not at all. With fine-grained locking and spinlocks-in-objects, cache hot spinlocks
>are a _very_ common case.

With fine-grained locking - I agree. But too fine-grained locking is probably a bad idea for other reason. Besides, how would you apply a fine-grain locking to something like [modifiable] balanced binary tree?

>Global cachelines get unscalable very easily, even if there is no contention.

Exactly. Although "very" sounds like exaggeration.

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