On 10/10/2011 07:01 AM, Stephan Diestelhorst wrote: > On Monday 10 October 2011, 07:00:50 Stephan Diestelhorst wrote: >> On Thursday 06 October 2011, 13:40:01 Jeremy Fitzhardinge wrote: >>> On 10/06/2011 07:04 AM, Stephan Diestelhorst wrote: >>>> On Wednesday 28 September 2011, 14:49:56 Linus Torvalds wrote: >>>>> Which certainly should *work*, but from a conceptual standpoint, isn't >>>>> it just *much* nicer to say "we actually know *exactly* what the upper >>>>> bits were". >>>> Well, we really do NOT want atomicity here. What we really rather want >>>> is sequentiality: free the lock, make the update visible, and THEN >>>> check if someone has gone sleeping on it. >>>> >>>> Atomicity only conveniently enforces that the three do not happen in a >>>> different order (with the store becoming visible after the checking >>>> load). >>>> >>>> This does not have to be atomic, since spurious wakeups are not a >>>> problem, in particular not with the FIFO-ness of ticket locks. >>>> >>>> For that the fence, additional atomic etc. would be IMHO much cleaner >>>> than the crazy overflow logic. >>> All things being equal I'd prefer lock-xadd just because its easier to >>> analyze the concurrency for, crazy overflow tests or no. But if >>> add+mfence turned out to be a performance win, then that would obviously >>> tip the scales. >>> >>> However, it looks like locked xadd is also has better performance: on >>> my Sandybridge laptop (2 cores, 4 threads), the add+mfence is 20% slower >>> than locked xadd, so that pretty much settles it unless you think >>> there'd be a dramatic difference on an AMD system. >> Indeed, the fences are usually slower than locked RMWs, in particular, >> if you do not need to add an instruction. I originally missed that >> amazing stunt the GCC pulled off with replacing the branch with carry >> flag magic. It seems that two twisted minds have found each other >> here :) >> >> One of my concerns was adding a branch in here... so that is settled, >> and if everybody else feels like this is easier to reason about... >> go ahead :) (I'll keep my itch to myself then.) > Just that I can't... if performance is a concern, adding the LOCK > prefix to the addb outperforms the xadd significantly: Hm, yes. So using the lock prefix on add instead of the mfence? Hm. J -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html