> I've never heard of failed CAS being cheaper I'm referring to this old (but good) article: https://blogs.oracle.com/dave/biased-locking-in-hotspot
Il giorno sabato 14 settembre 2019 21:41:56 UTC+2, Vitaly Davidovich ha scritto: > > On x86, I’ve never heard of failed CAS being cheaper. In theory, cache > snooping can inform the core whether it’s xchg would succeed without going > through the RFO dance. But, to perform the actual xchg it would need > ownership regardless (if not already owned/exclusive). > > Sharing ordinary mutable memory isn’t scalable, forget locks :) Just doing > plain loads/stores of shared memory will kill scalability (and perf) due to > cache coherence traffic. > > ARM uses LL/SC for CAS, and is much more susceptible to failures (eg even > if the value is expected, if the line has been refreshed after the LL part, > it fails). But, I’m not overly familiar with ARM internals/mechanics. > > On Sat, Sep 14, 2019 at 3:20 PM Francesco Nigro <nigr...@gmail.com > <javascript:>> wrote: > >> Although not mentioned (neither on Doug's cookbook) I have always >> supposed was more likely the c++ default for both weak/strong CAS ie >> seq_cst memory ordering. >> To make this question more mechanical sympathy focused: on hardware level >> what happen? I would be curious to know both x86/arm versions for this, >> what kind of memory reordering are guaranteed... >> >> Most people says that a failed CAS cost much less then one that succeed, >> but just like a load or more? >> Probably the success will cause invalidation of all the caches that >> reference the cache line changed, but the failed case should lock (on x86) >> it and AFAIK locks (hw or SW) are not well known to be scalable and >> performant :) >> >> -- >> You received this message because you are subscribed to the Google Groups >> "mechanical-sympathy" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to mechanical-sympathy+unsubscr...@googlegroups.com <javascript:>. >> To view this discussion on the web, visit >> https://groups.google.com/d/msgid/mechanical-sympathy/caebab48-f88f-477d-bc8e-95702e45bc76%40googlegroups.com >> . >> > -- > Sent from my phone > -- You received this message because you are subscribed to the Google Groups "mechanical-sympathy" group. To unsubscribe from this group and stop receiving emails from it, send an email to mechanical-sympathy+unsubscr...@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/mechanical-sympathy/54efeb7e-04a2-48eb-a4f8-8e97556ccc03%40googlegroups.com.
