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 <nigro....@gmail.com> 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. > 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/CAHjP37HDQLSMhX6oriRWS%3DWkfUDy9fGe41zjFWwzm9zO%3DFJbZg%40mail.gmail.com.
