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The MESI or MOESI protocols need the LLC directory in order to work... and the directory needs to synchronize parallel RFO + snoop-invalidation calls in order for it to work

(in TSO architectures that is... but not necessarily?? IMO a TSO architecture COULD be achieved with an LL/SC based exclusivity acquisition protocol... the ordering flexibility is orthogonal to the cache exclusivity acquisition mechanic... maybe this is just a proprietary choice... maybe not...).

Action which is mostly referred to as "serialize" in the literature... not synchronize (why?).

So, if high-level synchronization (MCS/CLH queues) need MESI... then

What synchronizes the synchronizer?

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    and the directory needs to synchronize parallel RFO + snoop-invalidation calls in order for it to work - You mean arbitrate between simultaneous requests for the same line? If you mean different lines, I don't think any ordering is needed; merely having the store become visible to all other threads at the same time (other-multicopy-atomic) by committing to coherent cache is sufficient to avoid threads disagreeing about store orders (no IRIW reordering), and the rest of TSO comes from no local LoadLoad/LoadStore/StoreStore reordering (i.e. program order + a store buffer with store-fwding.) Commented Aug 27 at 2:35
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    I've read some mentions about interconnects between cores needing to maintain some ordering of something, but I didn't fully grok what. Commented Aug 27 at 2:36
  • Yes, for the same line. It seems one misconception is that the processor does not incur in "bubbling" during directory lookups during M/E... but what I've found most people refer to during assertions like these is that they are referring that the LLC <-> MM pipeline is the one not getting stalled... while the interaction I'm asking about is the L1x <-> LLC <-> L1y pipeline... in which case I'd argue there must be some bubbling going on between L1's and LLC in order to serialize... maybe even some hardware level MCS-like queuing for stores... Commented Aug 27 at 13:25
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