> BTW a basic understanding of coherence might lead one to doubt the claims about lack of scalability. When you hit the cache, you know that you own the data so it's fast. When you miss the cache, it's a very slow operation anyway, the overhead of checking whether to fetch the data from external memory or another cache can't be that bad. And if data travels a lot from one cache to another because of true or false sharing, that's the fault of the code doing that. Coherence is not supposed to make sharing blindingly fast, just to make it correct, and to make false sharing correct as well.

The problem isn't so much that missing the cache is slow, but rather how do you make a processor know that it has to look the data value up in another cache instead of main memory? The two main options are either to basically broadcast every request to every cache it might be in, or to store in main memory which cache it's in--you have a design tradeoff between high interconnect traffic or high memory metadata usage.