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Comment by dragontamer

8 hours ago

Hyperthreading actually shares L1 caches between two threads (after all, two threads are running in the same L1 cache and core).

I believe SMT4 and SMT8 cores from IBM Power10 also have L1 caches shared (8 threads on one core), and thus benefit from communication speeds.

But you're right in that this is a very obscure performance quirk. I'm honestly not aware of any practical code that takes advantage of this. E-cores are perhaps the most "natural" implementation of high-speed core-to-core communications though.