[m0zg]

>> Dirtying cache lines across different cores/threads is slow

Very succinct and correct. This is particularly pernicious with atomics, which people use for lock-free stuff such as queues and work stealing thread pools. If atomics used by different threads/cores share a cache line and you mutate them a lot, perf gets instantly fucked, sometimes worse than if you used a mutex in the first place. And if you don't benchmark, you aren't going to notice.