Ah - thank you. "True sharing" makes way more sense to describe the situation. The general problem with locks I've seen again and again - and maybe CAS doesn't improve it, but I thought it did. It is challenging to profile this stuff carefully. I've been disappointed many times by my attempts at multithreaded programming when locks are involved. I've implemented a multithreaded Common Lisp programming environment (https://github.com/clasp-developers/clasp.git) that interoperates with C++. Two locks that cause me massive headaches are (1) in the unwinder when we do a lot of stack unwinding in multiple threads and (2) in the memory manager when we do a lot of memory allocation in multiple threads. Parallel code runs multiple times slower than serial code. In another situation where we do parallel non-linear optimization without allocating memory, we can get 20 CPU working in parallel with no problems.