Funnily enough, there is another version of the slides these numbers come from in which mutex lock/unlock is listed as the same 100ns as memory access: http://s

Funnily enough, there is another version of the slides these numbers come from in which mutex lock/unlock is listed as the same 100ns as memory access: http://static.googleusercontent.com/media/research.google.com/en/us/people/jeff/stanford-295-talk.pdf http://static.googleusercontent.com/media/research.google.co.... I can't tell which came first, or if one is a typo, or what's going on there. In any case, assuming the mutex in cache, and appears to not currently be locked, the core performing the atomic operation needs to: (1) broadcast an invalidate/exclusive hold on the cache line to the other cores, (2) update its own cache, (3) eventually write back to memory. (1) is faster than main memory access, since CPU cores are closer to each other, physically, than they are to the memory bus, and have hard-wired access specifically for this operation. The slowest part is (3), but with a write-back cache the writer doesn't really pay the latency cost, because it gets amortized into the next cache flush.