False Sharing and Atomic Variables

Question

When different variables are inside the same cache line, you can experience False Sharing, which means that even if two different threads (running on different cores) are ac

Answer 1

A clarification: for negative consequences at least some accesses to "falsely shared" variables should be writes. If writes are rare, performance impact of false sharing is rather negligible; the more writes (and so cache line invalidate messages) the worse performance.

Even with atomics, cache line sharing (either false or true) still matters. Look for some evidence here: http://www.1024cores.net/home/lock-free-algorithms/first-things-first. Thus, the answer is - yes, placing atomic variables used by different threads to the same cache line may make application slower compared to placing them to two different lines. However, I think it will be mostly unnoticed, unless the app spends a significant portion of its time updating these atomic variables.