False Sharing

False sharing is a performance degradation that occurs when multiple threads access different variables that happen to reside on the same CPU cache line. Because cache coherence protocols operate at the cache-line granularity, a write to one variable causes the entire line to be invalidated in other cores' caches, forcing them to re-fetch the data even though they are not sharing the variable itself. This transforms logically independent data into a source of cache contention.

False sharing is the hardware's revenge on programmer intuition. We think in variables; the CPU thinks in cache lines. The gap between these two granularities is where performance dies silently, and no profiler will tell you that your perfectly independent threads are secretly fighting over memory they do not even share.