False sharing is a well-know performance issue on SMP systems, where each processor has a local cache. it occurs when treads on different processors modify varibles that reside on th the same cache line like so.

In Figure, threads 0 and 1 require variables that are adjacent in memory and reside on the same cache line. The cache line is loaded into the caches of CPU 0 and CPU 1. Even though the threads modify different variables, the cahche line is invalidatedm forcing a memory update to maintain cache coherency.