False sharing kills throughput

A C++ high-frequency packet-counter service regressed after a 'cache-friendly' refactor that packed per-thread counters into a tight contiguous array (counters[thread_id]++, no locks, each thread touches only its own index). On a 32-core box, throughput is now ~40% LOWER than before, and it gets worse the more threads you add even though there are zero locks and zero data races (each thread writes a different element). perf shows a huge spike in L2/L3 cache misses and 'HITM' (cache-line transfer) events, and CPU is busy but unproductive. Triage and explain why a lock-free design got slower.