[repstosb]

Amdahl's Law, like most 20th-century performance "wisdom" and metrics, focuses excessively on instruction count, neglecting memory and I/O pressure. 64 cores doesn't mean 64 independent cache pyramids and memory buses. In real life, the difference between CPU cycle frequency and memory latency is so great that memory pressure primarily determines performance, whereas core count really only matters to the extent that it contributes to that memory pressure.