Zen 5's Op Cache Disabled: A Deep Dive into its Clustered Decoders
This article delves into the instruction fetch and decode mechanism of AMD's Zen 5 processor. Zen 5 uses a unique dual-decoder cluster architecture, with each cluster serving one of the core's two SMT threads. Normally, Zen 5 relies on a 6KB op cache to deliver instructions, with the decoders only activating on cache misses. The author disables the op cache, forcing the decoders to handle all instructions, to evaluate their performance. Tests reveal significant performance drops in single-threaded mode with the op cache disabled; however, in multi-threaded mode, the dual-decoder clusters effectively compensate for the performance loss, even showing performance gains in some multi-threaded workloads. The author concludes that Zen 5's dual-decoder cluster design isn't the primary instruction source but acts as a secondary mechanism, boosting performance in high-IPC and multi-threaded scenarios, complementing the op cache for a balanced performance and power consumption.