The benefit of SMT in the multi-core era: flexibility towards degrees of thread-level parallelism

The number of active threads in a multi-core processor varies over time and is often much smaller than the number of supported hardware threads. This requires multi-core chip designs to balance core count and per-core performance. Low active thread counts benefit from a few big, high-performance cores, while high active thread counts benefit more from a sea of small, energy-efficient cores. This paper comprehensively studies the trade-offs in multi-core design given dynamically varying active thread counts. We find that, under these workload conditions, a homogeneous multi-core processor, consisting of a few high-performance SMT cores, typically outperforms heterogeneous multi-cores consisting of a mix of big and small cores (without SMT), within the same power budget. We also show that a homogeneous multi-core performs almost as well as a heterogeneous multi-core that also implements SMT, as well as a dynamic multi-core, while being less complex to design and verify. Further, heterogeneous multi-cores that power-gate idle cores yield (only) slightly better energy-efficiency compared to homogeneous multi-cores. The overall conclusion is that the benefit of SMT in the multi-core era is to provide flexibility with respect to the available thread-level parallelism. Consequently, homogeneous multi-cores with big SMT cores are competitive high-performance, energy-efficient design points for workloads with dynamically varying active thread counts.