Exploring Fine-Grained Heterogeneity with Composite Cores

Heterogeneous multicore systems- comprising multiple cores with varying performance and energy characteristics-have emerged as a promising approach to increasing energy efficiency. Such systems reduce energy consumption by identifying application phases and migrating execution to the most efficient core that meets performance requirements. However, the overheads of migrating between cores limit opportunities to coarse-grained phases (hundreds of millions of instructions), reducing the potential to exploit energy efficient cores. We propose Composite Cores, an architecture that reduces migration overheads by bringing heterogeneity into a core. Composite Cores pairs a big and little compute μEngine that together achieve high performance and energy efficiency. By sharing architectural state between the μEngines, the migration overhead is reduced, enabling fine-grained migration and increasing the opportunities to utilize the little μEngine without sacrificing performance. An intelligent controller migrates the application between μEngines to maximize energy efficiency while constraining performance loss to a configurable bound. We evaluate Composite Cores using cycle accurate microarchitectural simulations and a detailed power model. Results show that, on average, Composite Cores are able to map 30 percent of the execution time to the little μEngine, achieving a 21 percent energy savings while maintaining 95 percent performance.