MFLUSH: Handling Long-Latency Loads in SMT On-Chip Multiprocessors

Carmelo Acosta, Francisco J. Cazorla, Alex Ramírez, Mateo Valero

Abstract: Nowadays, there is a clear trend in industry towards employing the growing amount of transistors on chip in replicating execution cores (CMP), where each core is simultaneous multithreading (SMT). State-of-the-art high-performance processors like the IBM POWER5 and POWER6 corroborate this CMP+SMT trend. Within each SMT core any of the well-known SMT mechanisms may be applied to face SMT related challenges. Among them, probably the most important issue in an SMT execution pipeline concerns the instruction fetch (IFetch) Policy. The FLUSH IFetch Policy represents a choice for throughput-oriented scenarios. It handles L2 cache misses in order to avoid hardware resource monopolization by any given execution thread; involving an additional energy cost via instruction refetching. However, the new constraints imposed by the CMP+SMT scenario may affect well-known SMT mechanisms, like the FLUSH mechanism. In this paper we revisit the FLUSH mechanism and analyze its application in the emerging CMP+SMT scenario. The included analysis points out the new difficulties to be faced by the FLUSH mechanism in the emerging CMP+SMT scenario. Then we propose a novel IFetch Policy designed to cope with the CMP+SMT scenario: the MFLUSH. We also include a complete evaluation of the MFLUSH policy, both in terms of throughput and energy consumption. Our results indicate that the MFLUSH, specifically designed for the emerging CMP+SMT scenario, succeeds not only in overcoming the specific CMP+SMT constraints but also allowing a 20% energy consumption reduction without a significant system throughput loss.