A Fetch Policy Maximizing Throughput and Fairness for Two-Context SMT Processors

In Simultaneous Multithreading (SMT) processors, co-scheduled threads share the processor’s resources, but at the same time compete for them. A thread missing in L2 cache may hold a large number of resources which other threads could be using to make forward progress. And as a result, the overall performance of SMT processors is degraded. Currently, many instruction fetch policy focus on this problem. However, these policies are not perfect, and each has its own disadvantages. Especially, these policies are designed for processors implementing any ways simultaneous multithreading. The disadvantages of these policies may become more serious when they are used in two-context SMT processors.

In this paper, we propose a novel fetch policy called RG-FP (Resource Gating based on Fetch Priority), which is specially designed for two-context SMT processors. RG-FP combines reducing fetch priority with controlling shared resource allocation to prevent the negative effects caused by loads missing in L2 cache. Simulation results show that our RG-FP policy outperforms previously proposed fetch policies for all types of workloads in both throughput and fairness, especially for memory bounded workloads. Results also tell that our policy shows different degrees of improvement over other fetch policies. The increment over PDG is greatest, reaching 41.8% in throughput and 50.0% in Hmean on average.