Disjoint out-of-order execution processor

Abstract

High-performance superscalar architectures used to exploit instruction level parallelism in single-thread applications have become too complex and power hungry for the multicore processors era. We propose a new architecture that uses multiple small latency-tolerant out-of-order cores to improve single-thread performance. Improving single-thread performance with multiple small out-of-order cores allows designers to place more of these cores on the same die. Consequently, emerging highly parallel applications can take full advantage of the multicore parallel hardware without sacrificing performance of inherently serial and hard to parallelize applications. Our architecture combines speculative multithreading (SpMT) with checkpoint recovery and continual flow pipeline architectures. It splits single-thread program execution into disjoint control and data threads that execute concurrently on multiple cooperating small and latency-tolerant out-of-order cores. Hence we call this style of execution Disjoint Out-of-Order Execution (DOE). DOE uses latency tolerance to overcome performance issues of SpMT caused by interthread data dependences. To evaluate this architecture, we have developed a microarchitecture performance model of DOE based on PTLSim, a simulation infrastructure of the x86 instruction set architecture. We evaluate the potential performance of DOE processor architecture using a simple heuristic to fork control independent threads in hardware at the target addresses of future procedure return instructions. Using applications from SpecInt 2000, we study DOE under ideal as well as realistic architectural constraints. We discuss the performance impact of key DOE architecture and application variables such as number of cores, interthread data dependences, intercore data communication delay, buffers capacity, and branch mispredictions. Without any DOE specific compiler optimizations, our results show that DOE outperforms conventional SpMT architectures by 15%, on average. We also show that DOE with four small cores can perform on average equally well to a large superscalar core, consuming about the same power. Most importantly, DOE improves throughput performance by a significant amount over a large superscalar core, up to 2.5 times, when running multitasking applications.