A comparison of two approaches to parallel simulation of multiprocessors

The design trend towards CMPs has made the simulation of multiprocessor systems a necessity and has also made multiprocessor systems widely available. While a serial multiprocessor simulation necessarily imposes a linear slowdown, running such a simulation in parallel may help mitigate this effect. In this paper we document our experiences with two different methods of parallelizing Sparc Sulima, a simulator of UltraSPARC IIICu-based multiprocessor systems. In the first approach, a simple interconnect model within the simulator is parallelized non-deterministically using careful locking. In the second, a detailed interconnect model is parallelized while preserving determinism using parallel discrete event simulation (PDES) techniques. While both approaches demonstrate a threefold speedup using 4 threads on workloads from the NAS parallel benchmarks, speedup proved constrained by loadbalancing between simulated processors. A theoretical model is developed to help understand why observed speedup is less than ideal. An analysis of the related speed-accuracy tradeoff in the first approach with respect to the simulation time quantum is also given; the results show that, for both serial and parallel simulation, a quantum in the order of a few hundreds of cycles represents a 'sweet-spot', but parallel simulation is significantly more accurate for a given quantum size. As with the speedup analysis, these effects are workload dependent.