Limiting Optimism: Time or Event Count?

In optimistically synchronized parallel discrete event simulators, unlimited optimism can lead to excessive rollbacks and simulation thrashing. Artificially throttling the simulation is a well-known technique for improving the performance by avoiding the effects of uncontrolled optimism. Simulation throttling has been attempted based on time or event count beyond global virtual time (GVT). In this paper, we carry out a simulation study of both approaches within time warp as well as breathing time warp (BTW) synchronization algorithms in the context of the SPEEDES simulation framework. We discover that anomalies arise when limiting optimism based on event count (as is the case in the default SPEEDES BTW algorithm) and that this gives rise to forced rollbacks that generally do not arise without simulation throttling. We implement a version of BTW that limits optimism based on time beyond GVT and show that it outperforms BTW for two large scale applications. We discuss initial experiences with adaptively estimating the optimism limit