Virtual time III: Unification of conservative and optimistic synchronization in parallel discrete event simulation

There has long been a divide in synchronization approaches for parallel discrete event simulation, between conservative methods requiring lookahead and optimistic methods requiring rollback. These are usually seen as dichotomous, so that a model writer must make an early, static design decision between them. An optimistic simulator does not need lookahead information but is unable to take advantage of it even if it were available, whereas a conservative simulator may perform poorly or even deadlock without good lookahead information. Here we introduce unified virtual time (UVT) synchronization which provides the advantages of both conservative and optimistic synchronization dynamically for all models. Conservative synchronization becomes an accelerator for optimistic synchronization. When lookahead information is available the simulation will execute conservatively. Otherwise it will execute optimistically. In this paper we present UVT, argue for its correctness, and show adaptations of Time Warp, YAWNS, and Null Messages which cooperatively synchronize a single simulation.