In this paper is revisit the notion of event simultaneity in the context of parallel and distributed simulation. Although the simulation community has recognized this problem for years, it has focused mainly on the mechanics of breaking event-time ties, and has neither measured its extent nor considered its implications. Extant simulators (both serial and parallel) prohibit simultaneity either by user-specified event priorities or by an arbitrary (but well-documented) tie-breaking mechanism. We shall argue that, in many cases, these strategies lead to an invalid simulation.In doing so, we shall introduce the threshold of event simultaneity and use it to understand the semantics of simultaneity. We shall also present empirical results measuring the magnitude of deviation from logical correctness that occurs when a parallel simulation explicitly allows simultaneous events.
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