Demand-Driven PDES: Exploiting Locality in Simulation Models

Abstract

Traditional parallel discrete event simulation (PDES) systems treat each simulation thread in the same manner, regardless of whether a thread has events to process in its input queue or not. At the same time, many real-life simulation models exhibit significant execution locality, where only part of the model (and thus a subset of threads) are actively sending or receiving messages in a given time period. These inactive threads still continuously check their queues and participate in simulation-wide time synchronization mechanisms, such as computing Global Virtual Time (GVT). This wastes resources, ties up CPU cores with threads that offer no contribution to event processing and limits the performance and scalability of the simulation. In this paper, we propose a new paradigm for managing PDES threads that we call Demand-Driven PDES (DD-PDES). The key idea behind DD-PDES is to identify threads that have no events to process and de-schedule them from the CPU until they receive a message requiring event processing. Furthermore, these inactive threads are also excluded from participation in the GVT computation, accelerating that process as a result. DD-PDES ensures that the CPU cycles are mostly spent on actual event processing, resulting in performance improvements. This architecture allows for significant over-subscription of threads by exceeding the number of available hardware thread contexts on the chip. We demonstrate that on a Knights Landing processor, DD-PDES significantly outperforms the traditional simulation equipped with the best currently proposed GVT algorithms.