Urgent Partial Order Reduction for Extended Timed Automata

We propose a partial order reduction method for reachability analysis of networks of timed automata interacting via synchronous channel communication and via shared variables. Our method is based on (classical) symbolic delay transition systems and exploits the urgent behavior of a system, where time does not introduce dependencies among actions. In the presence of urgent behavior in the network, we apply partial order reduction techniques for discrete systems based on stubborn sets. We first describe the framework in the general setting of symbolic delay time transition systems and then instantiate it to the case of timed automata. We implement our approach in the model checker Uppaal and observe a substantial reduction in the reachable state space for case studies that exhibit frequent urgent behaviour and only a moderate slowdown on models with limited occurrence of urgency.