Solving the ignoring problem for partial order reduction

Partial order reduction limits the state explosion problem that arises in model checking by limiting the exploration of redundant interleavings. A state space search algorithm based on this principle may ignore some interleavings by delaying the execution of some actions provided that an equivalent interleaving is explored. However, if one does not choose postponed actions carefully, some of these may be infinitely delayed. This pathological situation is commonly referred to as the ignoring problem. The prevention of this phenomenon is not mandatory if one wants to verify if the system halts but it must be resolved for more elaborate properties like, for example, safety or liveness properties. We present in this work some solutions to this problem. In order to assess the quality of our propositions, we included them in our model checker Helena. We report the result of some experiments which show that our algorithms yield better reductions than state of the art algorithms like those implemented in the Spin tool.