State-Space Caching Revisited

State-space caching is a veri cation technique for nite-state concurrent systems. It performs an exhaustive exploration of the state space of the system being checked while storing only all states of just one execution sequence plus as many other previously visited states as available memory allows. So far, this technique has been of little practical signi cance: it allows one to reduce memory usage by only two to three times, before an unacceptable blow-up of the run-time overhead sets in. The explosion of the run-time requirements is due to redundant multiple explorations of unstored parts of the state space. Indeed, almost all states in the state space of concurrent systems are typically reached several times during the search. In this paper, we present a method to tackle the main cause of this prohibitive state matching: the exploration of all possible interleavings of concurrent executions of the system, which all lead to the same state. Then, we show that, in many cases, with this method, most reachable states are visited only once during state-space exploration. This enables one not to store most of the states that have already been visited without incurring too much redundant explorations of parts of the state space, and makes therefore state-space caching a much more attractive veri cation method. As an example, we were able to completely explore a state space of 250,000 states while storing simultaneously no more than 500 states and with only a three-fold increase of the run-time requirements.