A Modest Approach to Markov Automata

Markov automata are a compositional modelling formalism with continuous stochastic time, discrete probabilities, and nondeterministic choices. In this article, we present extensions to MODEST, an expressive high-level language with roots in process algebra, that allow large Markov automata models to be specified in a succinct, modular way. We illustrate the advantages of MODEST over alternative languages. Model checking Markov automata models requires dedicated algorithms for time-bounded and long-run average reward properties. We describe and evaluate the state-of-the-art algorithms implemented in the mcsta model checker of the MODEST TOOLSET. We find that mcsta improves the performance and scalability of Markov automata model checking compared to earlier and alternative tools. We explain a partial-exploration approach based on the BRTDP method designed to mitigate the state space explosion problem of model checking, and experimentally evaluate its effectiveness. This problem can be avoided entirely by purely simulation-based techniques, but the nondeterminism in Markov automata hinders their straightforward application. We explain how lightweight scheduler sampling can make simulation possible, and provide a detailed evaluation of its usefulness on several benchmarks using the MODEST TOOLSET’s modes simulator.