Ph.D. Defense
César Rodríguez


Verification Based on Unfoldings of Petri Nets with Read Arcs

Date & Venue

On Thursday December 12, 2013, at 2pm.

Lecture hall Tocqueville, École Normale Supérieure de Cachan.


See a map and directions from the RER station Bagneux.


Humans make mistakes, especially when faced to complex tasks, such as the construction of modern hardware or software. This thesis is about machine-assisted techniques to guarantee that computers behave correctly.

Modern computer systems are large and complex. Automated formal verification stands as an alternative to testing or simulation to ensuring their reliability. It essentially proposes to employ computers to exhaustively check the system behavior. Unfortunately, automated verification suffers from the state-space explosion problem: even relatively small systems can reach a huge number of states. Using the right representation for the system behavior seems to be a key step to tackle the inherent complexity of the problems that automated verification solves.

The verification of concurrent systems poses additional issues, as their analysis requires to evaluate, conceptually, all possible execution orders of their concurrent actions. Petri net unfoldings are a well-established verification technique for concurrent systems. They represent behavior by partial orders, which not only is natural but also efficient for automatic verification.

This dissertation focuses on the verification of concurrent systems, employing Petri nets to formalize them, and studies two prominent verification techniques: model checking and fault diagnosis.

We investigate the unfoldings of Petri nets extended with read arcs. The unfoldings of these so-called contextual nets seem to be a better representation for systems exhibiting concurrent read access to shared resources: they can be exponentially smaller than conventional unfoldings on these cases.

Theoretical and practical contributions are made. We first study the construction of contextual unfoldings, introducing algorithms and data structures that enable their efficient computation. We integrate contextual unfoldings with merged processes, another representation of concurrent behavior that alleviates the explosion caused by non-determinism. The resulting structure, called contextual merged processes, is often orders of magnitude smaller than unfoldings, as we experimentally demonstrate.

Next, we develop verification techniques based on unfoldings. We define SAT encodings for the reachability problem in contextual unfoldings, thus solving the problem of detecting cycles of asymmetric conflict. Also, an unfolding-based decision procedure for fault diagnosis under fairness constraints is presented, in this case only for conventional unfoldings.

Finally, we implement our verification algorithms, aiming at producing a competitive model checker intended to handle realistic benchmarks. We subsequently evaluate our methods over a standard set of benchmarks and compare them with existing unfolding-based techniques. The experiments demonstrate that reachability checking based on contextual unfoldings outperforms existing techniques on a wide number of cases.

This suggests that contextual unfoldings, and asymmetric event structures in general, have a rightful place in research on concurrency, also from an efficiency point of view.



Here is the thesis manuscript and the defense slides.

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