Gigliola Vaglini

Development of a debugger for a concurrent language

The authors discuss issues related to the debugging of concurrent programs. A set of desirable characteristics for a debugger for concurrent languages is deduced from a review of the differences between the debugging of concurrent programs and that of sequential ones. A debugger for concurrent language based upon CSP is then described. The debugger makes it possible to compare a description of the expected program behavior to the actual behaviour. The description of the behavior is given in terms of expressions composed by events and/or assertions on the process state. The developed formalism is able to describe behaviors at various levels of abstraction. Lastly, some guidelines for the implementation of the debugger are given and a detailed example of program debugging is analyzed.

Selective Mu-Calculus and Formula-Based Equivalence of Transition Systems

In model checking for temporal logic, the correctness of a system with respect to a desired behavior is verified by checking whether a structure that models the system satisfies a formula describing the behavior. Most existing verification techniques are based on a representation of the system by means of a labeled transition system. In this approach to verification, the efficiency of the model checking is essentially influenced by the number of states of the transition system. In this paper we present a new temporal logic, the selective mu-calculus, and an equivalence between transition systems based on the formulae of this logic. This property preserving equivalence can be used to reduce the size of transition systems. The equivalence (called ?-equivalence) is based on the set, ?, of actions occurring inside the modal operators of a selective mu-calculus formula. We prove that the ?-equivalence coincides with the equivalence induced by the set of the selective mu-calculus formulae with occurring actions in ?. Thus, a formula can be more efficiently checked on a transition system ?-equivalent to the standard one, but smaller than it, since all the actions not in ? are “discarded.”

Monitoring elderly behavior via indoor position-based stigmergy

In this paper we present a novel approach for monitoring elderly people living alone and independently in their own homes. The proposed system is able to detect behavioral deviations of the routine indoor activities on the basis of a generic indoor localization system and a swarm intelligence method. For this reason, an in-depth study on the error modeling of state-of-the-art indoor localization systems is presented in order to test the proposed system under different conditions in terms of localization error. More specifically, spatiotemporal tracks provided by the indoor localization system are augmented, via marker-based stigmergy, in order to enable their self-organization. This allows a marking structure appearing and staying spontaneously at runtime, when some local dynamism occurs. At a second level of processing, similarity evaluation is performed between stigmergic marks over different time periods in order to assess deviations. The purpose of this approach is to overcome an explicit modeling of users activities and behaviors that is very inefficient to be managed, as it works only if the user does not stray too far from the conditions under which these explicit representations were formulated. The effectiveness of the proposed system has been experimented on real-world scenarios. The paper includes the problem statement and its characterization in the literature, as well as the proposed solving approach and experimental settings.

Optimal joint routing and link scheduling for real-time traffic in TDMA Wireless Mesh Networks

We investigate the problem of joint routing and link scheduling in Time-Division Multiple Access (TDMA) Wireless Mesh Networks (WMNs) carrying real-time traffic. We propose a framework that always computes a feasible solution (i.e. a set of paths and link activations) if there exists one, by optimally solving a mixed integer-nonlinear problem. Such solution can be computed in minutes or tens thereof for e.g. grids of up to 4x4 nodes. We also propose heuristics based on Lagrangian decomposition to compute suboptimal solutions considerably faster and/or for larger WMNs, up to about 50 nodes. We show that the heuristic solutions are near-optimal, and we exploit them to gain insight on the schedulability in WMN, i.e. to investigate the optimal placement of one or more gateways from a delay bound perspective, and to investigate how the schedulability is affected by the transmission range.

Ant Colony Optimization for Deadlock Detection in Concurrent Systems

Ensuring deadlock freedom is one of the most critical requirements in the design and validation of concurrent systems. The biggest challenge toward the development of effective deadlock detection schemes remains the state-space explosion problem when model checking is used for proving the correctness of a system with respect to a desired behavior. In this paper we propose the use of the Ant Colony Optimization (ACO) to reduce the state explosion problem arising when finding deadlocks in complex networks described using Calculus of Communicating Systems (CCS). Moreover, ACO is used to provide minimal counterexamples. In fact, although one of the strongest advantages of model checking is the generation of counterexamples when verification fails, traditional model checkers may return very long counterexamples. We present an implementation of our technique and encouraging experimental results on several benchmarks. These results are then compared with other heuristic-based search strategies, retaining the advantages of our approach.

State space reduction by non-standard semantics for deadlock analysis

In recent years many techniques have been developed for automatically verifying concurrent systems and most of them are based on the representation of the concurrent system by means of a transition system. State explosion is one of the most serious problems of this approach: often the prohibitive number of states renders the verification inefficient and, in some cases, impossible. We propose a method for reducing the state space of the transition system corresponding to a CCS process that suites deadlock analysis. The reduced transition system is generated by means of a non-standard operational semantics containing a set of rules which are, in some sense, an abstraction, preserving deadlock freeness, of the inference rules of the standard semantics. Our method does not build the standard transition system, but directly generates an abstract system with a fewer number of states, so saving memory space. We characterize a class of processes whose abstract transition system is not exponential in the number of parallel components.

Selective µ-calculus: New Modal Operators for Proving Properties on Reduced Transition Systems

In model checking for temporal logic, the correctness of a (concurrent) system with respect to a desired behavior is verified by checking whether a structure that models the system satisfies a formula describing the behaviour. Most existing verification techniques, and in particular those defined for concurrent calculi like as CCS, are based on a representation of the concurrent system by means of a labelled transition system. In this approach to verification, state explosion is one of the most serious problems. In this paper we present a new temporal logic, the selective mu-calculus, with the property that only the actions occurring in a formula are relevant to check the formula itself. We prove that the selective mu-calculus is as powerful as the mu-calculus. We define the notion of ρ-bisimulation between transition systems: given a set of actions p,a transition system ρ-bisimulates another one if they have the same behaviour with respect to the actions in p. We prove that, if two transition systems are ρ-equivalent, they preserve all the selective mu-calculus formulae with occurring actions in ρ. Consequently, a formula with occurring actions ρ can be more efficiently checked on a transition system ρ-equivalent to the standard one, but smaller than it.

Incremental construction of systems: An efficient characterization of the lacking sub-system

Abstract Software engineering research is driven by the aim of making software development more dynamic, flexible and evolvable. Nowadays the emphasis is on the evolution of pre-existing sub-systems and component and service-based development, where often only a part of the system is totally under control of the designer, most components being remotely operated by external vendors. In this context, we tackle the following problem: given the formal specification of the (incomplete) system, say it p , already built, how to characterize collaborators of p to be selected, based on a given communication interface L , so that a given property φ is satisfied. Using properties described by temporal logic formulae and systems by CCS processes, if φ is the formula to be satisfied by the complete system, an efficient and automatic procedure is defined to identify a formula ψ such that, for each existing process q satisfying ψ , the process ( p ∣ q ) ∖ L satisfies φ . Important features of this result are simplicity of the derived property ψ , compared to the original one, and scalability of the verification process. Such characteristics are necessary for applying the method to both incremental design and system evolution scenarios where p is already in place, and one needs to understand the specification of the functionality of the new component that should correctly interact with p . Indeed, in general, finding a suitable partner for p is easier than finding a complete system satisfying the global property. Moreover, in this paper it is shown how ψ can be used also to select a set of possible candidate processes q through a property-directed and structural heuristic. From the verification point of view, the description of the lacking component through a logic formula guarantees correctness of the integration with p of any process that exhibits a behaviour compliant with the inferred formula.

LORETO: a tool for reducing state explosion in verification of LOTOS programs

LOTOS is a formal specification language for concurrent and distributed systems. Basic LOTOS is the version of LOTOS without value‐passing. A widely used approach to the verification of temporal properties is model checking. Often, in this approach the formal specification is translated into a labeled transition system on which formulae expressing properties are checked. A problem with this verification technique is state explosion: concurrent systems are often represented by automata with a prohibitive number of states. In this paper we show how, given a set ρ of actions, it is possible to automatically obtain for a Basic LOTOS program a reduced transition system to which only the arcs labeled by actions in ρ belong. The set ρ of actions plays a fundamental role in conjunction with a temporal logic defined by the authors in a previous paper: selective mu‐calculus. The reduced system with respect to ρ preserves the truth value of all selective mu‐calculus formulae with actions from the set ρ. We act at both syntactic and semantic levels. From a syntactic point of view, we define a set of transformation rules obtaining a smaller program. On the semantic side, we define a non‐standard semantics which dynamically reduces the transition system during generation. We present a tool implementing both the syntactic and the semantic reduction. Copyright

Abstract reduction in directed model checking CCS processes

Model checking tools face a combinatorial blow up of the state-space (commonly known as the state explosion problem) that must be addressed to formally verify concurrent systems. We propose an approach combining abstraction techniques and heuristic search to overcome the problem above. In particular, heuristic search can avoid the bottleneck of the exhaustive exploration of the global state graph of a system, while retaining the advantages of abstraction techniques.