Towards Reliable Benchmarks of Timed Automata

The verification of the time-dependent behavior of safety-critical systems is important, as design problems often arise from complex timing conditions. One of the most common formalisms for modeling timed systems is the timed automaton, which introduces clock variables to represent the elapse of time. Various tools and algorithms have been developed for the verification of timed automata. However, it is hard to decide which one to use for a given problem as no exhaustive benchmark of their effectiveness and efficiency can be found in the literature. Moreover, there does not exist a public set of models that can be used as an appropriate benchmark suite. In our work we have collected publicly available timed automaton models and industrial case studies and we used them to compare the efficiency of the algorithms implemented in the Theta model checker. In this paper, we present our preliminary benchmark suite, and demonstrate the results of the performed measurements

Activity-Based Abstraction Refinement for Timed Systems

Formal analysis of real time systems is important as they are widely used in safety critical domains. Such systems combine discrete behaviours represented by locations and timed behaviours represented by clock variables. The counterexample-guided abstraction refinement (CEGAR) algorithm utilizes the fundamental technique of abstraction to system verification. We propose a CEGAR-based algorithm for reachability analysis of timed systems. The algorithm is specialized to handle the time related behaviours efficiently by introducing a refinement technique tailored specially to clock variables. The performance of the presented algorithm is demonstrated by runtime measurements on models commonly used for benchmarking such algorithms

Adaptive Step Size Control for Hybrid CT Simulation without Rollback

The Hybrid CT approach for simulating cyber-physical systems uses continuous time simulation and provides wrappers for discrete event components that implement the required interfaces. Besides the general obstacles of continuous time simulation, Hybrid CT introduces new challenges, such as creating wrappers, detecting discrete events (with minimal latency), and finding the correct balance between the simulation step sizes required by different components. We propose an adaptive step size controller that uses high level information of the model and the simulation (e.g. types of components, critical values of variables) to adjust the step size based on the possibility of the detection of a discrete event in the following step. Besides overcoming the challenges of Hybrid CT simulation the component also improves threshold-crossing detection. The proposed approach does not require step rejection (rollback), that discrete event components often fail to support. In this paper we present the step size controller, demonstrate its usability on industrial case studies and evaluate the component both theoretically and based on measurements performed on our implementation that was integrated to the OMSimulator. We show that adaptive step size control can be used to bridge the gap between continuous time and discrete event simulation

Diversity of graph models and graph generators in mutation testing

When custom modeling tools are used for designing complex safety-critical systems (e.g., critical cyber-physical systems), the tools themselves need to be validated by systematic testing to prevent tool-specific bugs reaching the system. Testing of such modeling tools relies upon an automatically generated set of models as a test suite. While many software testing practices recommend that this test suite should be diverse, model diversity has not been studied systematically for graph models. In the paper, we propose different diversity metrics for models by generalizing and exploiting neighborhood and predicate shapes as abstraction. We evaluate such shape-based diversity metrics using various distance functions in the context of mutation testing of graph constraints and access policies for two separate industrial DSLs. Furthermore, we evaluate the quality (i.e., bug detection capability) of different (random and consistent) model generation techniques for mutation testing purposes

Backward Reachability Analysis for Timed Automata with Data Variables

Efficient techniques for reachability analysis of timed automata are zone-based methods that explore the reachable state space from the initial state, and SMT-based methods that perform backward search from the target states. It is also possible to perform backward exploration based on zones, but calculating predecessor states for systems with data variables is computationally expensive, prohibiting the successful application of this approach so far. In this paper we overcome this limitation by combining zone-based backward exploration with the weakest precondition operation for data variables. This combination allows us to handle diagonal constraints efficiently as opposed to zone-based forward search where most approaches require additional operations to ensure correctness. We demonstrate the applicability and compare the efficiency of the algorithm to existing forward exploration approaches by measurements performed on industrial case studies. Although the large number of states often prevents successful verification, we show that data variables can be efficienlty handled by the weakest precondition operation. This way our new approach complements existing techniques

Mosses and lichens in dynamics of acidic sandy grasslands: specific response to grazing exclosure = Mohok és zuzmók legelelés kizárásra adott válasza savanyú homoki gyepekben

Cryptogams, a neglected segment in vegetation dynamics, can form a large part of biomass in sandy grasslands. Since the pioneer work of Verseghy (early 1970s) their biomass and productivity has not been analyzed in Hungary. We intended to contribute to the knowledge of dynamics of cryprogams in dry acidic grasslands. Study objects were permanent plots in endangered open sandy grasslands ‘Pannonic inland dunes’ (EU Habitat Directive, 2340). Vegetation of CORYNEPHORETUM CANESCENTIS (CC) and FESTUCETUM VAGINATAE (FV), characteristic vegetation types of the Nyírség region (NE Hungary) has been monitored in grazed and experimentally fenced stands. Fencing was used to model the overall trend of falling stocking densities. Biomass of cryptogams has been sampled in 2013, five years after grazing exclosure then hand-sorted into species. Morphological studies, spot tests and HPTLC have also been applied to identify critical lichen taxa. Fencing has led to an increased biomass of cryptogams. Biomass of mosses exceeded those of lichens irrespective of the community or management. Cryptogamic biomass tended to be lower in CC than in FV and lower in grazed stands compared to fenced ones. Most frequent cryptogams (Brachythecium albicans, Cladonia rangiformis, C. rei, Polytrichum piliferum) have been supported by exclosure. Lichens benefitted relatively more from exclosure than did the mosses. The only lichen favoured by moderate grazing has been Cladonia magyarica. Soil traits (pH, organic material, P2O5) had contrasting effects on biomass fractions under different management (Spearman’s rank correlation). More significant correlations have been revealed in fenced stands than in the grazed ones. FV had more correlations than the CC. The few strict correlations included preference of Syntrichia ruralis for higher while that of Polytrichum piliferum for lower pH values. Low phosphorous content has been positively correlated with Cladonia rangiformis, Polytrichum piliferum, total moss and total of cryptogams while higher values only supported Brachythecium albicans. This suggests more organized assemblages in ungrazed stands and in the FV, respectively. Compared to soil traits microtopography had a minor effect on cryptogam distribution. Initial establishment pattern or random patch formation seem to be as important as all above-mentioned factors together. Long-term grazing exclosure is likely to promote further spread of the dominants, Cladonia rangiformis and Polytrichum piliferum. Short spells of grazing are likely to promote cryptogam biodiversity. Chemical analyses have been supported by OTKA K81232 and NKFIH K_17/124341