Johan Lilius

vUML: a tool for verifying UML models

The Unified Modelling Language (UML) is a standardised notation for describing object oriented software designs. We present vUML, a tool that automatically verifies UML models where the behaviour of the objects is described using UML Statecharts diagrams. The tool uses the SPIN model checker to perform the verification, but the user does not have to know how to use SPIN or the PROMELA language. If an error is found during the verification, the tool creates a UML sequence diagram showing how to reproduce the error in the model.

Formalising UML state machines for model checking

The paper discusses a complete formalisation of UML state machine semantics. This formalisation is given in terms of an operational semantics and it can be used as the basis for code-generation, simulation and verification tools for UML Statecharts diagrams. The formalisation is done in two steps. First, the structure of a UML state machine is translated into a term rewriting system. In the second step, the operational semantics of state machines is defined. In addition, some problematic situations that may arise are discussed. Our formalisation is able to deal with all the features of UML state machines and it has been implemented in the vUML tool, a tool for model-checking UML models.

Partial Order Reductions for Timed Systems

In this paper, we present a partial-order reduction method for timed systems based on a local-time semantics for networks of timed automata. The main idea is to remove the implicit clock synchronization between processes in a network by letting local clocks in each process advance independently of clocks in other processes, and by requiring that two processes resynchronize their local time scales whenever they communicate. A symbolic version of this new semantics is developed in terms of predicate transformers, which enjoys the desired property that two predicate transformers are independent if they correspond to disjoint transitions in different processes. Thus we can apply standard partial order reduction techniques to the problem of checking reachability for timed systems, which avoid exploration of unnecessary interleavings of independent transitions. The price is that we must introduce extra machinery to perform the resynchronization operations on local clocks. Finally, we present a variant of DBM representation of symbolic states in the local time semantics for efficient implementation of our method.

A survey on ontologies for human behavior recognition

Describing user activity plays an essential role in ambient intelligence. In this work, we review different methods for human activity recognition, classified as data-driven and knowledge-based techniques. We focus on context ontologies whose ultimate goal is the tracking of human behavior. After studying upper and domain ontologies, both useful for human activity representation and inference, we establish an evaluation criterion to assess the suitability of the different candidate ontologies for this purpose. As a result, any missing features, which are relevant for modeling daily human behaviors, are identified as future challenges.

Efficient State Space Search for Time Petri Nets

Partial-order reduction methods provide a number of well studied methods that have been succesfully applied to the state-space explosion problem that arises when analysing state based models of concurrent and reactive systems. The techniques have mainly been studied within the context of untimed systems, in the context of real-time systems little progress has been made. The main problem seems to be the global nature of time, that makes all clocks in the system dependent on each other. Typically this is manifested in the semantics of real-time models in which the ordering of events is implicitely stored in the state of the system. In this work we propose a new semantics for time Petri nets that, does not store the firing order into the timing constraints. This allows us to lift the notion of independence from untimed Petri nets, and it becomes possible to directly apply the theory of partial-order reductions to time Petri nets. We show that our semantics is finite and that it preserves reachable markings. As a second contribution we propose to exploit the branching prefix originally introduced by McMillan in the state-space search of time Petri nets. To make sure that the state-space search algorithm terminates, one needs a list of states that have already been seen during the search. This list is searched each time a new state is generated. We propose to only store states after cutoff events.The cutoff events in a branching prefix describe events after which the system will return to a state in which it has previously been. Finally we also show how to extract persistent sets from the branching prefix.

Timing Analysis of UML Sequence Diagrams

For real-time systems, UML sequence diagrams describe interaction among objects, which show the scenarios of system behaviour. In this paper, we give the solution for timing analysis of simple UML sequence diagrams which describe exactly one scenario without any alternatives and loops, and develop an algorithm for checking the compositions of UML sequence diagrams, which describe multiple scenarios, for timing consistency.

Prediction-Based Dynamic Resource Allocation for Video Transcoding in Cloud Computing

This paper presents prediction-based dynamic resource allocation algorithms to scale video transcoding service on a given Infrastructure as a Service cloud. The proposed algorithms provide mechanisms for allocation and deallocation of virtual machines (VMs) to a cluster of video transcoding servers in a horizontal fashion. We use a two-step load prediction method, which allows proactive resource allocation with high prediction accuracy under real-time constraints. For cost-efficiency, our work supports transcoding of multiple on-demand video streams concurrently on a single VM, resulting in a reduced number of required VMs. We use video segmentation at group of pictures level, which splits video streams into smaller segments that can be transcoded independently of one another. The approach is demonstrated in a discrete-event simulation and an experimental evaluation involving two different load patterns.

A fuzzy ontology for semantic modelling and recognition of human behaviour

We propose a fuzzy ontology for human activity representation, which allows us to model and reason about vague, incomplete, and uncertain knowledge. Some relevant subdomains found to be missing in previous proposed ontologies for this domain were modelled as well. The resulting fuzzy OWL 2 ontology is able to model uncertain knowledge and represent temporal relationships between activities using an underlying fuzzy state machine representation. We provide a proof of concept of the approach in work scenarios such as the office domain, and also make experiments to emphasize the benefits of our approach with respect to crisp ontologies. As a result, we demonstrate that the inclusion of fuzzy concepts and relations in the ontology provide benefits during the recognition process with respect to crisp approaches.

A causal semantics for time Petri nets

The objective of this work is to give time Petri nets a partial order semantics, akin to the nonsequential processes of untimed net systems. To this end a time process of a time Petri net is defined as a traditionally constructed causal process with a valid timing. A timing is a labelling that attaches occurrence times to the events of the process that must satisfy specific validness criteria. The main result of the paper is the bijective correspondence between firing schedules (the classical interleaving semantics of time Petri nets) and linearizations of time processes. The result shows that time processes correctly represent the behavior of the system. Using the definition of validness, an efficient algorithm for checking validness of given timings is derived. Also a sufficient condition is given for when the invalidity of timings for a process can be inferred from its initial subprocess. To compute, e.g. the maximum time separation between two events in a time process an alternative characterization of validness is developed. This definition is used to derive an algorithm for constructing the set of all valid timings for a process. The set of valid timings is presented as sets of alternative linear constraints, which can be used in optimization problems. It is shown that the existence of a valid timing for a given process can be decided in NP time.

Stream-Based Admission Control and Scheduling for Video Transcoding in Cloud Computing

This paper presents a novel approach for stream-based admission control and job scheduling for video transcoding called SBACS (Stream-Based Admission Control and Scheduling). SBACS uses queue waiting time of transcoding servers to make admission control decisions for incoming video streams. It implements stream-based admission control with per stream admission. To ensure efficient utilization of the transcoding servers, video streams are segmented at the Group of Pictures level. In addition to the traditional rejection policy, SBACS also provides a stream deferment policy, which exploits cloud elasticity to allow temporary deferment of the incoming video streams. In other words, the admission controller can decide to admit, defer, or reject an incoming stream and hence reduce rejection rate. In order to prevent transcoding jitters in the admitted streams, we introduce a job scheduling mechanism, which drops a small proportion of video frames from a video segment to ensure continued delivery of video contents to the user. The approach is demonstrated in a discrete-event simulation with a series of experiments involving different load patterns and stream arrival rates.