Andrea Bondavalli

The meaning and role of value in scheduling flexible real-time systems

The real-time community is devoting considerable attention to flexible scheduling and adaptive systems. One popular means of increasing the flexibility, and hence effectiveness, of real-time systems is to use value-based scheduling. It is surprising however, how little attention has been devoted, in the scheduling field, to the actual assignment of value. This paper deals with value assignment and presents a framework for undertaking value-based scheduling and advises on the different methods that are available. A distinction is made between ordinal and cardinal value functions. Appropriate techniques from utility theory are reviewed. An approach based on constant value modes is introduced and evaluated via a case example.

Threshold-based mechanisms to discriminate transient from intermittent faults

This paper presents a class of count-and-threshold mechanisms, collectively named /spl alpha/-count, which are able to discriminate between transient faults and intermittent faults in computing systems. For many years, commercial systems have been using transient fault discrimination via threshold-based techniques. We aim to contribute to the utility of count-and-threshold schemes, by exploring their effects on the system. We adopt a mathematically defined structure, which is simple enough to analyze by standard tools. /spl alpha/-count is equipped with internal parameters that can be tuned to suit environmental variables (such as transient fault rate, intermittent fault occurrence patterns). We carried out an extensive behavior analysis for two versions of the count-and-threshold scheme, assuming, first, exponentially distributed fault occurrencies and, then, more realistic fault patterns.

Markov regenerative stochastic petri nets to model and evaluate phased mission systems dependability

This study deals with model-based dependability transient analysis of phased mission systems. A review of the studies in the literature showed that several aspects of multiphased systems pose challenging problems to the dependability evaluation methods and tools. To attack the weak points of the state-of-the-art we propose a modeling methodology that exploits the power of the class of Markov regenerative stochastic Petri net models. By exploiting the techniques available in the literature for the analysis of the Markov Regenerative Processes, we obtain an analytical solution technique with a low computational complexity, basically dominated by the cost of the separate analysis of the system inside each phase. Last, the existence of analytical solutions allows us to derive the sensitivity functions of the dependability measures, thus providing the dependability engineer with additional means for the study of phased mission systems.

GUARDS: a generic upgradable architecture for real-time dependable systems

The development and validation of fault-tolerant computers for critical real-time applications are currently both costly and time consuming. Often, the underlying technology is out-of-date by the time the computers are ready for deployment. Obsolescence can become a chronic problem when the systems in which they are embedded have lifetimes of several decades. This paper gives an overview of the work carried out in a project that is tackling the issues of cost and rapid obsolescence by defining a generic fault-tolerant computer architecture based essentially on commercial off-the-shelf (COTS) components (both processor hardware boards and real-time operating systems). The architecture uses a limited number of specific, but generic, hardware and software components to implement an architecture that can be configured along three dimensions: redundant channels, redundant lanes, and integrity levels. The two dimensions of physical redundancy allow the definition of a wide variety of instances with different fault tolerance strategies. The integrity level dimension allows application components of different levels of criticality to coexist in the same instance. The paper describes the main concepts of the architecture, the supporting environments for development and validation, and the prototypes currently being implemented.

Failure classification with respect to detection

A comprehensive classification of failures in computing systems is discussed. The underlying guidelines are: (1) a failure classification must be independent of any particular system; that is, it should be able to be applied to every system; (2) a failure classification must be as detailed as possible; (3) treatment of failures following detection and other parameters, such as the severity of failures, are to be considered as a second step, since they may require a defined categorization. The main advantage of the failure classification is the possibility of characterizing systems with respect to their failure modes, providing designers with a way to choose the most appropriate detection techniques for each particular system.<<ETX>>

Stochastic dependability analysis of system architecture based on UML models

The work in this paper is devoted to the definition of a dependability modeling and model based evaluation approach based on UML models. It is to be used in the early phases of the system design to capture system dependability attributes like reliability and availabiUty, thus providing guidelines for the choice among different architectural and design solutions. We show how structural UML diagrams can be processed to filter out the dependability related information and how a system-wide dependability model is constructed. Due to the modular construction, this model can be refined later as more detailed information becomes available. We discuss the model refinement based on the General Resource Model, an extension of UML. We show that the dependability model can be constructed automatically by using graph transformation techniques.

Dependable Computing EDCC-4

Standard network QoS analysis usually accounts for the infrastructure performance/availability only, with scarce consideration of the user perspective. With reference to the General Packet Radio Service (GPRS), this paper addresses the problem of how to evaluate the impact of system unavailability periods on QoS measures, explicitly accounting for user characteristics. In fact, the ultimate goal of a service provider is user satisfaction, therefore it is extremely important to introduce the peculiarities of the user population when performing system analysis in such critical system conditions as during outages. The lack of service during outages is aggravated by the collision phenomenon determined by accumulated users requests, which (negatively) impacts on the QoS provided by the system for some time after its restart. Then, depending on the specific behavior exhibited by the variety of users, such QoS degradation due to outages may be perceived differently by different user categories. We follow a compositional modeling approach, based on the GPRS and user models; the focus is on the GPRS random access procedure on one side, and different classes of users behavior on the other side. Quantitative analysis, performed using a simulation approach, is carried out, showing the impact of outages on a relevant QoS indicator, in relation with the considered user characteristics and network load.

Automated dependability analysis of UML designs

The paper deals with the automatic dependability analysis of systems designed using UML. An automatic transformation is defined for the generation of models to capture systems dependability attributes, like reliability. The transformation concentrates on structural UML views, available early in the design, to operate at different levels of refinement, and tries to capture only the information relevant for dependability to limit the size (state space) of the models. Due to the modular construction, these models can be refined later as more detailed, relevant information becomes available. Moreover a careful selection of those critical parts to be detailed allows one to avoid explosion of the size. An implementation of the transformation is in progress and will be integrated in the toolsets available for the ESPRIT LTR HIDE project.

Dependability modeling and evaluation of phased mission systems: a DSPN approach

We focus on analytical modeling for the dependability evaluation of phased-mission systems. Because of their dynamic behavior, systems showing a phased behavior offer challenges in modeling. We propose the modeling and evaluation of phased-mission system dependability through the Deterministic and Stochastic Petri Nets (DSPN). The DSPN approach to the phased-mission systems offers many advantages, concerning both the modeling and the solution. The DSPN model of the mission can be a very concise one, and it can be efficiently solved for dependability evaluation purposes. The solution procedure is supported by the existence of an analytical solution for the transient probabilities of the marking process underlying the DSPN model. This analytical solution can be fully automated. We show how the DSPN model capabilities are able to deal with various peculiar features of phased-mission systems, including those systems where the next phase to be performed can be chosen at the time the preceding phase ends.

Experimental evaluation of the QoS of failure detectors on wide area network

This paper describes an experiment performed on wide area network to assess and fairly compare the quality of service provided by a large family of failure detectors. Failure detectors are a popular middleware mechanism used for improving the dependability of distributed systems and applications. Their QoS greatly influences the QoS that upper layers may provide. It is thus of uttermost importance to equip a system with an appropriate failure detector and to properly tune its parameters for the most desirable QoS to be provided. The paper first analyzes the QoS indicators and the structure of push-style failure detectors and then introduces the choices for estimators and safety margins used to build several (30) failure detectors. The experimental setup designed and implemented to allow a fair comparison of QoS of the several alternatives in a real representative experimental setting is then described. Finally the results obtained through the experiments and their interpretation are provided.