Erik J. Luit

A Static Scheduling Algorithm for Distributed Hard Real-Time Systems

A static scheduling algorithm is presented for off-line scheduling of tasks in distributed hard real-time systems. The tasks considered are instances of periodic jobs and have deadlines, resource requirements and precedence constraints. Tasks are divided into nonpreemptable blocks and all task characteristics are known a priori. The algorithm orders the tasks and iteratively schedules the tasks according to the order. Each task is scheduled globally by selecting a node to which it is assigned. Then, the task is scheduled locally by adding the task to the schedule of the selected node. Heuristics are used for both task ordering and node selection in order to guide the algorithm to a feasible schedule. Whenever local scheduling leads to an infeasible schedule, backtracking is used.Results of simulation studies of randomly generated task sets are presented. Although the scheduling problem is NP-hard, the results show that time performance is acceptable for off-line scheduling, except for extremely difficult task sets which make extensive use of the available resources.

Dedos: a distributed real-time environment

Until now, little research has been done on methods to combine the seemingly incompatible paradigms of hard and soft real-time systems. To this end, we have developed Dedos, a dependable distributed operating system. The driving forces behind the project are twofold: to meet the demand for dependable distributed control systems, especially in the area of embedded systems and industrial control; and to increase the productivity and quality of application programming for distributed control. Our current focus is on hard real-time issues; soft real-time needs are handled by conventional techniques. However, our work has raised interesting questions about the communication between the soft and hard real-time tasks of the system, which is necessary to pass externally specified control parameters and control status information. The problem is that the data set must always be consistent (concurrency atomicity), but hard real-time activities can never be delayed by soft real-time ones. Other intriguing questions are related to the integration of the reliability and security concepts that are used in the two parts of the systems. in this paper, however, we limit our discussion to the Dedos development model, the Dedos programming model, hard real-time scheduling, and the distributed algorithms needed to implement the Dedos execution environment.<<ETX>>

Invariants for Non-Hierarchical Object Structures

We present a Hoare-style specification and verification approach for invariants in sequential OO programs. It allows invariants over non-hierarchical object structures, in which update patterns that span several objects and methods occur frequently. This gives rise to invalidating and subsequent re-establishing of invariants in a way that compromises standard data induction, which assumes invariants hold when a method is called. We provide specification constructs (inc and coop) that identify objects and methods involved in such patterns, allowing a refined form of data induction. The approach now handles practical designs, as illustrated by a specification of the Observer Pattern.

Incorporating Temporal Considerations during Assignment and Pre-run-time Scheduling of Objects and Processes

Object-based programming techniques help to reduce the cost of software development and maintenance due to the benefits of reuse, information hiding, and encapsulation. This is especially helpful in large, real-time systems that are highly parallel and distributed. The paper reviews asemi-preemptionexecution model of object-based real-time systems that simplifies reasoning about the quality of process-to-processor assignment. The model is used to define system properties such as interprocess parallelism, processor utilization, and interprocessor communication. Additionally, an innovative assignment algorithm is presented which incorporates feasibility constraints. The algorithm is guided by an objective that balances minimum communication against maximum parallelism. Experimental results show that the process assignment algorithm performs extremely well with respect to finding process assignments in isolation. The algorithm easily finds process assignments for which a feasible schedule exists as long as the number of items to be scheduled does not exceed approximately 500.

Cooperation-based Invariants for OO Languages

In general, invariants may depend on the state of other objects. The approach introduced in this paper allows this for objects of mutually visible classes, in a way that supports modular verification. To this end, dependencies are made explicit by cooperation. In particular, invariants expressing non-hierarchical object relations are supported. Furthermore, an inc-set allows a method to specify explicitly that it does not depend on the validity of a certain invariant. This way, it can be called even when that invariant is violated.

The POLIPO Security Framework

Systems of systems are dynamic coalitions of distributed, autonomous and heterogeneous systems that collaborate to achieve a common goal. While offering several advantages in terms of scalability and flexibility, the systems of systems paradigm has a significant impact on systems interoperability and on the security requirements of the collaborating systems. In this chapter we introduce POLIPO, a security framework that protects the information exchanged among the systems in a system of systems, while preserving systems’ autonomy and interoperability. Information is protected from unauthorized access and improper modification by combining context-aware access control with trust management. Autonomy and interoperability are enabled by the use of ontology-based services. More precisely, each authority may refer to different ontologies to define the semantics of the terms used in the security policy of the system it governs and to describe domain knowledge and context information. A semantic alignment technique is then employed to map concepts from different ontologies and align the systems’ vocabularies. We demonstrate the applicability of our solution with a prototype implementation of the framework for a scenario in the maritime safety and security domain.

Assignment and pre-run-time scheduling of object-based, parallel real-time processes

The use of object-based programming techniques helps to reduce the cost of software development and maintenance, due to the benefits of reuse, information hiding and encapsulation. This is especially helpful in complex, real-time systems that are highly parallel and distributed, due to their magnitude. The paper presents a semi-preemption execution model of object-based real-time systems that simplifies reasoning about the quality of particular process-to-processor assignments. The model is used to define system properties such as inter-process parallelism, processor utilization and inter-processor communication. Additionally, we present an innovative assignment algorithm that incorporates feasibility constraints, and is guided by an objective that balances minimum communication against maximum parallelism. This work is part of a complete assignment and pre-run-time scheduling approach for distributed real-time systems.<<ETX>>