J.P.C. Verhoosel

A model for scheduling of object-based, distributed real-time systems

This paper describes a general model for pre-run-time scheduling of distributed real-time systems that are composed of abstract data types (definable in languages such as Ada, Clu and Modula-2) and abstract data objects (which can be defined in C++, Eiffel and RT Euclid). An architecture model, a programming paradigm, and execution and communication paradigms form the basis for this general model. The model includesabsolute timing constraints to represent periodicity and deadlines,relative timing constraints to model several kinds of timed precedence relations and synchronization requirements,independency constraints to capture non-determinism of conditionals and repetitions, andconsistency constraints to enforce consistent use of resources. In this paper, the model is formalized to obtain a mathematical foundation on which assignment (Verhoosel et al. 1993a, Welch 1992, Welch et al. 1993) and pre-run-time scheduling problems (Verhoosel et al. 1991, Verhoosel 1992, 1993a, 1993c) are defined. Additionally, the model is extended to allow exploitation of parallelism from programs, a technique that can be used during assignment and scheduling for meeting timing constraints.

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>>

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.

Pre-run-time scheduling for object-based, concurrent, real-time applications

The complexity of computer systems in many application areas has increased rapidly in the last decade. In order to deal with this complexity, application programs should be constructed by layering reusable modules. Therefore, an object-based design and implementation methodology is advocated. In addition, dependability of these computer systems is very important, and thus timeliness has to be guaranteed. In this paper, a pre-run-time scheduling approach is proposed to guarantee timeliness for computer systems in an almost fully predictable environment and in the presence of faults. The advantages of pre-run-time scheduling over other strategies to guarantee timeliness are discussed. The approach is used to schedule applications with a novel combination of timing and functional constraints. In addition, a novel characteristic of the approach is the enhancement of inherent parallelism of an application in order to find a schedule.<<ETX>>

Metrics for assessment of designs and implementations during reengineering of computer-based systems

Modern computer-based systems have many required characteristics, including performance, concurrency, timeliness, availability, dependability, safety and security. Aging computer-based systems are being reengineered to exploit distributed and highly parallel hardware platforms, in order to accommodate increased functional requirements and to achieve dependability. Simultaneously, reengineering is introducing modern software engineering principles such as component layering, encapsulation and information hiding, in order to reduce maintenance burdens and to simplify future enhancement. This paper summarizes a process for reengineering of computer-based systems to achieve these goals. The process incorporates experience gained through the reengineering of the Weapons Selection module and the Surface Operations module of the AEGIS Weapon System. A key component of the reengineering process is themission critical software architecture (MCSA), a hierarchical view of systems that includes the abstraction levels of programs, tasks, packages, procedures, and statements. To guide the reengineering processes of software transformation, concurrency metrics are defined at the procedure, package and task levels of the MCSA. The paper defines the intermediate representation (IR) needed to compute the metrics, and shows how to use the IR to compute several metrics: the percentage of concurrency within procedures and packages, and the amount of potential concurrency among tasks. The metrics are important in any reengineering approach that iteratively transforms design and code, or that deals with system configuration (the integration of software, hardware and humanware).

Off-Line Scheduling of Hard Real-Time Distributed Systems Using Windows

The last decade dependability of computer systems has become a more and more important topic in computing science. Dependability of a computer system includes besides correct functionality also timeliness, i.e., hard real-time constraints imposed on the tasks of the system must be satisfied. In our opinion, timeliness can only be guaranteed when the event rate of the environment is bound and all hard real-time tasks are scheduled off-line incorporating all resources. An approach for off-line scheduling of tasks in hard real-time distributed systems is presented.

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>>