Chapter 13 Components in Real-Time Systems

1. Chapter 13 Components in Real-Time Systems Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

2. Overview • Introduction • Characteristics and challenges of real-time systems • Real-time component models • Designing component-based real-time systems • Composition of components • Example: RT components in Rubus OS Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

3. Introduction • Embedded computers • Medical control equipment, mobile phones, and vehicle control systems. • Most of such embedded systems can also be characterized as real-time systems. • They must usually meet stringent specifications for safety, reliability, limited hardware capacity etc Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

4. Characteristics and Challenges of RTS • Real-time systems are computing systems in which the meeting of timing constraints is essential to correctness. • If the system delivers the correct answer, but after a certain deadline, it could be regarded as having failed. Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

5. Tasks • Real-time systems can be constructed of sequential programs, but typically they are built of concurrent programs, called tasks. • Tasks are usually divided into: • Periodic tasks: consist of an infinite sequence of identical activities, called instances, which are invoked within regular time periods. • Non-periodic : are invoked by the occurrence of an event. Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

6. Scheduling • Offline scheduling: • The scheduler has complete knowledge of the task set and its constraints. • Online scheduling: • Make their scheduling decisions during run-time. • Deadline: • Is the maximum time within which the task must complete its execution with respect to an event. • Real-time systems are divided into two classes, hard and soft real-time systems Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

7. Limited Resources • Common CBSE technologies (JavaBeans, CORBA and COM) are seldom used as they: • Require excessive processing requirements • Require excessive memory requirements • Provide unpredictable timing characteristics Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

8. System Level Analysis • At system level we analyze to determine if the system composed fulfils the timing requirements. • Several different mature analysis methods exist, for example, analysis for priority-based systems and pre-run-time scheduling techniques Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

9. Real-time Component Models • Using a standard operating system in a real-time application, such as windows NT must be done carefully, as it was designed to be used so. Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

10. Application-specific Component Models • Maintain a component library which the application engineer can use when developing an application. • In addition to infrastructure components, domain specific component models, which in fact have been used for many years for certain domains must be considered. Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

11. IEC 61131-3 Application Structure Configuration Variable Resource Resource access path Task Task Task Task FB Function Block Program Program Program Program Variable FB FB FB FB Global and direct variables Execution control path Access path Communication Function Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

12. A Configuration in IEC 61131-3 • Encapsulates all software for an application and consists of one or several resources which provide the computational mechanisms. Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

13. A Program in IEC 61131-3 • A program is written in any of the languages proposed in the standard, for example: • Instruction lists • Assembly languages • Structured text • A high level language similar to Pascal • Ladder diagrams • Function block diagrams (FBD) Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

14. Function Block Diagram A simple function block diagram representing a feedback control loop. FB1 FB2 Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

15. A Port-based Object Approach • The model is based upon the development of domain-specific components which maximize usability, flexibility and predictable temporal behavior. • Independent tasks are the bases for the PBO model. • Whenever a PBO needs data for its computation, it reads the most recent information from its in-ports, irrespective of its producer. • The PBOs are in their nature periodic and the system can be analyzed using traditional schedulability analysis. Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

16. A Port-based Object Configuration parameters Variable input ports Variable output ports Port-based object Resource ports for communication with sensors and actuators Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

17. Designing Component-based RTS System specification Component library Top-level design Detailed design Architecture analysis Create specifications for the new components Add new components to library Implement and verify new components using classical development methods Scheduling / interface check Obtain components timing behavior on target platform System verification Final product Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

18. Top-level Design • The first stage of the development process involves de-composition of the system into manageable components Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

19. Detailed Design • At this stage a detailed component design is performed, by selecting components to be used from the candidate set. Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

20. Architecture Analysis • At this stage it is time to check that the system under development satisfies extra-functional requirements such as: • Maintainability • Reusability • Modifiability • Testability Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

21. Scheduling • At this point we must check that the temporal requirements of the system can be satisfied, assuming time budgets assigned in the detailed design stage. • In other words, we need to make a schedulability analysis of the system based on the temporal requirements of each component Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

22. WCET Verification • Performing a worst-case analysis can either be based on measurements or on a static analysis of the source code. • What is more interesting in the test cases is the execution time behavior shown as a function of input parameters as shown in the following slide. Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

23. Execution time Input domain 2 domain 1 domain 3 An Execution Time Graph • The execution time shows different values for the different input sub-domains. Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

24. Execution time Input domain 2 domain 1 domain 3 Maximum execution time per sub-domain Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

25. Implementation of New Components • New components; Those not already in the library must be implemented. The designer of the component has two requirements: • The functional requirements • The assigned time budget Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

26. System Build and Test • Finally, we build the system using old and new components. • We must now verify the functional and temporal properties of the system obtained. • If the verification test fails, we must return to the relevant stage of the development process and correct the error. Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

27. Component Library • Is the most central part of any CBSE system as it contains binaries of components and their descriptions. • A component library containing real-time components should provide the following: • Memory requirements • WCET test cases • Dependencies • Environment assumptions Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

28. Composition of Components Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

29. End-To-End Deadlines • End-to-end deadlines • Are set such that the system requirements are fulfilled in the same way as the time budgets are set • Should be specified for the input to and output from the component since the WCET cannot be computed since its parts may be executing with different periods. Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

30. Specification Of Timing Attributes • We specify virtual timing attributes of the composed component, which are used to compute the timing attributes of sub-components, ie: IF virtual period is set to P, THEN the period of a sub-component A should be fA * P AND the period of B is fB * P, WHERE fA and fB are constants for the composed component, which are stored in the component library Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

31. RT Components in Rubus OS • Rubus: • Is one of a few real-time operating systems currently available which have some concept of components. • Is a hybrid operating system, in the sense that it supports both pre-emptive static scheduling and fixed priority scheduling. Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

32. A Task and Its Interfaces • The timing requirements are specified by release-time, deadline, WCET and period Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

33. A Composed System in the Red Model of Rubus • The task depicted below is required to execute before the outputBrakeValues task, (i.E. Task BrakeLeftRight precedes task outputBrakeValues). Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems

34. Composition of Components in Rubus Building Reliable Component-based SystemsChapter 13 -Components in Real-Time Systems