Environment: dictates timeliness requirements, to which the internal system has to react on time.

Transcription

1 1. Introduction 1.1 What is a Real-Time System? Environment: dictates timeliness requirements, to which the internal system has to react on time. Internal system: contains many processes that are executed repeatedly periodic readings of instruments evaluation of the measured data computation of derived data e.g. monitoring purposes generation of signals for adjusting the actuators appropriately 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-1

2 Real-Time System sensed data control data Environment "internal" "external" 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-2

3 General requirement: Technical systems demand from their control systems significant computation and control processing, and a guarantee of predictable, reliable and timely operation. Design problems the of real-time systems are manifold: Technical processes as presented by an external system are usually very complex. The corresponding real-time systems consist of a large number of dependent components. The notion of "system" is difficult to handle. Essential dependencies have to be realized. Both, the external system and the real-time system have to be partitioned into smaller components ("partial systems", "partial processes"). Knowledge about partial systems is usually incomplete K. H. Ecker, T.U. Clausthal. 1. Introduction 1-3

4 1.2 Characterization and Definition of Real-Time-Systems Definition of a real-time system [Koymans et al. 1988]: "A real-time system is defined as an interaction system that maintains an ongoing relationship with an asynchronous environment, i.e. an environment that progresses irrespectively of the real-time system in an uncooperative manner. The real-time system is fully responsible for the proper synchronization of its operation with respect to its environment." Alternative Definition of real-time systems: "... one which controls an environment by receiving data, processing them, and taking action or returning results sufficiently quick to affect the functioning of the environment at that time." J. Martin, Design of Real-Time Computer Systems. Prentice-Hall, K. H. Ecker, T.U. Clausthal. 1. Introduction 1-4

5 More detailed description: "Real-time operation is the operating mode of a computing system, in which the programs for the processing of data arriving from the outside are permanently ready in such a way, that the processing results become available within a priori given time frames. According to the application, the data may become available for the processing or at predetermined points of time." DIN 44300: Informationsverarbeitung. Beuth-Verlag, 1985 Closely related to real-time systems: embedded systems, mechatronic systems Some applications are industrial robots, NC-machines, engine control J. Wikander, B. Svensson (1998) 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-5

6 General questions that arise in connection with a realization concern which microcomputers or microprocessors should be used, the interfaces between the technical processes and the computational processes, the choice of bus systems for communication between closely connected real-time processes, which multiprocessor architecture should be chosen in case of a parallel or distributed realization. These questions lead to the general problem areas of requirement analysis, specification of real-time systems, computer architecture, operating systems, programming, others (data bases, control theory, fault tolerant systems, etc.) 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-6

7 In more detail, one has to deal with questions from different areas: Hardware architectures and distributed automation structures Special requirements of real-time computers concern local and global bus systems hardware architectures with special regard to achieving fault-tolerant systems hardware configurations centralized, decentralized, hierarchical Distributed systems External technical processes usually consist of a number of more or less independent sub-processes which operate in parallel. requirement for simultaneous processing of external process requests Real time systems must be and must also provide essentially distributed parallel processing capabilities K. H. Ecker, T.U. Clausthal. 1. Introduction 1-7

8 Real-time operating systems with special emphasis on task scheduling: Particular questions: deadlock handling and prevention synchronization concepts memory management priority and time driven task scheduling strategies under hard timing conditions predictability and guaranteed execution time bounds 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-8

9 Basic requirements with regard to the time behavior: Concerns the general properties of timeliness, simultaneousness, and predictability Special questions: fundamental concepts of real-time mode of computer operation, process types, process coupling, interrupt systems. In particular one has to deal with the concept of tasks as parallel program threads in a multiprogramming environment task scheduling operations synchronization primitives timing of task executions and organization of the task dispatcher 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-9

10 Integrated project development support systems: software tools for the realization of embedded and real-time automation projects need to jointly design integrated hardware and software systems Real-time software engineering and structured real-time program development High-level real-time languages: existing languages enhanced by special real-time features or new high-level languages that include real-time features or specialized languages 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-10

11 Process interfacing and peripherals: classification of processes, noise, and signals conditions on sampling, signal quality and signal conditioning. System reliability and fault tolerance: notions of "reliability" and "safety" have to be defined, erroneous and failure situations need to be analyzed, and concept of redundancy is required K. H. Ecker, T.U. Clausthal. 1. Introduction 1-11

12 Some Application Examples and Their Characteristics There is a great diversity of real-time applications Examples: control and safety systems for nuclear power plants, air traffic control, air defense systems, future space borne systems, computer integrated manufacturing systems, navigation system for ships, controllers in laundry machines, and even seat reservation in travel agencies K. H. Ecker, T.U. Clausthal. 1. Introduction 1-12

13 RT-Requirements in these applications differ considerably: Timing requirements can range from msec to hours, days,..., even within the same application. for operating a power plant, the boiler heat control requires seconds for fuel, air, and steam flow management, but hours for gaining maximum efficiency. Another extreme: aircraft control e.g. military planes such as X-29 highly maneuverable, fast, aerodynamically efficient, but unstable The pilot sets directions, and the X-29 computers monitor and adjust the positions of the airplane's control surfaces every 25 msec. Urgency of control actions: Hard real-time control: deadlines have to be observed Example: Aircraft control Soft real-time control: actions may by delayed up to a certain extent (due dates) 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-13

14 Alternative distinctions between soft and hard real-time control: the size of reaction time is taken as criterion hard real-time: reaction time is in the range of milliseconds soft real-time: reaction time in the range of seconds or more A system is called soft real-time if it is able to recover from timing errors (i.e. a proper synchronization is restored), and hard real time, if it is not able to recover R. Koymans, R. Kuiper, E. Zijlstra (1988) 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-14

15 hard real-time task completion not allowed deadline time of completion penalty soft real-time due date time of completion 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-15

16 1.3 Functional Requirements for Real-Time Systems Requirements in traditional systems: Usually optimal system throughput No crucial timing constraints, except that the average response time should be acceptable RT-operating system requirements: management of different types of processes, time-, event-, and priority-based interrupts synchronization primitives concept of tasks as a parallel program thread in a multiprogramming environment task scheduling operations timing of task executions is fundamental 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-16

17 Basic requirements for real-time systems: Timeliness: two general kinds of requirements relative timing constraints, actions have to be performed within a given interval of time absolute timing constraints specify the system behavior for globally given points in time Predictability: behavior of an environment may appear at random the controlling real-time system must handle each external event predictably this includes overload and other error situations: the system must degrade its performance gracefully in a transparent and foreseeable way K. H. Ecker, T.U. Clausthal. 1. Introduction 1-17

18 Dependability refers to the general requirement of trustworthiness: correctness: the system has to produce the right control signals at the right time robustness: the system remains in a predictable state, even if the environment does not correspond to the specification permanent readiness: the real-time system does not terminate tolerance against software or hardware faults Simplicity: A general observation is the increasing complexity of real-time systems and their software Dijkstra's fundamental design principle: Simplicity and understandability of software components are a precondition for dependable engineering, and for allowing later modifications K. H. Ecker, T.U. Clausthal. 1. Introduction 1-18

19 Analyzability: System behavior is modeled in mathematical terms Simultaneity: Several simultaneous actions e.g. inputs from independent sources require a quasi-parallel processing on a single processor, or parallel processing on several cooperating computers or processors in distributed or parallel real-time systems Criterion of maximal processor [resource] utilization: processor utilization is irrelevant cost of any processor involved in a failure of an external process is negligible compared to the cost o the failed process...this is true even in comparatively inexpensive environments for example: one hour of production stoppage of a media-sized chemical facility: 100 K$ loss comparable to the price of the entire computer system 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-19

20 Summary of Chapter 1 Informal understanding of a real-time system environment, internal system predictability, reliability, timeliness General questions requirement analysis, specification, hardware architecture, operating system, programming 2003 K. H. Ecker, T.U. Clausthal. 1. Introduction 1-20

21 Anwendungsbeispiele auf S. 12: Welche zeitkritische Aktionen können in diesen auftreten mit welchen geschätzten Reaktionszeiten, sind diese hard- oder soft real time, was die Konsequenzen im Fehlerfall sein würden K. H. Ecker, T.U. Clausthal. 1. Introduction 1-21