2 An Introduction to Real-Time Systems

1. An Introduction to Real-Time SystemsbyAnita ChoudharyAssistant Professor (NPIU)

2. Why Real-Time Systems ? Due to the increasing number of the applications having timing constraints, which are too complex or fast changing to be amenable to direct human control, a need for effective real-time systems has come up.

3. What is a Real-Time System? • Definition 1: Real-Time systems are systems in which the correctness of the system behavior depends • on the logical results of the computations, and • on the physical time when these results are produced. • Definition 2: Real-Time systems have to be designed according to the dynamics of a physical process. • Definition 3: A real-time system may be defined as one whose principal measure of performance is whether or not it meets prespecified task timing constraints or deadlines. • Definition 4: The term real-time can be used to describe any information processing activity or system which has to respond to externally generated input stimuli within a finite and specifiable delay

4. Real-Time Systems - An Introduction • “Time” is the most precious resource to be managed. • Tasks arrive endlessly in the computer system and initiate requests for their execution. • Every request carries a timing constraint for its completion called the “deadline”. • The real-time task must be assigned and scheduled to be completed before its deadline. • The correctness of computation not only depends on the logical correctness but also on the time at which results are produced.

5. Real-Time Tasks and Jobs Each unit of work that is scheduled and executed by the system is called job. A set of related jobs which jointly provide some system function is called task. A job executes or is executed by the (operating) system. Every job executes on some resource.

6. Processors and Resources We divide all the system resources into two major types: Active resource - processors and Passive resources or simply resources The jobs is execute on a CPU, a network, and a disk. These resources are called servers and, sometimes, active resources and processors in real-time systems. By resources, we specifically mean passive resources e.g. memory, sequence numbers, mutexes, database locks, semaphore etc.

7. Timing Constraints Release time and deadline of jobs are two parameters that distinguish jobs in real-time systems from those in nonreal-time systems. The release time of a job is the instant of time at which the job becomes available for execution. The job can be scheduled and executed at any time at or after its release time whenever its data and control dependency conditions are met. The deadline of a job is the instant of time by which its execution is required to be completed. We say that a job has no deadline if its deadline is at infinity.

8. Timing Constraints (contd…) Response time is the length of time from the release time of the job to the instant when it completes. The deadline of a job, sometimes called its absolute deadline, is equal to its release time plus its relative deadline. absolute deadline = release time + relative deadline Constraint imposed on the timing behavior of a job are called timing constraint. A timing constraint of a job can be specified in terms of its release time and relative or absolute deadlines

9. Hard and Soft Timing Constraints • Timing constraints are divided into two types: • hard real-time constraints and • Soft real-time constraints • Hard real-time constraints: A timing constraint or deadline is hard if the failure to meet it is considered to be a fatal fault. • A hard deadline is imposed on a job because a late result produced by the job after the deadline may have disastrous consequences. • Soft real-time constraints: The late completion of a job that has a soft deadline is undesirable. • A few misses of soft deadlines do no serious harm; only the system’s overall performance becomes poorer and poorer when more and more jobs with soft deadlines complete late.

10. Hard and Soft Timing Constraints(contd…) • The distinction between hard and soft timing constraints is sometimes stated quantitatively in terms of the usefulness of results (and therefore the overall system performance) as functions of the tardinesses of jobs. • The tardiness of a job measures how late it completes respective to its deadline. • If tardiness is zero: the job completes at or before its deadline.

11. Hard and Soft Timing Constraints(contd…) • If the job is late: its tardiness is equal to the difference between its completion time and its deadline. Tardiness= completion time – deadline • The usefulness of a result produced by a soft real-time job decreases gradually as the tardiness of the job increases. • The usefulness of a result produced by a hard real-time job falls off abruptly and may even become negative when the tardiness of the job becomes larger than zero. • On the basis of whether the timing constraint is expressed in deterministic or probabilistic terms then the • deadline is hard: if a job must never miss its deadline, or • deadline is soft: if deadline can be missed occasionally with some acceptably low probability.

12. Hard and Soft Timing Constraints(contd…) (tardiness)

13. Thank you