Real-Time System

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What is a Real-Time System?:

What is a Real-Time System? Real-time systems have been defined as: “those systems in which the correctness of the system depends not only on the logical result of the computation, but also on the time at which the results are produced”. J. Stankovic , 21 October 1988.



Real-Time Characteristics:

Real-Time Characteristics Real-time systems often are comprised of a controlling system, controlled system and environment . Timing constraints derived from physical impact of controlling systems activities. Hard and soft constraints.

Hard v/s Soft:

Hard v/s Soft Hard : failure to meet constraint is a fatal fault. Validation system always meets timing constraints. Deterministic constraints Probabilistic constraints Soft : late completion is undesirable but generally not fatal.

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Controlled Real-Time System Fred Kuhns () CS523S: Operating Systems Controlling System Environment sensor sensor sensor sensor actuator actuator actuator actuator

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Timing constraint: constraint imposed on timing behavior of a job hard or soft. Release Time : Instant of time job becomes available for execution Deadline : Instant of time a job's execution is required to be completed. Response time : Length of time from release time to instant job completes.

Validate Constraints:

Validate Constraints Validation : Demonstration by a provably correct, efficient procedure simulation and testing. Involves three steps :

Developing a Reference Model:

Developing a Reference Model Modeling the system to focus on timing properties and resource requirements. workload model. Resource model. Algorithm model.

Tasks and Jobs:

Tasks and Jobs Task : Set of related jobs jointly provide function. Job : Unit of work, scheduled and executed by system. characterized by the following parameters: Temporal parameters. Functional parameters. Resource parameters. Interconnection parameters.


Resources Active and Passive Active resources: Every job must have one or more processors Same type if functionally identical and used interchangeably. Passive resource: job may require Resources in addition to processor. reusable resources are not consumed.

Resource Parameters:

Resource Parameters Job resource parameters indicate processor and resource requirements. Preemptively of resources non-preemptive: serial access, typical case. preemptive - jobs can interleave access.


Scheduling Schedule : assignment of all jobs to available processors, produced by scheduler . Valid schedule : every processor assigned to at most one job at a time. every job assigned to at most one processor at a time. no job scheduled before its release time.

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Feasible schedule : Every job starts at or after release time and completes by deadline. Optimal : Scheduling algorithm optimal if it always produces a feasible schedule if such a schedule exists. Lateness : difference between completion time and deadline, can be negative if early.

Performance Measures:

Performance Measures Miss rate : percentage of jobs executed but completed late. Loss rate : percentage of jobs discarded. Invalid rate : sum of miss and loss rate. Max or average response times. Max or average tardiness/lateness.


FEATURES Sequential ,Concurrent ,Real Time and Execution  Programming. Predetermined order. Multiple sequential programs executing in  parallel ,Usually composed. EX: Accounting, payroll  , Air flight controller.


CONCLUSION Real-Time systems span a large part of computer industry. So far most of the real-time systems research has been mostly confined to single node systems and mainly for processor scheduling. It does not provide any means of verifying functional properties.

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