lect7 - Lecture 7 Lecture Real Time Task Scheduling Forrest...

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Lecture 7 Lecture 7 Real Time Task Scheduling Forrest Brewer
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Real Time Real Time ANSI defines real time as A Real time process is a process which delivers the results of processing in a given time span” A data may require processing at a priori known point in time, or it may be demanded without any priori knowledge Correctness of computation Deadlines (latest acceptable time) Soft deadline Diminished functionality as deadlines are missed System does not ‘fail’ Hard deadline System fails (X-29 wings fall off…)
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Real Time Real Time Processing guarantees for time-critical applications: Predictably fast response to time-critical events & accurate timing information Jitter issues High degree of schedulability High degree of resource utilization below which the processing guarantee is a question… Stability under transient overload Under system overload, critical jobs processing of must be ensured Priority Scheme, process preemption Sharing of Resources Management of Arbitration Low Overhead
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Five Characteristics of Real-Time Five Characteristics of Real-Time Operating Systems Operating Systems Determinism : concerned with how long an operating system delays before acknowledging an event Responsiveness : concerned with how long after acknowledgment, it takes an operating system to finish the event (interrupt) service Determinism and responsiveness together make up the response time to external events which are crucial for real- time systems User control : allow the user (dynamic?) fine-grained control over task priority Reliability : a transient failure may cause financial loss or major equipment damage or even loss of life. Fail-soft operation : during overload, continued operation at a reduced level of service
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System Modeling in RT Scheduling System Modeling in RT Scheduling Tasks are the schedulable unit of the system. A task is characterized by timing constraints and resource requirements. Periodic task (T) processing time deadline period Processing time of T Deadline of T Period of T Periodic task T 0
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Real time scheduling: Real time scheduling: Periodic Periodic system model system model Task: schedulable entity Processing of separate tasks are assumed mutually independent Timing constraints of a periodic task τ i is specified by ( s , e , D , p ) – s i -(scheduled) Starting Time of Task i – e i -Processing time of i – f i -Finish time of i – D i -Deadline of i – p i -Period of i – r i -Rate of i = ( 1/p i )
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Real time scheduling: Real time scheduling: Periodic Periodic system model system model Tasks can be Preemptive Nonpreemptive Guarantee ratio Processing time used by guaranteed tasks versus total processing time Utilization: = = n i i i p e U 1
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System Model - Assumptions and Notation System Model - Assumptions and Notation Assumptions: Periodic tasks without precedence relations
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This note was uploaded on 12/29/2011 for the course ECE 253 taught by Professor Brewer,f during the Fall '08 term at UCSB.

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lect7 - Lecture 7 Lecture Real Time Task Scheduling Forrest...

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