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Real time system tsp

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  • 1. Real Time System T S PRADEEP KUMAR VIT University
  • 2. Key points
    • Real Time System and its types
    • Characteristics
    • Scheduling Strategies
    • RTOS Concepts
  • 3. What is Real Time System (RTS)
    • A real-time system is any information processing system which has to respond to externally generated input stimuli within a finite and specified period
      • the correctness depends not only on the logical result but also the time it was delivered
      • failure to respond is as bad as the wrong response!
  • 4. Terms Associated with RTS
    • Tasks
      • Periodic
        • Time-driven. Characteristics are known a priori
        • E.g.: Task monitoring temperature of a patient in an ICU.
      • Aperiod
        • Event-driven. Characteristics are not known a priori
        • E.g.: Task activated upon detecting change in patient’s condition
      • Sporadic
        • Aperiodic tasks with known minimum inter-arrival time.
  • 5. Terms Associated with RTS
    • Release time of a job:
      • The time instant the task becomes ready to execute.
    • Deadline of a job:
      • The time instant by which the task must complete execution.
    • Relative deadline of a job:
      • Deadline - Release time.
    • Response time of a job:
      • Completion time - Release time.
  • 6. Example
  • 7. Types of RTS
    • Hard real-time — systems where the responses occur within the required deadline. Eg.Nuclear Reactor control Systems.
    • Soft real-time — systems where deadlines are important but which will still function correctly if deadlines are occasionally missed.
    • Firm real-time — systems which are soft real-time but in which there is no benefit from late delivery of service.
  • 8. Hard RTS Vs Soft RTS User Assisted Autonomous Error Detection Large Small Size of Data Non Critical Critical Safety Computer Environment Controlled by Degraded Predictable Peak Load Performance Soft- required Hard- Required Response Time Soft RTS Hard RTS Characteristics
  • 9. Real Time Spectrum
  • 10. Characteristics of RTS
    • Deterministic
      • Operations are performed at predetermined times or within predetermined time intervals
    • Responsiveness
      • How long, after acknowledgment, it takes the operating system to service the interrupt
      • Includes amount of time to begin execution of the interrupt
      • Includes the amount of time to perform the interrupt
  • 11. Characteristics…
    • User control
      • User specifies priority
      • What processes must always reside in main memory
      • Rights of processes
    • Reliability
      • Degradation of performance may have catastrophic consequences
  • 12. Characteristics…
    • Fail-soft operation
      • Ability of a system to fail in such a way as to preserve as much capability and data as possible
      • Stability
  • 13. Typical RTS Task Model
    • Each task a triplet: (execution time, period, deadline)
    • Usually, deadline = period
    • Can be initiated any time during the period
    • P=(2,8,8)
  • 14. Scheduling Algorithms
      • Cyclic Executive (Round Robin)
      • Static Priority or offline scheduling
        • Rate Monotonic Scheduling
          • Deadline=period
        • Deadline Monotonic Scheduling
          • Deadline ≠period
      • Dynamic Priority or online scheduling
        • Earliest Deadline First
  • 15. Preemptive and Non preemptive Schedule
    • Preemptive Schedule
      • The higher priority task activated when it requires the CPU while the low priority task suspended during the time in which the higher priority tasks run.
      • Most of the RTOS provides preemptive schedule
    • Non Preemptive Schedule
      • The Low priority task will never suspend when any higher priority task require the CPU time.
  • 16. Cyclic Executive
    • Simplest approach
      • for (;;)
      • {
      • do part of task 1
      • do part of task 2
      • do part of task 3
      • }
  • 17. Cyclic Executive
    • Advantages
      • Simple implementation
      • Very predictable
    • Disadvantages
      • Can’t handle sporadic events
      • Everything must operate in lockstep
      • Code must be scheduled manually
  • 18. Rate Monotonic Scheduling (RMS)
    • Processes with shorter period given higher priority, deadline=period
    • Works for Periodic tasks
    • RMS Schedule can exists if
        • Processor Utilization U < n(2 1/n -1),
          • where U= Σ c i /p i
        • if U > 1, then no processor can be running 110% of the time.
  • 19. Rate Monotonic Scheduling
    • Each process has a fixed priority based on its frames/second, value (hence, rate monotonic)
    • Select always the highest priority process
    • Rule:
      • Each periodic process must complete within its period
    • If a high priority process becomes ready for execution at any time, it preempts the running process if there is any
    • optimal, static, priority-driven real-time scheduling algorithm for preemptive, periodic jobs
      • optimal, in the sense that no other static algorithm can schedule a set of tasks that RM cannot schedule
  • 20. RMS Scheduling
    • Five assumptions necessary:
      • all tasks with deadlines are periodic;
      • each task must be completed, before the next request occurs;
      • all tasks are independent;
      • run-time for each request of a task is constant;
      • any non-periodic task has no required deadline
  • 21. RMS Example
  • 22. RMS Example 2 Task set: T i = (c i , di, p i ) T1 = (2,6,6) and T2 = (4,10,10) Schedulability check: 2/6 + 4/10 = 0.33 + 0.40 = 0.73 ≤ 2(√2 -1) = 0. 82 T 1 1 T 1 2 0 2 6 8 10 T 2 1 T 2 2 0 2 6 10 T1 T2 Hole Background scheduling: basic idea -- Scheduling aperiodic tasks in holes like this Schedule continues
  • 23. Earliest Deadline First (EDF)
    • Earliest Deadline First (EDF)
      • Online
      • Preemptive
      • Dynamic priorities
      • Always run the process that is closest to its deadline
    • • Requirements:
      • Pi has a max computation time ei
      • the process must be finished before its deadline
      • processes are independent (do not share resources)
      • the process with shortest absolute deadline (di) will run first
  • 24. EDF Example
  • 25. EDF Schedule
    • Earliest deadline first scheduling is optimal:
      • If a dynamic priority schedule exists, EDF will produce a feasible schedule
      • EDF schedule usually achieves 100% processor utilisation.
      • If a schedule is not feasible under EDF algorithm, then there is no other algorithm to give a feasible schedule
  • 26. Comparison between Static and dynamic Scheduling
    • RMA only guarantees feasibility at 69% utilization.
    • EDF guarantees it at 100%
    • EDF is complicated enough to have unacceptable overhead
    • More complicated than RMA
    • Less predictable: can’t guarantee which process runs when
  • 27.
    • Questions!!!