Real time-system


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what is real time system? how they can be scheduled? what is RMA? what is EDF? what is the difference between EDF and RMA?

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Real time-system

  1. 1. A Real Time System is any informationprocessing system which has to respond toexternally generated input stimuli within afinite & specified period.In real time computing , the correctness ofthe computation depends not only on thelogical results but also on the time atwhich the results are produced.
  2. 2. In RTS a substantialfraction of the designeffort goes intomaking sure that thetask deadlines aremet.A Deadline is agiven time after atriggering event , bywhich a response hasto be completed.
  3. 3. 1. Hard realtime systems 2. Soft real time systems
  4. 4. Hard Real Time SystemsSystem should meets the timings constraints. Response time overrun leads to disastrous damageExample : Flight Control SystemSOFT REAL TIME SYSTEMS Deadline overruns are tolerable, but not desired. There are no catastrophic consequences of missing one or more deadlines. Example : Multimedia System
  5. 5. Operating System IssuesArchitectural IssuesOther Issues
  6. 6.  By task scheduling , we means that a computer’s resources must be shared out equitably among the users. In general-purpose system , Round robin scheduling is used , which ensures that user does not get a disproportionate share of the computer’s services. But Round robin scheduling approach does not work in Real Time applications because priority scheduling makes the difference in the two.
  7. 7. Computation Priority Scheduling Offline Static Preemptive (precomputed) Non Online Dynamic Preemptive (Dynamically)
  8. 8. A1. No task has any non- preempt able section & cost of preemption is negligible.A2. Only processing requirements are significant; memory, I/O & other resource requirements are negligible.A3. All tasks are independent.A4. The deadline of a task is equal to its period.A5. All the tasks in the task set are periodic.
  9. 9.  Optimal uniprocessor algorithm Also known as Deadline Monotonic Scheduling algorithm Tasks are preemptable Dynamic priorities Scheduler picks task , whose deadline is due next A set of tasks is schedulable ( in hard real time sense ) , if the total processor utilization is not greater than 1.
  10. 10. NECESSARY & SUFFICIENT CONDITIONIn EDF , for a task set is said to beschedulable is if it satisfy the followingnecessary and sufficient condition : Σ (i=1 to n) e/p ≤ 1 i i
  11. 11. Consider 3 periodic processes scheduledusing EDF, the following acceptance test showsthat all deadlines will be met. Execution Time = ei Period = pi Process P1 1 8 P2 2 5 P3 4 10
  12. 12. The utilization will be:U = { ( 1/8 ) + ( 2/5 ) + ( 4/10 ) } = 0.925 ( or 92.5 % )The theoretical limit for any number ofprocesses is 100% ( or less than 1 ) andso the system is schedulable .
  13. 13. Different possible scenarios Sufficient but NOT necessary If di ≥ pi If di < piNecessaryand Necessarysufficient and sufficient ∑ ei/pi ≤ 1 ∑ ei/di ≤ 1 Fail Pass Schedulable Processor demand Based test
  14. 14. •OptimalityADVANTAGES •Reduces number of task switches •Deteriorates badly under overloadDRAWBACKS •Scheduling is more expensive •Needs smarter scheduler
  15. 15.  Static priority based algorithm : tasks scheduled according to priorities Classical uniprocessor scheduling algorithm Also called Liu-Layland algorithm or Clock driven uniprocessor algorithm Tasks are pre-emptable
  16. 16.  It says that if the total utilization of the task is not greater than n(21/n -1) where n stands for number of tasks to be scheduled then RM algorithm will schedule all the task to meet their respective deadlines. The RMA assigns priorities to the tasks based on their rate of occurrence, i.e. the lower the occurrence rate of the task the lower the priority assigned to it.
  17. 17.  In RMA the priority of the task is directly proportional to its rate of occurrence or it is inversely proportional to the period
  18. 18.  A set of periodic real time tasks would not be RMA schedulable unless they satisfy the following necessary condition:- Σ (i=1 to n) U =Σ i (i=1 to n) e /p ≤ 1 i i
  19. 19.  Also referred to as Liu and Layland’s condition. It implies that a set of ‘n’ real time periodic tasks are schedulable under RMA if : Σ (i=1 to n) U ≤ n i (21/n -1)
  20. 20.  If the set of tasks satisfies this sufficient condition then it is guaranteed that the set of tasks would be RMA schedulable.CASES :Consider the case when• there is only 1 task in the system, i.e., n=1 U1 = 1• there is 2 tasks in the system, i.e., n=2 U2 = 0.824• n=∞, applying L’Hospitals Rule ln 2 = 0.69CONCLUSION:From above computation it is clear that maximumCPU utilization that can be achieved under RMA is 1.
  21. 21. Consider 3 tasks to be scheduled using RMA, thefollowing acceptance test determines whether thatall deadlines will be met or not. TASK EXECUTION PERIOD TIME T1 20 100 T2 30 150 T3 60 200
  22. 22. Checking for the NECESSARY CONDITION : Σ(i=1 to n) U =Σ i (i=1 to n) e /p i i ≤ 1 U1 0.2 U2 0.2 U3 0.3 Σ(i=1 to 3) U 0.7
  23. 23. Checking for the SUFFICIENT CONDITION : Σ(i=1 to n) Ui ≤ n(21/n -1)For n=3, n(21/n -1) = 3(21/3 -1) = 0.78 < 1 Ui = 0.7 < 0.78Hence, sufficient condition is satisfied.Therefore, all the given three tasks areschedulable under RMA.
  24. 24. • possesses good transient Advantages overload handling • difficult to support aperiodic and sporadic tasksDisadvantages • not optimal when task periods and deadlines differ
  25. 25. EDF RMA EDF is also called Deadline-  RMA is also called Liu- Monotonic Scheduling Layland algorithm. algorithm  Static priority algorithm Dynamic priority algorithm  All tasks are assumed to be Tasks do not have to be periodic periodic Full processor can be  It lacks flexibility. For e.g. feasibly scheduled, if RMA assumes that tasks are utilization of task set is no infinitely preempt able. greater than 1  Simpler implementation, even It requires a run time in systems without explicit overhead. Deteriorates during support for timing constraints overload conditions (periods, deadlines)