1. Real Time Scheduling
Mrs. K.M. Sanghavi

2. Real Time Scheduling Times
Arrival Term
Ready Time
Scheduling Time
Burst / Run Time
Waiting Time
Completion Time
Deadline Time
Tardiness : When Task misses the deadline
Completion Time - Deadline
Laxity : Task follows Deadline
Deadline – Completion Time

3. RM DM

4. Rate Monotic
Rate Monotonic refers to assigning priorities as a monotonic function of the rate (frequency of occurrence) of those processes.
Rate Monotonic Scheduling (RMS) can be accomplished based upon rate monotonic principles.
Priority and Time period are inversely proportional
Higher the time period lower the priority.
This is a preemptive static priority algorithm.

5. Rate Monotonic Analysis: Assumptions
A1: Tasks are periodic (activated at a constant rate). Period = Interval between two consecutive activations of task
A2: All instances of a periodic task have the same computation time
A3: All instances of a periodic task have the same relative deadline, which is equal to the period
A4: All tasks are independent (i.e., no precedence constraints and no resource constraints)
Implicit assumptions:
A5: Tasks are preemptable
A6: No task can suspend itself
A7: All tasks are released as soon as they arrive
A8: All overhead in the kernel is assumed to be zero (or part of )
Frank Drews
Real-Time Systems

6. Rate Monotonic Algorithm
A Higher priority process will preempt a lower priority process
Algorithm to schedule priority tasks
It is based on priorities
Process
Burst Time
Period P1 2 10 P2 1 5 P3 30 P4 15

7. Utilization Bound (UB) Test
Ui = Ci Ti
Processor Utilization for a task, i
U(n) = n( ) 1 n
Utilization Bound for n tasks
Results:
If S Ui ≤ U(n) then the set of tasks is schedulable.
If S Ui > 1 then the set of tasks is unschedulable.
If U(n) < S Ui ≤ 1 then the test is inconclusive.

8. RMA
For that we need to use two formulas and check L.H.S (CPU Utilization) <= R.H.S (Utilization Bound Tasks) : ≤

9. RMA
Will never miss a deadline if RMA is applied

10. Now Schedule the task Process Burst Time Period P1 2 10 P2 1 5 P3 3015

11. Process
Burst Time
Period P1 2 10 P2 1 5 P3 30 P4 15

12. Example
Process
Burst Time
Period P1 2 10 P2 4 15 P3 35 ≤