1. Lecture 24: Process Scheduling Examples and for Real-time Systems

2. Review: Categories of Process Scheduling
Batch Systems (Compute Servers)
Many processes, not interactive, throughput is important
Interactive Systems PC
Real Time Systems
Guaranteed response times, meet deadlines

3. Scheduling Algorithm Goals
All Systems
Fairness – giving each process a fair share of the CPU
Policy enforcement – seeing that stated policy is carried out
Balance – keeping all parts of the system busy

4. Review: Scheduling Algorithms for Batch Systems
First come first serve
Shortest job first
Shortest remaining time next

5. Review: Scheduling Algorithms for Interactive Systems
Round-Robin scheduling
Priority scheduling
Multiple queues
Lottery scheduling

6. FCFS Arrival time CPU burst P1: 4 36 8 112 1 13 I/O burst 1613 16 20
P1:
I/O burst
P2:
P3:
P4:

7. FCFS 13 16 20
P1:
P2:
P3:
P4:
Which process goes next?
P1 idle P2 P3 P4

8. FCFS
Time next ready 40 76 60
192 13 16 20
P1:
P2:
P3:
P4:
Which process goes next?
P1 idle P2 P3 P4

9. FCFS
Time next ready 40 76 60
192 13 16 20
P1:
P2:
P3:
P4:
P1 idle P2 P3 P4 P1 idle P3

10. FCFS
162 76
212
192 13 16 20
P1:
P2:
P3:
P4:
P1 idle P2 P3 P4 P1 idle P3 P P idle P4

11. RR, Q = 10 Arrival time CPU burst P1: 4 36 8 112 1 13 I/O burst 1613 16 20
P1:
I/O burst
P2:
P3:
P4:
Gantt chart

12. RR, Q = 10 13 16 20
P1:
P2:
P3:
P4:
Which process goes next, and when?
Gantt chart
P1 idle P2 P3 P4

13. RR, Q = 10 40 82 47
179 13 16 20
P1:
P2:
P3:
P4:
Gantt chart
P1 idle P2 P3 P4 P2

14. RR, Q = 10
162
End
Run
179 13 16 20
P1:
P2:
P3:
P4:
Gantt chart
P1 idle P2 P3 P4 P2 P1 P3 P2 P3

15. RR, Q = 10
End
205
179 13 16 20
P1:
P2:
P3:
P4:
Gantt chart
P1 idle P2 P3 P4 P2 P1 P3 P2 P3 P1 P3 idle

16. MLFQ: Q0 = 8, Q1 = 16, Q2 =  40 q1 45
177 13 16 20
P1:
P2: 15
P3:
P4:
Gantt chart
1 idle

17. MLFQ: Q0 = 8, Q1 = 16, Q2 = 
160 q1
177 13 16 20
P1:
P2: 2
P3:
104
P4:
Gantt chart
1 idle

18. MLFQ: Q0 = 8, Q1 = 16, Q2 = 
160 98 q2
177 13 16 20
P1:
P2:
P3: 64
P4:
Gantt chart
1 idle

19. MLFQ: Q0 = 8, Q1 = 16, Q2 = 
end q2
190 13 16 20
P1:
P2:
P3: 4
P4:
Gantt chart
1 idle idle

20. Real-time Scheduling Definition Performance measure Key property
Systems whose correctness depends on their temporal aspects as well as their functional aspects
Performance measure
Timeliness on timing constraints (deadlines)
Speed/average case performance are less significant.
Key property
Predictability on timing constraints

21. Real-time Scheduling Absolute deadline Released Execution time
Relative deadline

22. Deadlines: Hard vs. Soft
Hard deadline
Disastrous or very serious consequences may occur if the deadline is missed
Validation is essential : can all the deadlines be met, even under worst-case scenario?
Deterministic guarantees
Soft deadline
Ideally, the deadline should be met for maximum performance. The performance degrades in case of deadline misses.
Best effort approaches / statistical guarantees

23. RM (Rate Monotonic) Optimal static-priority scheduling
It assigns priority according to period
A task with a shorter period has a higher priority
Executes a job with the shortest period T1
(4,1) T2
(5,2) 5 10 15 T3
(7,2)

24. EDF (Earliest Deadline First)
Optimal dynamic priority scheduling
A task with a shorter deadline has a higher priority
Executes a job with the earliest deadline T1
(4,1) T2
(5,2) 5 10 15 T3
(7,2)