Priority Inversion

1. Priority Inversion

2. Roadmap for Today • Priority Inversion • Resource protocols • Synchronization Mechanisms • Signaling • Semaphores • Monitors • Synchronization Problems • Producer / Consumer and • Reader / Writer

3. Example • 5 processes • process number equals priority • Priority of P5 > priority of P1 • Release and execution times as shown • No deadlines (only an example for later comparison) • Priority-driven preemptively scheduled P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

4. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

5. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

6. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

7. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

8. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

9. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

10. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

11. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

12. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

13. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

14. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

15. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

16. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

17. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

18. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

19. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

20. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

21. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

22. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

23. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

24. Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

25. Reality is more complex • Processes are not usually independent

26. Real-Time Traffic Scheduling • Two process streams • A high priority & a low priority

27. Problem • Intersection is a mutually exclusive resource

28. Mutual Exclusion • Can be solved by resource access protocols

29. Priorities and Resource Contention Main Reference Pane W. S. Liu “Real-time Systems”, Chapter 8

30. Resources • Processes require resources in order to execute. (e.g. locks, ports, memory, …) • Resource characteristics • Serially reusable, • Mutually exclusive • We ignore resources that • are infinitely available or exceed demand, • or can be pre-allocated.

31. P3 3 P2 2 R1 P1 1 Resource Contention Problem • Priority inversion. • We need to, at least, bound the length of priority inversion. • Preferably minimize the length of priority inversion. Famous example of priority inversion: Mars Pathfinder 1997

32. Marth Pathfinder Mars Path Finder and the famous Mars Rock YOGI

33. Resource Contention Problems • Timing anomaly • Deadlock P2 2 R1 R2 P1 1

34. Resource Contention Major Assumption • Single processor system

35. Nested Critical Section* *P2 first needs R1 and then later additionally R2 Example with Resources Our Example + 2 Resources Resource 1 P5 Resource 2 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

36. SPD Scheduling Simple Priority Driven Scheduling P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

37. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

38. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

39. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

40. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

41. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

42. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

43. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

44. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

45. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

46. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

47. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

48. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

49. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20

50. SPD Scheduling Example P5 P4 P3 P2 P1 0 2 4 6 8 10 12 14 16 18 20