Chapter 6 Concurrency: Deadlock and Starvation

1. Chapter 6Concurrency: Deadlock and Starvation Operating Systems:Internals and Design Principles Seventh Edition By William Stallings

2. Operating Systems:Internals and Design Principles When two trains approach each other at a crossing, both shall come to a full stop and neither shall start up again until the other has gone. Statute passed by the Kansas State Legislature, early in the 20th century. —A TREASURY OF RAILROAD FOLKLORE, B. A. Botkin and Alvin F. Harlow

3. Deadlock • The permanent blocking of a set of processes that either compete for system resources or communicate with each other • A set of processes is deadlocked when each process in the set is blocked awaiting an event that can only be triggered by another blocked process in the set • Permanent • No efficient solution

4. Potential Deadlock I need quad C and B I need quad B and C I need quad A and B I need quad D and A

5. Actual Deadlock HALT until D is free HALT until C is free HALT until B is free HALT until A is free

6. Resource Categories

7. Reusable Resources Example

8. Example 2:Memory Request P1 P2 . . . . . . Request 80 Kbytes; Request 70 Kbytes; . . . . . . Request60 Kbytes; Request 80 Kbytes; Space is available for allocation of 200Kbytes, and the following sequence of events occur: Deadlock occurs if both processes progress to their second request

9. Consumable Resources Deadlock Consider a pair of processes, in which each process attempts to receive a message from the other process and then send a message to the other process: Deadlock occurs if the Receive is blocking

10. Deadlock Detection, Prevention, and Avoidance

11. Resource Allocation Graphs

12. Resource Allocation Graphs

13. Conditions for Deadlock

14. Dealing with Deadlock Three general approaches exist for dealing with deadlock:

15. Deadlock Prevention Strategy • Design a system in such a way that the possibility of deadlock is excluded • Two main methods: • Indirect • prevent the occurrence of one of the three necessary conditions • Direct • prevent the occurrence of a circular wait

16. Deadlock ConditionPrevention

17. Deadlock Condition Prevention • No Preemption • if a process holding certain resources is denied a further request, that process must release its original resources and request them again • OS may preempt the second process and require it to release its resources • Circular Wait • define a linear ordering of resource types

18. Deadlock Avoidance A decision is made dynamically whether the current resource allocation request will, if granted, potentially lead to a deadlock Requires knowledge of future process requests

19. Two Approaches to Deadlock Avoidance

20. Resource Allocation Denial Referred to as the banker’s algorithm State of the system reflects the current allocation of resources to processes Safe state is one in which there is at least one sequence of resource allocations to processes that does not result in a deadlock Unsafe state is a state that is not safe

21. Determination of a Safe State Amount of existing resources Resources available after allocation State of a system consisting of four processes and three resources Allocations have been made to the four processes

22. P2 Runs to Completion

23. P1 Runs to Completion

24. P3 Runs to Completion Thus, the state defined originally is a safe state

25. Determination of an Unsafe State

26. Deadlock Avoidance Logic

27. Deadlock Avoidance Logic

28. Deadlock Avoidance Advantages It is not necessary to preempt and rollback processes, as in deadlock detection It is less restrictive than deadlock prevention

29. Deadlock Avoidance Restrictions Maximum resource requirement for each process must be stated in advance Processes under consideration must be independent and with no synchronization requirements There must be a fixed number of resources to allocate No process may exit while holding resources

30. Deadlock Strategies

31. Deadline Detection Algorithms • A check for deadlock can be made as frequently as each resource request or, less frequently, depending on how likely it is for a deadlock to occur • Advantages: • it leads to early detection • the algorithm is relatively simple • Disadvantage • frequent checks consume considerable processor time

32. Deadlock Detection Algorithm

33. Deadlock Approaches

34. Dining Philosophers Problem • No two philosophers can use the same fork at the same time (mutual exclusion) • No philosopher must starve to death (avoid deadlock and starvation)

35. Using Semaphores (deadlock possible) Solutions Cont.

36. A Second Solution . . . (4 people allowed)

37. Solution Using A Monitor

38. Summary • Deadlock: • the blocking of a set of processes that either compete for system resources or communicate with each other • blockage is permanent unless OS takes action • may involve reusable or consumable resources • Consumable = destroyed when acquired by a process • Reusable = not depleted/destroyed by use • Dealing with deadlock: • prevention – guarantees that deadlock will not occur • detection – OS checks for deadlock and takes action • avoidance – analyzes each new resource request