1. Chapter 6 Concurrency: Deadlock and Starvation Operating Systems: Internals and Design Principles, 6/E William Stallings Patricia Roy Manatee Community College, Venice, FL ©2008, Prentice Hall

2. Dining Philosophers Problem

8. Deadlock Permanent blocking of a set of processes that either compete for system resources or communicate with each other No efficient solution –prevention, avoidance, detection Involve conflicting needs for resources by two or more processes

9. Two Resource Types Reusable resources Consumable resources

10. Reusable Resources Used by only one process at a time and not depleted by that use Processes obtain resources that they later release for reuse by other processes

11. Reusable Resources Processors, I/O channels, main and secondary memory, devices, and data structures such as files and databases Deadlock occurs if each process holds one resource and requests the other

12. Reusable Resources Space is available for allocation of 200Kbytes, and the following sequence of events occur Deadlock occurs if both processes progress to their second request P1... Request 80 Kbytes; Request 60 Kbytes; P2... Request 70 Kbytes; Request 80 Kbytes;

13. Consumable Resources Created (produced) and destroyed (consumed) Interrupts, signals, messages, and information in I/O buffers

14. Example of Deadlock Deadlock occurs if receive is blocking P1... Receive(P2); Send(P2, M1); P2... Receive(P1); Send(P1, M2);

15. Deadlock

17. Conditions for Deadlock Mutual exclusion –Only one process may use a resource at a time Hold-and-wait –A process may hold allocated resources while awaiting assignment of others

18. Conditions for Deadlock No preemption –No resource can be forcibly removed from a process holding it Circular wait –A closed chain of processes exists, such that each process holds at least one resource needed by the next process in the chain

19. Resource Allocation Graphs Directed graph that depicts a state of the system of resources and processes

20. Resource Allocation Graphs

21. Resource Allocation Graph for Figure 6.1 (b)

23. Possibility of Deadlock Mutual Exclusion No preemption Hold and wait

24. Existence of Deadlock Mutual Exclusion No preemption Hold and wait Circular wait So, how to prevent deadlock?

25. Deadlock Prevention Mutual Exclusion –Must be supported by the OS Hold and Wait –Require a process request all of its required resources at one time

26. Deadlock Prevention No Preemption –OS may preempt a process to require it to release its resources, the process must release resource and request again Circular Wait –Define a linear ordering of resource types Deadlock prevention disadvantages –Conservative: undercommits resources

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

28. Two Approaches to Deadlock Avoidance Do not start a process if its demands might lead to deadlock –Start a process only if the maximum resource claim of all current processes plus those of the new process can be met. –Too pessimistic!

29. Two Approaches to Deadlock Avoidance Do not start a process if its demands might lead to deadlock –Too pessimistic! Do not grant an incremental resource request to a process if this allocation might lead to deadlock

30. Resource Allocation Denial Referred to as the bankers algorithm State of the system is the current allocation of resources to process Safe state is where there is at least one sequence that does not result in deadlock Otherwise, it is an unsafe state

31. Determination of a Safe State

35. Safe to Grant P1s Request? P1 requests one unit each of R1 and R3

36. Safe to Grant P1s Request?

37. Determination of an Unsafe State

38. Deadlock Avoidance Logic

40. In-Class Exercise Prob 6.15: consider a system consisting of four processes and a single resource. The current state of the claim and allocation matrices are –C = [3, 2, 9, 7] T –A = [1, 1, 3, 2] T What is the minimum number of units of the resource needed to be available for this state to be safe?

41. C = [3, 2, 9, 7] T A = [1, 1, 3, 2] T V = [?] C = [3, 2, 9, 7] T A = [1, 1, 3, 2] T R = [7] V = [0] X Cannot make progress on any process! need at least 1 extra instance of resource

42. C = [3, 2, 9, 7] T A = [1, 1, 3, 2] T R = [8] V = [1] Finish P2: C = [3, 2, 9, 7] T A = [1, 0, 3, 2] T V = [2] Finish P1: C = [3, 2, 9, 7] T A = [0, 0, 3, 2] T V = [3] X Cannot make progress on any process! need at least 2 extra instances of resource

43. C = [3, 2, 9, 7] T A = [0, 0, 3, 2] T R = [10] V = [5] Finish P4: C = [3, 2, 9, 7] T A = [0, 0, 3, 0] T V = [7] Finish P3: C = [3, 2, 9, 7] T A = [0, 0, 0, 0] T V = [10]

44. Deadlock Avoidance Maximum resource requirement must be stated in advance Processes under consideration must be independent; their execution order must be unconstrained by any synchronization requirements There must be a fixed number of resources to allocate No process may exit while holding resources

45. Deadlock Detection Available vector

46. Deadlock Detection Algorithm 1.Mark each process that has a row in the allocation matrix of all zeros 2.Initialize a temporary vector W to equal to available vector

47. Deadlock Detection Algorithm 3.Find an index i such that process i is currently unmarked and the ith row of request matrix is less than or equal to W. If no such row is found, terminate the algorithm and all unmarked processes are those involved in the deadlock. 4.If such a row is found, mark process i and add the corresponding row of the allocation matrix to W.

48. Strategies Once Deadlock Detected Abort all deadlocked processes Back up each deadlocked process to some previously defined checkpoint, and restart all processes –Original deadlock may occur

49. Strategies Once Deadlock Detected Successively abort deadlocked processes until deadlock no longer exists Successively preempt resources until deadlock no longer exists

50. Advantages and Disadvantages

51. In-Class Exercises Probs. 6.5, 6.6, 6.7, 6.8