When Deadlock Happens

Transcription

1 Deadlock Detection Notice: The slides for this lecture have been largely based on those accompanying an earlier edition of the course text Operating Systems Concepts, 9th ed., by Silberschatz, Galvin, and Gagne. Many, if not all, the illustrations contained in this presentation come from this source. Revised by X.M. based on Professor Perrone s notes. CSCI 315 Operating Systems Design 1

2 When Deadlock Happens Another way to deal with deadlock is not to use either prevention or avoidance. The system may enter a deadlock state; the OS will deal with that when [ if ] it happens. What is needed in such a system: a detection algorithm to determine when deadlock states are entered, and a recovery scheme to get the system back on a safe state. CSCI 315 Operating Systems Design 2

3 DEADLOCK DETECTION CSCI 315 Operating Systems Design 3

4 Single Instance of Each Maintain a wait-for graph Nodes are processes. Resource Type P i P j iff P i is waiting for P j to release a needed resource Periodically invoke an algorithm that searches for a cycle in the graph. (Recall the four necessary conditions for deadlock.) An algorithm to detect a cycle in a graph requires an order of n 2 operations, where n is the number of vertices (processes) in the graph. CSCI 315 Operating Systems Design 4

5 Resource-Allocation Graph and Wait-for Graph P 5 P 5 R 1 R 3 R 4 P 1 P 2 P 3 P 1 P 2 P 3 R 2 R 5 P 4 P 4 Resource-Allocation Graph Corresponding wait-for graph CSCI 315 Operating Systems Design 5

6 How To Detect a Cycle? You will study the subject in greater detail in CSCI 311 Basic ideas Represent the graph in a data structure (e.g., adjacent list) Perform depth-first-search (DFS) traversal, while recording the nodes being visited If a node to be visited is already in the list of nodes visited, a cycle exists. CSCI 315 Operating Systems Design 6

7 Link To Algorithm And Example ctedcycle.java.html A B B D D C C B Since B has already been visited, we declare there is a cycle. The nodes on the cycle can be recovered using a stack. CSCI 315 Operating Systems Design 7

8 Several Instances of a Resource Type Available: A vector of length m indicates the number of available resources of each type. Allocation: An n x m matrix defines the number of resources of each type currently allocated to each process. Request: An n x m matrix indicates the current request of each process. If Request [i j ] = k, then process P i is requesting k more instances of resource type. R j. CSCI 315 Operating Systems Design 8

9 Detection Algorithm 1. Let Work and Finish be vectors of length m and n, respectively Initialize: (a) Work = Available (b) For i = 1,2,, n, if Allocation i 0, then Finish[i] = false, otherwise, Finish[i] = true. 2. Find an index i such that both: (a) Finish[i] == false (b) Request i Work If no such i exists, go to step Work = Work + Allocation i Finish[i] = true Go to step If Finish[i] == false, for some i, 1 i n, then the system is in deadlock state. Moreover, if Finish[i] == false, then P i is deadlocked. CSCI 315 Operating Systems Design 9

10 Example of Detection Algorithm Five processes P 0 through P 4 ; three resource types A (7 instances), B (2 instances), and C (6 instances). Snapshot at time T 0 : Allocation Request Available A B C A B C A B C P P P P P Sequence <P 0, P 2, P 3, P 1, P 4 > will result in Finish[i] = true for all i. CSCI 315 Operating Systems Design 10

11 Example (Cont.) P 2 requests an additional instance of type C. State of the system? Request A B C P P P P P Can reclaim resources held by process P 0, but have insufficient resources to fulfill the requests of other processes. Deadlock exists, consisting of processes P 1, P 2, P 3, and P 4. CSCI 315 Operating Systems Design 11

12 Detection-Algorithm Usage When, and how often, to invoke depends on: How often a deadlock is likely to occur? How many processes will need to be rolled back? (one for each disjoint cycle) If detection algorithm is invoked arbitrarily, there may be many cycles in the resource graph and so we would not be able to tell which of the many deadlocked processes caused the deadlock. CSCI 315 Operating Systems Design 12

13 Recovery from Deadlock: Process Termination Abort all deadlocked processes. (How do we know which processes are in the deadlock group?) Abort one process at a time until the deadlock cycle is eliminated. In which order should we choose to abort? Priority of the process. How long process has computed, and how much longer to completion. Resources the process has used. Resources process needs to complete. How many processes will need to be terminated. Is process interactive or batch? CSCI 315 Operating Systems Design 13

14 Recovery from Deadlock: Resource Preemption Selecting a victim minimize cost. (In what regard?) Rollback return to some safe state, restart process for that state. Starvation same process may always be picked as victim, include number of rollback in cost factor. CSCI 315 Operating Systems Design 14

15 Combined Approach to Deadlock Handling Combine the three basic approaches prevention avoidance detection allowing the use of the optimal approach for each of resources in the system. Partition resources into hierarchically ordered classes. Use most appropriate technique for handling deadlocks within each class. CSCI 315 Operating Systems Design 15