Concurrency: Deadlock Detection

1. Concurrency: Deadlock Detection Fred Kuhns (fredk@arl.wustl.edu, http://www.arl.wustl.edu/~fredk) Department of Computer Science and Engineering Washington University in St. Louis

2. Deadlock Detection • Allow system to enter deadlock state • Detection algorithm • Recovery scheme Cs422 – Operating Systems Organization

3. 1 Instance of each Resource Type • Maintain wait-for graph • Nodes are processes. • Pi Pj if Piis waiting for Pj to release some resource Rk • Periodically invoke an algorithm that searches for a cycle in the graph. • An algorithm to detect a cycle in a graph requires an order of n2 operations, where n is the number of vertices in the graph. Cs422 – Operating Systems Organization

4. P0 P0 R1 R2 P3 P2 P1 P1 P2 P3 R3 R4 P4 P4 Wait-For Graph Resource Allocation Graph Wait-for Graph Cs422 – Operating Systems Organization

5. >1 Instance of each Resource Type • Assume m resource types and n processes • Available[i]: A vector of length m indicates the number of available resources of each type. • Allocation[i,j]: An n x m matrix defines the number of resources of each type currently allocated to each process. • Allocation[i, j] = number of resources of type j currently allocated to Process i. • Allocationi = allocation vector for Process i • Request[i,j]: An n x m matrix indicates the current per process requist vector. • Request[i,j] = number of resources of type j requested by Process i. • Requesti = request vector for Process i Cs422 – Operating Systems Organization

6. Detection Algorithm 0. Work and Finish are vectors of length m and n, respectively. Initialize: (a) Work = Available (b) if Allocationi 0, then Finish[i] = false; else Finish[i] = true. 1. Find an index i such that both: • Finish[i] == false // has allocated resources • Requesti Work// request can be filled If no such i exists, go to step 4. 2. Work = Work + Allocationi; Finish[i] = true; go to step 1 3. If Finish[i] == false, for some i, 1  i  n, then the system is in deadlock state. Moreover, if Finish[i] == false, then Piis deadlocked. Cs422 – Operating Systems Organization

7. Detection Algorithm (Cont.) • Algorithm requires an order of O(m x n2) operations to detect whether the system is in deadlocked state. Cs422 – Operating Systems Organization

8. Example of Detection Algorithm • Five processes P0 through P4 • three resource types A (7), B (2), and C (6). • Snapshot at time T0: AllocationRequestAvailable A B C A B C A B C P0 0 1 0 0 0 00 0 0 P1 2 0 0 2 0 2 P2 3 0 3 0 0 0 P3 2 1 1 1 0 0 P4 0 0 2 0 0 2 • Sequence <P0, P2, P3, P1, P4> will result in Finish[i] = true for all i. Cs422 – Operating Systems Organization

9. Step 0 • Total Resource {A = 7, B = 2, C = 6} AllocationRequestAvailable A B C A B C A B C P0 0 1 0 0 0 00 0 0 P1 2 0 0 2 0 2 P2 3 0 3 0 0 0 P3 2 1 1 1 0 0 P4 0 0 2 0 0 2 • Step 0: • Work = Available = {0, 0, 0} • Finish = {false, false, false, false, false} Cs422 – Operating Systems Organization

10. Steps 1, 2 and 3 Steps 1 & 2: • Request0 < work // {0 0 0}Work = {0 1 0}Finish = {true, false, false, false, false} • Request2 < work // {0 1 0}Work = {3 1 3}Finish = {true, false, true, false, false} • Request1 < work // {3 1 3}Work = {5 1 3}Finish = {true, true, true, false, false} • Request3 < work // {5 1 3}Work = {7 2 4}Finish = {true, true, true, true, false} • Request4 < work // {7 2 4}Work = {7 2 6}Finish = {true, true, true, true, true} AllocationRequest A B C A B C P0 0 1 0 0 0 0 P1 2 0 0 2 0 2 P2 3 0 3 0 0 0 P3 2 1 1 1 0 0 P4 0 0 2 0 0 2 Step 3: • No deadlock since all elements of Finish = true Cs422 – Operating Systems Organization

11. New Request from P2 • P2 requests an additional instance of type C • AllocationRequestAvailable • A B C A B C A B C • P0 0 1 0 0 0 00 0 0 • P1 2 0 0 2 0 2 • P2 3 0 3 0 0 1 • P3 2 1 1 1 0 0 • P4 0 0 2 0 0 2 • State of system? • Can reclaim resources held by process P0, but insufficient resources to fulfill other processes; requests. • Deadlock exists, consisting of processes P1, P2, P3, and P4. Cs422 – Operating Systems Organization

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 process 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. Cs422 – Operating Systems Organization

13. Recovery: Process Termination • Abort all deadlocked processes. • 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? Cs422 – Operating Systems Organization

14. Recovery: Resource Preemption • Selecting a victim – minimize cost. • 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. Cs422 – Operating Systems Organization

15. Combined Approach • Combine the three basic approaches • prevention • avoidance • detection allowing the use of the optimal approach for each resource type in the system. • Partition resources into hierarchically ordered classes. • Use most appropriate technique for handling deadlocks within each class. Cs422 – Operating Systems Organization