1. 02/19/2008CSCI 315 Operating Systems Design1 Deadlock Notice: The slides for this lecture have been largely based on those accompanying the textbook Operating Systems Concepts with Java, by Silberschatz, Galvin, and Gagne (2007). Many, if not all, the illustrations contained in this presentation come from this source.

2. 02/19/2008CSCI 315 Operating Systems Design2 A System Model Resource types R 1, R 2,..., R m (CPU cycles, memory space, I/O devices) Each resource type R i has W i instances. Each process utilizes a resource as follows: –request –use –release

3. 02/19/2008CSCI 315 Operating Systems Design3 Deadlock Characterization Mutual exclusion: only one process at a time can use a resource. Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes. No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task. Circular wait: there exists a set {P 0, P 1, …, P 0 } of waiting processes such that P 0 is waiting for a resource that is held by P 1, P 1 is waiting for a resource that is held by P 2, …, P n–1 is waiting for a resource that is held by P n, and P 0 is waiting for a resource that is held by P 0. Deadlock can arise if four conditions hold simultaneously:

4. 02/19/2008CSCI 315 Operating Systems Design4 Resource Allocation Graph The nodes in V can be of two types (partitions): –P = {P 1, P 2, …, P n }, the set consisting of all the processes in the system. –R = {R 1, R 2, …, R m }, the set consisting of all resource types in the system. request edge – directed edge P 1  R j assignment edge – directed edge R j  P i Graph: G=(V,E)

5. 02/19/2008CSCI 315 Operating Systems Design5 Resource Allocation Graph Example 1

6. 02/19/2008CSCI 315 Operating Systems Design6 Resource Allocation Graph Example 2

7. 02/19/2008CSCI 315 Operating Systems Design7 Basic Facts If graph contains no cycles  no deadlock. If graph contains a cycle  –if only one instance per resource type, then deadlock. –if several instances per resource type, possibility of deadlock.

8. 02/19/2008CSCI 315 Operating Systems Design8 Methods for Handling Deadlocks Ensure that the system will never enter a deadlock state. Allow the system to enter a deadlock state and then recover. Ignore the problem and pretend that deadlocks never occur in the system; used by most operating systems, including UNIX.

9. 02/19/2008CSCI 315 Operating Systems Design9 Deadlock Prevention Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources. Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources. –Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none. –Low resource utilization; starvation possible. Restrain the ways request can be made.

10. 02/19/2008CSCI 315 Operating Systems Design10 Deadlock Prevention No Preemption – –If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released. –Preempted resources are added to the list of resources for which the process is waiting. –Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting. Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration. Restrain the ways request can be made.

11. 02/19/2008CSCI 315 Operating Systems Design11 Deadlock Avoidance Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need. The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition. Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes. The system has additional a priori information.

12. 02/19/2008CSCI 315 Operating Systems Design12 Safe States Sequence is safe if for each P i, the resources that P i can still request can be satisfied by currently available resources plus the resources held by all the P j, with j<I. –If P i resource needs are not immediately available, then P i can wait until all P j have finished. –When P j is finished, P i can obtain needed resources, execute, return allocated resources, and terminate. –When P i terminates, P i+1 can obtain its needed resources, and so on. The system is in a safe state if there exists a safe sequence for all processes. When a process requests an available resource, the system must decide if immediate allocation leaves the system in a safe state.

13. 02/19/2008CSCI 315 Operating Systems Design13 Basic Facts If a system is in a safe state there can be no deadlock. If a system is in unsafe state, there exists the possibility of deadlock. Avoidance strategies ensure that a system will never enter an unsafe state.

14. 02/19/2008CSCI 315 Operating Systems Design14 Safe, Unsafe, and Deadlock States safe unsafe deadlock

15. 02/19/2008CSCI 315 Operating Systems Design15 Resource-Allocation Graph Algorithm Goal: not to allow the system to enter an unsafe state. Applicable only when there is a single instance of each resource type. Claim edge P i  R j indicated that process P j may request resource R j ; represented by a dashed line. Claim edge converts to request edge when a process requests a resource. When a resource is released by a process, assignment edge reconverts to a claim edge. Resources must be claimed a priori in the system.

16. 02/19/2008CSCI 315 Operating Systems Design16 Resource-Allocation Graph for Deadlock Avoidance

17. 02/19/2008CSCI 315 Operating Systems Design17 Unsafe State In Resource-Allocation Graph

18. 02/19/2008CSCI 315 Operating Systems Design18 Banker’s Algorithm Applicable when there are multiple instances of each resource type. In a bank, the cash must never be allocated in a way such that it cannot satisfy the need of all its customers. Each process must state a priori the maximum number of instances of each kind of resource that it will ever need. When a process requests a resource it may have to wait. When a process gets all its resources it must return them in a finite amount of time.

19. 02/19/2008CSCI 315 Operating Systems Design19 Banker’s Algorithm: Data Structures Available: Vector of length m. If available [j] = k, there are k instances of resource type R j available. Max: n x m matrix. If Max [i,j] = k, then process P i may request at most k instances of resource type R j. Allocation: n x m matrix. If Allocation[i,j] = k then P i is currently allocated k instances of R j. Need: n x m matrix. If Need[i,j] = k, then P i may need k more instances of R j to complete its task. Need[i,j] = Max[i,j] – Allocation [i,j] Let n = number of processes, and m = number of resources types.

20. 02/19/2008CSCI 315 Operating Systems Design20 Safety Algorithm 1.Let Work and Finish be vectors of length m and n, respectively. Initialize: Work = Available Finish [i] = false for i - 1,3, …, n. 2.Find an i such that both: (a) Finish [i] = false (b) Need i  Work If no such i exists, go to step 4. 3.Work = Work + Allocation i Finish[i] = true go to step 2. 4.If Finish [i] == true for all i, then the system is in a safe state.

21. 02/19/2008CSCI 315 Operating Systems Design21 Resource-Request Algorithm for Process P i Request = request vector for process P i. If Request i [j] = k then process P i wants k instances of resource type R j. 1.If Request i  Need i go to step 2. Otherwise, raise error condition, since process has exceeded its maximum claim. 2.If Request i  Available, go to step 3. Otherwise P i must wait, since resources are not available. 3.Pretend to allocate requested resources to P i by modifying the state as follows: Available = Available = Request i ; Allocation i = Allocation i + Request i ; Need i = Need i – Request i;; If safe  the resources are allocated to P i. If unsafe  P i must wait, and the old resource-allocation state is restored

22. 02/19/2008CSCI 315 Operating Systems Design22 Example of Banker’s Algorithm 5 processes P 0 through P 4 ; 3 resource types A (10 instances), B (5instances, and C (7 instances). Snapshot at time T 0 : AllocationMaxAvailable A B CA B C A B C P 0 0 1 07 5 3 3 3 2 P 1 2 0 0 3 2 2 P 2 3 0 2 9 0 2 P 3 2 1 1 2 2 2 P 4 0 0 24 3 3

23. 02/19/2008CSCI 315 Operating Systems Design23 Example (Cont.) The content of the matrix. Need is defined to be Max – Allocation. Need A B C P 0 7 4 3 P 1 1 2 2 P 2 6 0 0 P 3 0 1 1 P 4 4 3 1 The system is in a safe state since the sequence satisfies safety criteria.

24. 02/19/2008CSCI 315 Operating Systems Design24 Example P 1 Request (1,0,2) (Cont.) Check that Request  Available (that is, (1,0,2)  (3,3,2)  true. AllocationNeedAvailable A B CA B CA B C P 0 0 1 0 7 4 3 2 3 0 P 1 3 0 20 2 0 P 2 3 0 1 6 0 0 P 3 2 1 1 0 1 1 P 4 0 0 2 4 3 1 Executing safety algorithm shows that sequence satisfies safety requirement. Can request for (3,3,0) by P4 be granted? Can request for (0,2,0) by P0 be granted?