13/03/07Week 21 CENG334 Introduction to Operating Systems Erol Sahin Dept of Computer Eng. Middle East Technical University Ankara, TURKEY URL: Handling Deadlocks Topics: Deadlock prevention Deadlock avoidance Deadlock detection

2 A Deadlock ● P1 holds an instance of R2 and waits for R1. ● P2 holds R1 and and instance of R2, while waiting for R3. ● P3 holds R3 and waits for and instance of R2. ● P1, P2 and P3 have a circular wait condition, hence a deadlock. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

3 Deadlock Characterization Deadlock can arise if four conditions hold simultaneously. 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. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

4 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. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

5 Deadlock Prevention Restrain the ways request can be made. Ensure that at least one of the four conditions NOT HOLD! Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources, such as a printer. 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. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

6 Deadlock Prevention (Cont.) 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. ●Can be applied to resources whose state can be saved such as CPU, and memory. Not applicable to resources such as printer and tape drives. Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

7 Circular Wait - 1 Each resource is given an ordering: ●F(tape drive) = 1 ●F(disk drive) = 2 ●F(printer) = 3 ●F(mutex1) = 4 ●F(mutex2) = 5 ●……. Each process can request resources only in increasing order of enumeration. A process which decides to request an instance of Rj should first release all of its resources that are F(Ri) >= F(Rj).

8 Circular Wait - 2 For instance an application program may use ordering among all of its synchronization primitives: ●F(semaphore1) = 1 ●F(semaphore2) = 2 ●F(semaphore3) = 3 ●……. After this, all requests to synchronization primitives should be made only in the increasing order: ●Correct use: ●P(semaphore1); ●P(semaphore2); ●Incorrect use: ●P(semaphore3); ●P(semaphore2); Keep in mind that it’s the application programmer’s responsibility to obey this order.

9 Deadlock Avoidance Requires that the system has some additional a priori information available. 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. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

10 Safe, unsafe and deadlock states ●If a system is in safe state  no deadlocks. ●If a system is in unsafe state  possibility of deadlock. ●Avoidance  ensure that a system will never enter an unsafe state. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

11 Safe State When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state. System is in safe state if there exists a safe sequence of all processes. Sequence is safe if for each P i, the resources that P i can still request can be satisfied by currently available resources + 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. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

12 Example Maximum NeedsCurrent Needs P0105 P142 P292 The system has three processes and 12 tape drives. t=t0 The system at t0 is safe since the sequence exists.

13 Example Maximum NeedsCurrent Needs P0105 P142 P292 The system has three processes and 12 tape drives. t=t0 The system at t1 is no longer safe since ●P1 requests 2 more tape drives, finishes and releases 4 drives. ●However 4 drives are not sufficient for P0 or P2 complete its operation and would result in a deadlock. P2 requests one more drive Maximum NeedsCurrent Needs P0105 P142 P293 t=t1

14 Resource-Allocation Graph Algorithm 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. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

15 Resource-Allocation Graph Algorithm 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. Note that the cycle detection algorithm does not work with resources that have multiple instances. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides. Cycle => Unsafe

16 Banker’s Algorithm While giving credits, a banker should ensure that it never allocates all of its cash in such a way that none of its creditors can finish their work and pay back the loan.

17 Banker’s Algorithm Handles multiple instances of a resource. Each process must a priori claim maximum use. 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. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

18 Data Structures for the Banker’s Algorithm Let n = number of processes, and m = number of resources types. 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]. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

19 Safety Algorithm 1.Let Work and Finish be vectors of length m and n, respectively. Initialize: Work = Available Finish [i] = false for 0 to n-1. 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. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

20 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 Pi. If unsafe  Pi must wait, and the old resource-allocation state is restored Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

21 Example of Banker’s Algorithm 5 processes P 0 -P 4 ; 3 resource types – A (10 instances), – B (5instances, and – C (7 instances). snapshot at time T 0 : Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides. AllocationMaxAvailable ABCABCABC P P P P P

22 Example of Banker’s Algorithm Need = Max – Allocation; The system is in a safe state since the sequence satisfies safety criteria. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides. AllocationNeedAvailable ABCABCABC P P P P P

23 Example P 1 Request (1,0,2) Executing safety algorithm shows that sequence satisfies safety requirement. Can request for (3,3,0) by P4 be granted? Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides. AllocationNeedAvailable ABCABCABC P P P P P Check that ●Request <= Available ●that is, (1,0,2) <= (3,3,2) true. After allocation

24 Deadlock Detection ●Allow system to enter deadlock state ●Detection algorithm ●Recovery scheme Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

25 Single Instance of Each Resource Type Maintain wait-for graph ●Nodes are processes. ●P i  P j if P i is waiting for P j. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides. Resource-Allocation GraphCorresponding wait-for graph

26 Single Instance of Each Resource Type ●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 n 2 operations, where n is the number of vertices (that is processes) in the graph. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

27 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. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

28 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 4. 3.Work = Work + Allocation i Finish[i] = true go to step 2. 4.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. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides. Algorithm requires an order of O(m x n 2) operations to detect whether the system is in deadlocked state.

29 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. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

30 Recovery from Deadlock: 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? Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.

31 Recovery from Deadlock: 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. Adapted from Operating System Concepts (Silberschatz, Galvin, Gagne) slides.