Deadlocks. Operating System Concepts - 7 th Edition, Feb 14, 2005

Transcription

1 Deadlocks

2 Deadlocks The Deadlock Problem System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock 7.2 Silberschatz, Galvin and Gagne 2005

3 The Deadlock Problem A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set. Example System has 2 disk drives. P 1 and P 2 each hold one disk drive and each needs another one. 7.3 Silberschatz, Galvin and Gagne 2005

4 Bridge Crossing Example Traffic only in one direction. Each section of a bridge can be viewed as a resource. If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback). Several cars may have to be backed up if a deadlock occurs. Starvation is possible. 7.4 Silberschatz, Galvin and Gagne 2005

5 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 7.5 Silberschatz, Galvin and Gagne 2005

6 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 Silberschatz, Galvin and Gagne 2005

7 Resource-Allocation Graph A set of vertices V and a set of edges E. V is partitioned into two types: 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 multi-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 7.7 Silberschatz, Galvin and Gagne 2005

8 Resource-Allocation Graph (Cont.) Process Resource Type with 4 instances P i requests instance of R j P i P i is holding an instance of R j R j P i R j 7.8 Silberschatz, Galvin and Gagne 2005

9 Example of a Resource Allocation Graph 7.9 Silberschatz, Galvin and Gagne 2005

10 Resource Allocation Graph With A Deadlock 7.10 Silberschatz, Galvin and Gagne 2005

11 Graph With A Cycle But No Deadlock 7.11 Silberschatz, Galvin and Gagne 2005

12 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 Silberschatz, Galvin and Gagne 2005

13 Methods for Handling Deadlocks We can use protocol to prevent or avoid deadlocks, ensuring that the system will never enter a deadlock state. We can allow the system to enter a deadlock state, detect it, and recover. Deadlock prevention and deadlock detection algorithm is used for ignoring the deadlocks. Deadlock prevention is a set of methods for ensuring that at least one of the necessary condition cannot hold. These methods prevent deadlocks by constraining how requests for resources can be made. Deadlocks avoidance requires that the operating system be given in advance, additional information concerning which resources, a process will request and use during its lifetime. If a system does not employ either a deadlock prevention or a deadlock avoidance algorithm, then a deadlock situation may occur. If a system does not ensure that a deadlock will never occur and also does not provide a mechanism for deadlock detection and recovery, then the system is in a deadlock state Silberschatz, Galvin and Gagne 2005

14 Deadlock Prevention Methods for preventing deadlock are of two classes : indirect method and direct method. An indirect method is to prevent the occurrence of one of the three necessary condition i.e. mutual exclusion, hold and wait and no preemption. A direct method is to prevent the occurrence of a circular wait. Mutual Exclusion Mutual exclusion condition must hold for non-sharable resource. If access to a resource requires mutual exclusion, then mutual exclusion must be supported by the operating system. Some resources, such as files, may allow multiple accesses for reads but only exclusive access for writes. In this case, deadlock can occur if more than one process requires write permission Silberschatz, Galvin and Gagne 2005

15 Deadlock Prevention (Cont.) Hold & Wait- The hold and wait condition can be eliminated by forcing a process to release all resources held by it whenever it requests a resource that is not available. For example, process copies data from a floppy disk to a hard disk, sort a disk file and then prints the results to printer. If all the resources must be requested at the beginning of the process, then the process must initially request the floppy disk, hard disk and a printer. It will hold the printer for its entire execution, even though it needs the printer only at then end. In these two method, resource utilization is low in the first method and second method is affected by the starvation Silberschatz, Galvin and Gagne 2005

16 Deadlock Prevention (Cont.) No Preemption -If a process holding certain resources is denied a further request. That process must release its original resources and if necessary request them again together with additional resource. If a process requests a resource that is currently held by another process, the operating system may preempt the second process and require it to release its resources. In general, sequential I/O device cannot be preempted. Preemption is possible for certain types of resources, such as CPU and main memory. Circular-Wait-One way to prevent the circular-wait condition is by linear ordering of different types of system resources. In this system resources are divided into different classes. If a process has been allocated resources of type R, then it may subsequently request only those resource types. Following R in the ordering 7.16 Silberschatz, Galvin and Gagne 2005

17 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 Silberschatz, Galvin and Gagne 2005

18 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 sequence <P 1, P 2,, P n > of ALL the processes is the system such that 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. That is: 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 Silberschatz, Galvin and Gagne 2005

19 Safe, Unsafe, Deadlock State 7.19 Silberschatz, Galvin and Gagne 2005

20 Basic Facts System is in a safe state no deadlocks If system is deadlocked state is unsafe System is in unsafe state possibility of deadlock. OS cannot prevent processes from requesting resources in a sequence that leads to deadlock Deadlock Avoidance Ensures system will never enter an unsafe state Thereby avoids the possibility of deadlock 7.20 Silberschatz, Galvin and Gagne 2005

21 Safe States: An Example Suppose processes P0, P1, and P2 share 12 magnetic tape drives Currently 9 drives are held among the processes and 3 are available Question: Is this system currently in a safe state? Answer: Yes! Safe Sequence: <P1, P0, P2> 7.21 Silberschatz, Galvin and Gagne 2005

22 How to reach an Unsafe State 3 Suppose process P2 requests and is allocated 1 more tape drive. Question: Is the resulting state still safe? Answer: No! Because there does not exist a safe sequence anymore. Only P1 can be allocated its maximum needs. IF P0 and P2 request 5 more drives and 6 more drives, respectively, then the resulting state will be deadlocked Silberschatz, Galvin and Gagne 2005

23 Deadlock Avoidance - Concepts Key Ideas: Initially the system is in a safe state Whenever a process requests an available resource, system will allocate resource immediately only if the resulting state is still safe! Otherwise, requesting process must wait. Why does this work? By induction, all reachable states are safe states By definition, all safe states are not deadlocked 7.23 Silberschatz, Galvin and Gagne 2005

24 Avoidance Algorithms Single instance of a resource type. Use a resource-allocation graph Cycles are necessary are sufficient for deadlock Multiple instances of a resource type. Use the banker s algorithm Cycles are necessary, but not sufficient for deadlock 7.24 Silberschatz, Galvin and Gagne 2005

25 Resource-Allocation Graph Scheme Claim edge P i R j indicates 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. Request edge converted to an assignment edge when the resource is allocated to the process. When a resource is released by a process, assignment edge reconverts to a claim edge. Resources must be claimed a priori in the system Silberschatz, Galvin and Gagne 2005

26 Resource-Allocation Graph P2 requesting R1, but R1 is already allocated to P1. Both processes have a claim on resource R2 What happens if P2 now requests resource R2? 7.26 Silberschatz, Galvin and Gagne 2005

27 Unsafe State In Resource-Allocation Graph Cannot allocate resource R2 to process P2 Why? Because resulting state is unsafe P1 could request R2, thereby creating deadlock! 7.27 Silberschatz, Galvin and Gagne 2005

28 Resource-Allocation Graph Algorithm Use only when there is a single instance of each resource type Suppose that process P i requests a resource R j The request can be granted only if converting the request edge to an assignment edge does not result in the formation of a cycle in the resource allocation graph 7.28 Silberschatz, Galvin and Gagne 2005

29 Banker s Algorithm Multiple instances. Each process claims maximum resource needs a priori. When a process requests a resource it may have to wait. When a process gets all of its resources it must return them in a finite amount of time Silberschatz, Galvin and Gagne 2005

30 Data Structures for the Banker s Algorithm Let n = number of processes 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] Silberschatz, Galvin and Gagne 2005

31 Safety Algorithm 1. Let Work and Finish be vectors of length m and n, respectively. Initialize: Work = Available Finish [i] = false for i = 0, 1,, n Find and i such that both: (a) Finish [i] = false (b) Need i Work If no such i exists, go to step Work = Work + Allocation i Finish[i] = true go to step If Finish [i] == true for all i, then the system is in a safe state Silberschatz, Galvin and Gagne 2005

32 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; 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 7.32 Silberschatz, Galvin and Gagne 2005

33 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 : Allocation Max Available A B C A B C A B C P P P P P Silberschatz, Galvin and Gagne 2005

34 Example (Cont.) The content of the matrix Need is defined to be Max Allocation. Need A B C P P P P P The system is in a safe state since the sequence < P 1, P 3, P 4, P 2, P 0 > satisfies safety criteria Silberschatz, Galvin and Gagne 2005

35 Example: P 1 Request (1,0,2) Check that Request Available (that is, (1,0,2) (3,3,2) true. Allocation Need Available A B C A B C A B C P P P P P Executing safety algorithm shows that sequence < P 1, P 3, P 4, P 0, P 2 > satisfies safety requirement. Can request for (3,3,0) by P 4 be granted? Can request for (0,2,0) by P 0 be granted? 7.35 Silberschatz, Galvin and Gagne 2005

36 Deadlock Detection If a system does not employ either a deadlock prevention or a deadlock-avoidance algorithm, then a deadlock situation may occur. In this environment the system must provide An algorithm tat examines the state of the system to determine whether a deadlock has occurred An algorithm to recover from the deadlock Silberschatz, Galvin and Gagne 2005

37 Single Instance of Each Resource Type If all resources have only a single instance, then deadlock detection algorithm that uses a variant of the resource allocation graph, called wait-for graph. Wait-for graph is obtained from resourceallocation graph. Nodes of resource is removed and collapsing the appropriate edge. i.e. assignment and request edge Silberschatz, Galvin and Gagne 2005

38 Resource-Allocation Graph and Wait-for Graph Resource-Allocation Graph Corresponding wait-for graph 7.38 Silberschatz, Galvin and Gagne 2005

39 Several Instances of a Resource Type The wait for graph scheme is not applicable to a resource allocation system with multiple instances of each 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 Silberschatz, Galvin and Gagne 2005

40 Recovery from Deadlock: Process Termination Abort all deadlocked processes. Abort one process at a time until the deadlock cycle is eliminated. Aborting a process may not easy. If the process was in the midst of updating a file, terminating it will leave that file in an incorrect state. Similarly, if the process was in the midst of printing data on the printer, the system must reset the printer to a correct state before printing the next job. 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 Silberschatz, Galvin and Gagne 2005

41 Recovery from Deadlock: Resource Preemption 1. Selecting a Victim : - Which resources and which processes are to be preempted? As in process termination, we must determine the order of preemption to minimize cost. Cost factors may include such parameters as the number of resources a deadlock process is holding, and the amount of time a deadlocked process has thus far consumed during its execution. 2. Rollback return to some safe state, restart process for that state. 3. Starvation same process may always be picked as victim, include number of rollback in cost factor 7.41 Silberschatz, Galvin and Gagne 2005

42 End of Chapter