Chapter 7: Deadlocks

Transcription

1 Chapter 7: Deadlocks System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock Combined Approach to Deadlock Handling 1

2 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 tape drives. P 1 and P 2 each hold one tape drive and each needs another one. Example semaphores A and B, initialized to 1 P 0 P 1 wait (A); wait(b) wait (B); wait(a) 2

3 Bridge Crossing Example Traffic only in one direction at each time. 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. 3

4 Potential Deadlock I need quad C and D I need quad B and C I need quad D and A I need quad A and B

5 Actual Deadlock HALT until D is free HALT until C is free HALT until A is free HALT until B is free

6 Reusable Resources Example

7 Example 2: Memory Request Space is available for allocation of 200Kbytes, and the following sequence of events occur: P1... Request 80 Kbytes;... Request 60 Kbytes; P2... Request 70 Kbytes;... Request 80 Kbytes; Deadlock occurs if both processes progress to their second request

8 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 8

9 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 n-1 } 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 0. 9

10 Resource-Allocation Graph V is partitioned into two types: A set of vertices V and a set of edges E." 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 Pi R j Assignment edge directed edge R j P i 10

11 Resource-Allocation Graph (Cont.) Process Resource Type with 4 instances P i requests instance of R j P i! R j! P i is holding an instance of R j P i " R j! 11

12 Example of a Resource Allocation Graph 1. Request 2. Use 3. Release 12

13 Resource Allocation Graph With A Deadlock 1. Request 2. Use 3. Release Deadlock condition Mutual Exclusion Hold & Wait No Preemption Circular Wait 13

14 Resource Allocation Graph With A Cycle But No Deadlock 1. Request 2. Use 3. Release Deadlock condition Mutual Exclusion Hold & Wait No Preemption Circular Wait 14

15 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. 15

16 Methods for Handling Deadlocks 1. Ensure that the system will never enter a deadlock state. 2. Allow the system to enter a deadlock state and then recover. 3. Ignore the problem and pretend that deadlocks never occur in the system; used by most operating systems, including UNIX. 16

17 Deadlock Prevention Break one of the 4 deadlock conditions Violating Mutual Exclusion condition Not required for sharable resources; but must hold for nonsharable resources. We can not do much about it. Violating Hold and Wait condition Constrain the ways request can be made." 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. 17

18 Deadlock Prevention (Cont.) Violating No Preemption condition 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. Violating Circular Wait condition impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration. 18

19 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. 19

20 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 <P 1, P 2,, P n > is safe if for each P i, the resources that Pi 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 i-1 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. 20

21 Basic Facts 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. 21

22 Safe, Unsafe, Deadlock State 22

23 Resource-Allocation Graph Algorithm Claim edge P i R j indicated that process P i 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. 23

24 Resource-Allocation Graph For Deadlock Avoidance 24

25 Unsafe State In Resource-Allocation Graph 25

26 Banker s Algorithm Features: Multiple instances of each resource type. Each process must claim maximum use beforehand. 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. 26

27 Data Structures for the Banker s Algorithm Let n = number of processes, and m = number of resource 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] 27

28 We need two algorithms: Safety & resource-request Safety Algorithm 1. Let Work[m] and Finish[n] be vectors of lengths m and n, respectively. Initialize: Work := Available Finish [i] := false for i = 1,2,, n. loop 2. Find an i such that both conditions below are held: (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; otherwise the system is in an unsafe state. 28

29 Example of Banker s Algorithm 5 processes P 0 through P 4 ; A : 10 instances, B : 5 instances, and C : 7 instances. 3 resource types: Snapshot at time T 0 : Allocation Max Available A B C A B C A B C P P P P P

30 Example (Cont.) The content of the matrix. Need is defined to be Need := Max Allocation. Allocation Max Available A B C A B C A B C P P P P P 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 0, P 2 > satisfies safety criteria. Work = Available = > > > > >

31 Initial check Resource-Request Algorithm for Process P i Request i := 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 31

32 Example P 1 Request (1,0,2) (Cont.) P 1 Allocation Need Max Available

33 Example of Banker s Algorithm 33

34 Deadlock Detection Allow system to enter deadlock state Detection algorithm Recovery scheme 34

35 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. 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 in the graph. 35

36 Resource-Allocation Graph and Wait-for Graph Resource-Allocation Graph" Corresponding wait-for graph" 36

37 Several Resource Types 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. 37

38 Detection Algorithm 1. Let Work and Finish be vectors of length m and n, respectively Initialize: (a) Work := Available (b) For i := 1 to n, if Allocation i 0, then Finish[i] := false; else 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. 38

39 Detection Algorithm (Cont.) 3. 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. Algorithm requires an order of O(m x n 2) operations to detect whether the system is in deadlocked state. " 39

40 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 4, P 1 > will result in Finish[i] = true for all i. 40

41 Example (Cont.) P 2 requests an additional instance of type C. State of system? Request A B C P P P P P OLD Allocatio Request Available A B C A B C A B C P P P P P Can reclaim resources held by process P 0 (since P 0 finishes without need to get any resources), but insufficient resources to fulfill other processes requests. Deadlock exists, consisting of processes P 1, P 2, P 3, and P 4. 41

42 Example 1: Deadlock Detection 42

43 Example 1: Deadlock Detection 43

44 Example 2: Deadlock Detection Consider five processes P 0 through P 4 that use four resource types with the following snapshot at time T0. Answer the following questions: Allocation Request Available A B CD A B CD A B CD P P P P P

45 Example 2: Deadlock Detection Question 1: At time T1 > T0 process P3 requests 2 resources of type A and process P0 requests 3 resources of type C. The operating system does not grant the resource requests before time T2 > T1. Below draw two tables that show the changes that will occur at time T1 and at time T2. T1: T2: Allocation Request Available A B CD A B CD A B CD P P P P P A B CD A B CD A B CD P P P P P Question 2: At time T2 the operating system applies the deadlock detection algorithm. If the system is in the deadlock situation, determine the processes involved in the deadlock and the value of the vector Available. If the system is not in deadlock indicate the sequence of changing the values of vector Available and its final value. Available: (P3 can terminate without any additional resource) (P2 can terminate with resources from Available) (P1 can terminate with resources from Available) (DEADLOCK, see below) None of the processes P0 and P4 can terminate since the available resources can not satisfy any of the requests from these processes. Therefore <P0, P4> are involved in a deadlock situation. 45

46 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. 46

47 Recovery from Deadlock: Process Termination Option 1. Abort all deadlocked processes. Option 2. Abort one process at a time until the deadlock cycle is eliminated. In which order should we choose to abort? What is the 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? Solution: define a cost function in which the above criteria have different weights; then choose a process to abort that minimizes the cost 47

48 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. 48

49 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. 49

50 Traffic Deadlock 50