Deadlock PDF

Chapter 8: Deadlocks Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018  Outline ▪ System Model ▪ Deadlock Characterization ▪ Methods for Handling Deadlocks ▪ Deadlock Prevention...

Chapter 8: Deadlocks Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018  Outline ▪ System Model ▪ Deadlock Characterization ▪ Methods for Handling Deadlocks ▪ Deadlock Prevention ▪ Deadlock Avoidance ▪ Deadlock Detection ▪ Recovery from Deadlock Operating System Concepts – 10th Edition 8.2 Silberschatz, Galvin and Gagne ©2018  Chapter Objectives ▪ Illustrate how deadlock can occur when mutex locks are used ▪ Define the four necessary conditions that characterize deadlock ▪ Identify a deadlock situation in a resource allocation graph ▪ Evaluate the four different approaches for preventing deadlocks ▪ Apply the banker’s algorithm for deadlock avoidance ▪ Evaluate approaches for recovering from deadlock Operating System Concepts – 10th Edition 8.3 Silberschatz, Galvin and Gagne ©2018  System Model ▪ System consists of resources ▪ Resource types R1, R2,..., Rm CPU cycles, memory space, I/O devices ▪ Each resource type Ri has Wi instances. ▪ Each process utilizes a resource as follows: request use release Operating System Concepts – 10th Edition 8.4 Silberschatz, Galvin and Gagne ©2018  Deadlock with Semaphores ▪ Data: A semaphore S1 initialized to 1 A semaphore S2 initialized to 1 ▪ Two processes P1 and P2 ▪ P1: wait(s1) wait(s2) ▪ P2: wait(s2) wait(s1) Operating System Concepts – 10th Edition 8.5 Silberschatz, Galvin and Gagne ©2018  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 {P0, P1, …, Pn} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and Pn is waiting for a resource that is held by P0. Operating System Concepts – 10th Edition 8.6 Silberschatz, Galvin and Gagne ©2018  Resource-Allocation Graph A set of vertices V and a set of edges E. ▪ V is partitioned into two types: P = {P1, P2, …, Pn}, the set consisting of all the processes in the system R = {R1, R2, …, Rm}, the set consisting of all resource types in the system ▪ request edge – directed edge Pi → Rj ▪ assignment edge – directed edge Rj → Pi Operating System Concepts – 10th Edition 8.7 Silberschatz, Galvin and Gagne ©2018  Resource Allocation Graph Example ▪ One instance of R1 ▪ Two instances of R2 ▪ One instance of R3 ▪ Three instance of R4 ▪ T1 holds one instance of R2 and is waiting for an instance of R1 ▪ T2 holds one instance of R1, one instance of R2, and is waiting for an instance of R3 ▪ T3 is holds one instance of R3 Operating System Concepts – 10th Edition 8.8 Silberschatz, Galvin and Gagne ©2018  Resource Allocation Graph with a Deadlock Operating System Concepts – 10th Edition 8.9 Silberschatz, Galvin and Gagne ©2018  Graph with a Cycle But no Deadlock Operating System Concepts – 10th Edition 8.10 Silberschatz, Galvin and Gagne ©2018  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 Operating System Concepts – 10th Edition 8.11 Silberschatz, Galvin and Gagne ©2018  Methods for Handling Deadlocks ▪ Ensure that the system will never enter a deadlock state: Deadlock prevention Deadlock avoidance ▪ Allow the system to enter a deadlock state and then recover ▪ Ignore the problem and pretend that deadlocks never occur in the system. Operating System Concepts – 10th Edition 8.12 Silberschatz, Galvin and Gagne ©2018  Deadlock Prevention Invalidate one of the four necessary conditions for deadlock: ▪ Mutual Exclusion – not required for sharable resources (e.g., read-only files); must hold for non-sharable 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 allocated to it. Low resource utilization; starvation possible Operating System Concepts – 10th Edition 8.13 Silberschatz, Galvin and Gagne ©2018  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 ▪ Circular Wait: Impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration Operating System Concepts – 10th Edition 8.14 Silberschatz, Galvin and Gagne ©2018  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 Operating System Concepts – 10th Edition 8.15 Silberschatz, Galvin and Gagne ©2018  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 of ALL the processes in the systems such that for each Pi, the resources that Pi can still request can be satisfied by currently available resources + resources held by all the Pj, with j < I ▪ That is: If Pi resource needs are not immediately available, then Pi can wait until all Pj have finished When Pj is finished, Pi can obtain needed resources, execute, return allocated resources, and terminate When Pi terminates, Pi +1 can obtain its needed resources, and so on Operating System Concepts – 10th Edition 8.16 Silberschatz, Galvin and Gagne ©2018  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. Operating System Concepts – 10th Edition 8.17 Silberschatz, Galvin and Gagne ©2018  Safe, Unsafe, Deadlock State Operating System Concepts – 10th Edition 8.18 Silberschatz, Galvin and Gagne ©2018  Avoidance Algorithms ▪ Single instance of a resource type Use a resource-allocation graph ▪ Multiple instances of a resource type Use the Banker’s Algorithm Operating System Concepts – 10th Edition 8.19 Silberschatz, Galvin and Gagne ©2018  Resource-Allocation Graph Scheme ▪ Claim edge Pi → Rj indicated that process Pj may request resource Rj; 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 Operating System Concepts – 10th Edition 8.20 Silberschatz, Galvin and Gagne ©2018  Resource-Allocation Graph Operating System Concepts – 10th Edition 8.21 Silberschatz, Galvin and Gagne ©2018  Unsafe State In Resource-Allocation Graph Operating System Concepts – 10th Edition 8.22 Silberschatz, Galvin and Gagne ©2018  Resource-Allocation Graph Algorithm ▪ Suppose that process Pi requests a resource Rj ▪ 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 Operating System Concepts – 10th Edition 8.23 Silberschatz, Galvin and Gagne ©2018  Banker’s Algorithm ▪ Multiple instances of resources ▪ 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 Operating System Concepts – 10th Edition 8.24 Silberschatz, Galvin and Gagne ©2018  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 Rj available ▪ Max: n x m matrix. If Max [i,j] = k, then process Pi may request at most k instances of resource type Rj ▪ Allocation: n x m matrix. If Allocation[i,j] = k then Pi is currently allocated k instances of Rj ▪ Need: n x m matrix. If Need[i,j] = k, then Pi may need k more instances of Rj to complete its task Need [i,j] = Max[i,j] – Allocation [i,j] Operating System Concepts – 10th Edition 8.25 Silberschatz, Galvin and Gagne ©2018  Example of Banker’s Algorithm ▪ 5 processes P0 through P4; 3 resource types: A (10 instances), B (5instances), and C (7 instances) ▪ Snapshot at time T0: Allocation Max Available ABC ABC ABC P0 010 753 332 P1 200 322 P2 302 902 P3 211 222 P4 002 433 Operating System Concepts – 10th Edition 8.26 Silberschatz, Galvin and Gagne ©2018  Example (Cont.) ▪ The content of the matrix Need is defined to be Max – Allocation Need ABC P0 743 P1 122 P2 600 P3 011 P4 431 ▪ The system is in a safe state since the sequence < P1, P3, P4, P2, P0> satisfies safety criteria Operating System Concepts – 10th Edition 8.27 Silberschatz, Galvin and Gagne ©2018  Example: P1 Request (1,0,2) ▪ Check that Request  Available (that is, (1,0,2)  (3,3,2)  true Allocation Need Available ABC ABC ABC P0 010 743 230 P1 302 020 P2 302 600 P3 211 011 P4 002 431 ▪ Executing safety algorithm shows that sequence < P1, P3, P4, P0, P2> satisfies safety requirement ▪ Can request for (3,3,0) by P4 be granted? ▪ Can request for (0,2,0) by P0 be granted? Operating System Concepts – 10th Edition 8.28 Silberschatz, Galvin and Gagne ©2018  Deadlock Detection ▪ Allow system to enter deadlock state ▪ Detect ▪ Recover Operating System Concepts – 10th Edition 8.29 Silberschatz, Galvin and Gagne ©2018  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? 1. Priority of the process 2. How long process has computed, and how much longer to completion 3. Resources the process has used 4. Resources process needs to complete 5. How many processes will need to be terminated 6. Is process interactive or batch? Operating System Concepts – 10th Edition 8.30 Silberschatz, Galvin and Gagne ©2018  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 Operating System Concepts – 10th Edition 8.31 Silberschatz, Galvin and Gagne ©2018  End of Chapter 8 Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018