ch8.pptx

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

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  Apply the deadlock detection algorithm  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 threads T1 and T2  T1: wait(s1) wait(s2)  T2: 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 thread at a time can use a resource  Hold and wait: a thread holding at least one resource is waiting to acquire additional resources held by other threads  No preemption: a resource can be released only voluntarily by the thread holding it, after that thread has completed its task  Circular wait: there exists a set {T0, T1, …, Tn} of waiting threads such that T0 is waiting for a resource that is held by T1, T1 is waiting for a resource that is held by T2, …, Tn–1 is waiting for a resource that is held by Tn, and Tn is waiting for a resource that is held by T0. 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: T = {T1, T2, …, Tn}, the set consisting of all the threads in the system. R = {R1, R2, …, Rm}, the set consisting of all resource types in the system  request edge – directed edge Ti Rj  assignment edge – directed edge Rj Ti 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 thread requests a resource, it does not hold any other resources Require threads to request and be allocated all its resources before it begins execution or allow thread to request resources only when the thread 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 thread is waiting Thread 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 thread requests resources in an increasing order of enumeration Operating System Concepts – 10th Edition 8.14 Silberschatz, Galvin and Gagne ©2018  Circular Wait  Invalidating the circular wait condition is most common.  Simply assign each resource (i.e., mutex locks) a unique number.  Resources must be acquired in order.  If: first_mutex = 1 second_mutex = 5 code for thread_two could not be written as follows: Operating System Concepts – 10th Edition 8.15 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 thread 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 8.16 Silberschatz, Galvin and Gagne ©2018  Safe State  When a thread 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 threads in the systems such that for each Ti, the resources that Ti can still request can be satisfied by currently available resources plus resources held by all the Tj, with j < i  That is: If the resources that Ti needs are not immediately available, then Ti can wait until all Tj have finished When Tj is finished, Ti can obtain needed resources, execute, return allocated resources, and terminate When Ti terminates, Ti +1 can obtain its needed resources, and so on Operating System Concepts – 10th Edition 8.17 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 8.18 Silberschatz, Galvin and Gagne ©2018  Safe, Unsafe, Deadlock State Operating System Concepts – 10th Edition 8.19 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 8.20 Silberschatz, Galvin and Gagne ©2018  Resource-Allocation Graph Scheme  Claim edge Ti Rj indicated that process Tj may request resource Rj; represented by a dashed line  Claim edge converts to request edge when a thread requests a resource  Request edge converted to an assignment edge when the resource is allocated to the thread  When a resource is released by a thread, assignment edge reconverts to a claim edge  Resources must be claimed a priori in the system Operating System Concepts – 10th Edition 8.21 Silberschatz, Galvin and Gagne ©2018  Resource-Allocation Graph Operating System Concepts – 10th Edition 8.22 Silberschatz, Galvin and Gagne ©2018  Unsafe State In Resource-Allocation Graph Operating System Concepts – 10th Edition 8.23 Silberschatz, Galvin and Gagne ©2018  Resource-Allocation Graph Algorithm  Suppose that thread Ti 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 8.24 Silberschatz, Galvin and Gagne ©2018  Banker’s Algorithm  Multiple instances of resources  Each thread must a priori claim maximum use  When a thread requests a resource, it may have to wait  When a thread gets all its resources it must return them in a finite amount of time Operating System Concepts – 10th Edition 8.25 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 Ti may request at most k instances of resource type Rj  Allocation: n x m matrix. If Allocation[i,j] = k then Ti is currently allocated k instances of Rj  Need: n x m matrix. If Need[i,j] = k, then Ti 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 8.26 Silberschatz, Galvin and Gagne ©2018  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- 1 2. Find an i such that both: (a) Finish [i] = false (b) Needi Work If no such i exists, go to step 4 3. Work = Work + Allocationi Finish[i] = true go to step 2 4. If Finish [i] == true for all i, then the system is in a safe state Operating System Concepts – 10th Edition 8.27 Silberschatz, Galvin and Gagne ©2018  Resource-Request Algorithm for Process Pi Requesti = request vector for process Ti. If Requesti [j] = k then process Ti wants k instances of resource type Rj 1. If Requesti Needi go to step 2. Otherwise, raise error condition, since process has exceeded its maximum claim 2. If Requesti Available, go to step 3. Otherwise Ti must wait, since resources are not available 3. Pretend to allocate requested resources to Ti by modifying the state as follows: Available = Available – Requesti; Allocationi = Allocationi + Requesti; Needi = Needi – Requesti; If safe the resources are allocated to Ti If unsafe Ti must wait, and the old resource-allocation state is restored Operating System Concepts – 10th Edition 8.28 Silberschatz, Galvin and Gagne ©2018  Example of Banker’s Algorithm  5 threads T0 through T4; 3 resource types: A (10 instances), B (5instances), and C (7 instances)  Snapshot at time T0: Allocation Max Available ABC ABC ABC T0 0 1 0 753 332 T1 2 0 0 322 T2 3 0 2 902 T3 2 1 1 222 T4 0 0 2 433 Operating System Concepts – 10th Edition 8.29 Silberschatz, Galvin and Gagne ©2018  Example (Cont.)  The content of the matrix Need is defined to be Max – Allocation Need ABC T0 7 4 3 T1 1 2 2 T2 6 0 0 T3 0 1 1 T4 4 3 1  The system is in a safe state since the sequence < T1, T3, T4, T2, T0> satisfies safety criteria Operating System Concepts – 10th Edition 8.30 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 T0 010 743 230 T1 302 020 T2 302 600 T3 211 011 T4 002 431  Executing safety algorithm shows that sequence < T1, T3, T4, T0, T2> satisfies safety requirement  Can request for (3,3,0) by T4 be granted?  Can request for (0,2,0) by T0 be granted? Operating System Concepts – 10th Edition 8.31 Silberschatz, Galvin and Gagne ©2018  Deadlock Detection  Allow system to enter deadlock state  Detection algorithm  Recovery scheme Operating System Concepts – 10th Edition 8.32 Silberschatz, Galvin and Gagne ©2018  Single Instance of Each Resource Type  Maintain wait-for graph Nodes are threads Ti Tj if Ti is waiting for Tj  Periodically invoke an algorithm that searches for a cycle in the graph. If there is a cycle, there exists a deadlock  An algorithm to detect a cycle in a graph requires an order of n2 operations, where n is the number of vertices in the graph Operating System Concepts – 10th Edition 8.33 Silberschatz, Galvin and Gagne ©2018  Resource-Allocation Graph and Wait-for Graph Resource-Allocation Graph Corresponding wait-for graph Operating System Concepts – 10th Edition 8.34 Silberschatz, Galvin and Gagne ©2018  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 thread.  Request: An n x m matrix indicates the current request of each thread. If Request [i][j] = k, then thread Ti is requesting k more instances of resource type Rj. Operating System Concepts – 10th Edition 8.35 Silberschatz, Galvin and Gagne ©2018  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 Allocationi 0, then Finish[i] = false; otherwise, Finish[i] = true 2. Find an index i such that both: a) Finish[i] == false b) Requesti Work If no such i exists, go to step 4 Operating System Concepts – 10th Edition 8.36 Silberschatz, Galvin and Gagne ©2018  Detection Algorithm (Cont.) 3. Work = Work + Allocationi 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 Ti is deadlocked Algorithm requires an order of O(m x n2) operations to detect whether the system is in deadlocked state Operating System Concepts – 10th Edition 8.37 Silberschatz, Galvin and Gagne ©2018  Example of Detection Algorithm  Five threads T0 through T4; three resource types A (7 instances), B (2 instances), and C (6 instances)  Snapshot at time T0: Allocation Request Available ABC ABC ABC T0 010 000 000 T1 200 202 T2 303 000 T3 211 100 T4 002 002  Sequence will result in Finish[i] = true for all i Operating System Concepts – 10th Edition 8.38 Silberschatz, Galvin and Gagne ©2018  Example (Cont.)  T2 requests an additional instance of type C Request ABC T0 0 0 0 T1 2 0 2 T2 0 0 1 T3 1 0 0 T4 0 0 2  State of system? Can reclaim resources held by thread T0, but insufficient resources to fulfill other processes; requests Deadlock exists, consisting of processes T1, T2, T3, and T4 Operating System Concepts – 10th Edition 8.39 Silberschatz, Galvin and Gagne ©2018  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 threads “caused” the deadlock. Operating System Concepts – 10th Edition 8.40 Silberschatz, Galvin and Gagne ©2018  Recovery from Deadlock: Process Termination  Abort all deadlocked threads  Abort one process at a time until the deadlock cycle is eliminated  In which order should we choose to abort? 1. Priority of the thread 2. How long has the thread computed, and how much longer to completion 3. Resources that the thread has used 4. Resources that the thread needs to complete 5. How many threads will need to be terminated 6. Is the thread interactive or batch? Operating System Concepts – 10th Edition 8.41 Silberschatz, Galvin and Gagne ©2018  Recovery from Deadlock: Resource Preemption  Selecting a victim – minimize cost  Rollback – return to some safe state, restart the thread for that state  Starvation – same thread may always be picked as victim, include number of rollback in cost factor Operating System Concepts – 10th Edition 8.42 Silberschatz, Galvin and Gagne ©2018  End of Chapter 8 Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018