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ch8.pptx
1.
Silberschatz, Galvin and
Gagne ©2018 Operating System Concepts – 10th Edition Chapter 8: Deadlocks
2.
8.2 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Outline System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock
3.
8.3 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
4.
8.4 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
5.
8.5 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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)
6.
8.6 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Deadlock Characterization 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. Deadlock can arise if four conditions hold simultaneously.
7.
8.7 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Resource-Allocation Graph 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 A set of vertices V and a set of edges E.
8.
8.8 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
9.
8.9 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Resource Allocation Graph with a Deadlock
10.
8.10 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Graph with a Cycle But no Deadlock
11.
8.11 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
12.
8.12 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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.
13.
8.13 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Deadlock Prevention 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 Invalidate one of the four necessary conditions for deadlock:
14.
8.14 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
15.
8.15 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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:
16.
8.16 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Deadlock Avoidance 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 Requires that the system has some additional a priori information available
17.
8.17 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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 <T1, T2, …, Tn> 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 + resources held by all the Tj, with j < I That is: • If Ti resource 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
18.
8.18 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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.
19.
8.19 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Safe, Unsafe, Deadlock State
20.
8.20 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Avoidance Algorithms Single instance of a resource type • Use a resource-allocation graph Multiple instances of a resource type • Use the Banker’s Algorithm
21.
8.21 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
22.
8.22 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Resource-Allocation Graph
23.
8.23 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Unsafe State In Resource-Allocation Graph
24.
8.24 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
25.
8.25 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
26.
8.26 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Data Structures for the Banker’s Algorithm 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] Let n = number of processes, and m = number of resources types.
27.
8.27 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
28.
8.28 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
29.
8.29 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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 A B C A B C A B C T0 0 1 0 7 5 3 3 3 2 T1 2 0 0 3 2 2 T2 3 0 2 9 0 2 T3 2 1 1 2 2 2 T4 0 0 2 4 3 3
30.
8.30 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Example (Cont.) The content of the matrix Need is defined to be Max – Allocation Need A B C 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
31.
8.31 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Example: P1 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 T0 0 1 0 7 4 3 2 3 0 T1 3 0 2 0 2 0 T2 3 0 2 6 0 0 T3 2 1 1 0 1 1 T4 0 0 2 4 3 1 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?
32.
8.32 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Deadlock Detection Allow system to enter deadlock state Detection algorithm Recovery scheme
33.
8.33 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
34.
8.34 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Resource-Allocation Graph and Wait-for Graph Resource-Allocation Graph Corresponding wait-for graph
35.
8.35 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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.
36.
8.36 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
37.
8.37 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
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8.38 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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 A B C A B C A B C T0 0 1 0 0 0 0 0 0 0 T1 2 0 0 2 0 2 T2 3 0 3 0 0 0 T3 2 1 1 1 0 0 T4 0 0 2 0 0 2 Sequence <T0, T2, T3, T1, T4> will result in Finish[i] = true for all i
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8.39 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition Example (Cont.) T2 requests an additional instance of type C Request A B C 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
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8.40 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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.
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8.41 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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?
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8.42 Silberschatz, Galvin
and Gagne ©2018 Operating System Concepts – 10th Edition 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
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Silberschatz, Galvin and
Gagne ©2018 Operating System Concepts – 10th Edition End of Chapter 8
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