Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Silberschatz, Galvin and Gagne ©2007 Chapter 7: Deadlocks

7.2 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Chapter 7: Deadlocks  The Deadlock Problem  System Model  Deadlock Characterization  Methods for Handling Deadlocks  Deadlock Prevention  Deadlock Avoidance  Deadlock Detection  Recovery from Deadlock

7.3 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Chapter Objectives  To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks  To present a number of different methods for preventing or avoiding deadlocks in a computer system.

7.4 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 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 1 Procs P 1 and P 2 each hold one disk drive, and each needs the other drive.  Example 2 semaphores A and B, initialized to 1 P 0 P 1 A.wait ();B.wait(); B.wait ();A.wait();

7.5 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Java Deadlock Example Thread A Thread B

7.6 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Java Deadlock Example Deadlock is possible if: threadA -> lockY -> threadB -> lockX -> threadA java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/locks/ReentrantLo ck.html

7.7 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 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.

7.8 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 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

7.9 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Necessary Conditions for Deadlock  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 is a set {P 0, P 1, …, P 0 } of waiting processes such that P 0 is waiting for a resource held by P 1 ; P 1 is waiting for a resource held by P 2 ; …; P n–1 is waiting for a resource held by P n ; and P 0 is waiting for a resource held by P 0. Deadlock may only arise if four conditions hold:

7.10 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Resource-Allocation Graph  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 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 A set of vertices V and a set of edges E.

7.11 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Resource-Allocation Graph (Cont.)  Process  Resource Type with 4 instances  P i requests instance of R j  P i is holding an instance of R j PiPi PiPi RjRj RjRj

7.12 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Example of a Resource Allocation Graph

7.13 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Resource Allocation Graph With A Deadlock

7.14 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Graph With A Cycle But No Deadlock

7.15 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 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.

7.16 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Handling Deadlocks in Java plain, simple lock

7.17 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Handling Deadlocks in Java

7.18 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Methods for Handling Deadlocks  Ensure the system will never deadlock. #1 Prevention (break one condition for deadlock) #2 Avoidance (keep in safe state)  #3 Allow system to enter deadlock and then recover.  #4 Ignore the problem and pretend that deadlocks never occur; used by most operating systems, including UNIX.  For m resources and n procs, ##1, 3 take O(mn 2 ) time, #2 takes O(mn) time.

7.19 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Safe, Unsafe Deadlock State

Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Silberschatz, Galvin and Gagne ©2007 End of April 4, 2007 Lecture

7.21 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Suppose a bank has 5 customers  It has been loaning money. At this instant,  the amount loaned, the max need and remaining  available money are as follows (in billions of $$): AllocationMaxAvailable $ $ $ C C C C C 4 04 Question: Can the bank satisfy all the customers? How?

7.22 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Suppose a bank has 5 customers  It loans money in three currencies: $, £, and ¥  The amounts loaned, max need, and available currencies are as follows: AllocationMaxAvailable $ £ ¥ $ £ ¥ $ £ ¥ C C C C C Question: Can the bank satisfy all the customers? How?

7.23 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Deadlock Prevention  Mutual Exclusion – (assumed to be needed, as small as possible).  Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources. Possible solution: Aquire all resources all at once before proc begins execution. Problems: Low resource utilization; starvation possible. Restrain the ways request can be made.

7.24 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006  No Preemption – One possible solution: 1 If a proc that is holding some resources requests another that cannot be immediately allocated to it, then release all resources. 2 Requested resource is added to the list of resources for which the proc is waiting. 3 Proc will be restarted only when it can regain its old resources as well as the new ones that it needs.  Circular Wait – One possible solution: Impose a total ordering of all resource types; require that each proc request resources in increasing order. Deadlock Prevention (Cont.) Another possible solution: Dijkstra

7.25 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Edsgar Dijkstra’s Deadlock Avoidance  E.g. each proc declare the max 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.  Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the procs. Requires that the system has a priori information.

7.26 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Deadlock Avoidance: Safe State  When a proc 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 procs 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.

7.27 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Safe, Unsafe Deadlock State

7.28 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Deadlock Avoidance: 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.

7.29 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Avoidance algorithms  Single instance of a resource type. Use a resource-allocation graph  Multiple instances of a resource type. Use the banker’s algorithm.

7.30 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Resource-Allocation Graph Scheme  Claim edge P i  R j indicated 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.

7.31 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Resource-Allocation Graph

7.32 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Unsafe State In Resource-Allocation Graph

7.33 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Resource-Allocation Graph Algorithm  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

7.34 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Banker’s Algorithm  Multiple instances.  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.

7.35 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Data Structures for the Banker’s Algorithm  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]. Let n = number of processes, and m = number of resources types.

7.36 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 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 4. 3.Work = Work + Allocation i Finish[i] = true go to step 2. 4.If Finish [i] == true for all i, then the system is in a safe state.

7.37 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 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 ; l If safe  the resources are allocated to Pi. l If unsafe  Pi must wait, and the old resource- allocation state is restored

7.38 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Dijkstra’s 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 : AllocationMaxAvailable A B CA B C A B C P P P P P

7.39 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Dijkstra’s Banker’s Algorithm (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 satisfies safety criteria.

7.40 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Example: P 1 Request (1,0,2)  Check that Request  Available (that is, (1,0,2)  (3,3,2)  true. AllocationNeedAvailable A B CA B CA B C P P P P P  Executing safety algorithm shows that sequence 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?

7.41 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Deadlock Detection  Allow system to enter deadlock state  Detection algorithm  Recovery scheme

7.42 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 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. If there is a cycle, there exists a deadlock.  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.

7.43 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Resource-Allocation Graph and Wait-for Graph Resource-Allocation GraphCorresponding wait-for graph

7.44 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 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 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.

7.45 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 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 Allocation i  0, then Finish[i] = false;otherwise, 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.

7.46 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Detection Algorithm (Cont.) 3.Work = Work + Allocation i 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 P i is deadlocked. Algorithm requires an order of O(m x n 2) operations to detect whether the system is in deadlocked state.

7.47 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 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 : AllocationRequestAvailable A B C A B C A B C P P P P P  Sequence shows that this is a safe state [will result in Finish[i] = true for all i in the Banker’s algorithm].

7.48 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Detection Algorithm (Cont.)  But: P 2 requests an additional instance of type C: AllocationRequestAvailable A B C A B C A B C P P P P P  State of system? Can reclaim resources held by process P 0, but insufficient resources to fulfill other processes; requests. Deadlock! Procs P 1, P 2, P 3, and P 4.

7.49 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 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  There may be many cycles in the resource graph, so we don’t expect to be able to tell which of the many deadlocked procs “caused” the deadlock.

7.50 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 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? 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?

7.51 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Recovery from Deadlock: Resource Preemption  Selecting a victim – minimize some “cost.”  Rollback – return to some safe state, restart proc for that state.  Starvation – same process may always be picked as victim, so include number of rollbacks in the cost factor.

Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Silberschatz, Galvin and Gagne ©2007 End of Chapter 7