Chapter 8 Deadlocks Chapter 8 Deadlocks System Model

Chapter 8 Deadlocks

Chapter 8: Deadlocks System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock 2

System Model § 8. 1 A system consists of a finite number of resources to be distributed among competing processes. Resources are partitioned into several types, each of which consists of multiple identical instances. CPU cycles, memory space, files, I/O devices, object locks 3

System Model Each process utilizes a resource as following sequence: request • If the request cannot be granted immediately, then the requesting process must wait until it can acquire the resource. use • The process can operate on the resource. release • The process releases the resource. 4

Deadlock Characterization § 8. 2 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 0} 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, …, Pn– 1 is waiting for a resource that is held by Pn, and Pn is waiting for a resource that is held by P 0. 5

Deadlock Characterization § 8. 2 Deadlock can arise if four conditions hold simultaneously. Mutual exclusion: only one process at a time can use a Multiple Choices Question: resource. (Hold)and If the processes the system canresource be wait: a process in holding at least one is waiting to acquire additional resources by other preempted, no deadlock can held happen. processes. No preemption: a resource can be released only Answer: by ○ the process holding it, after that process voluntarily has completed its task. Circular wait: there exists a set {P 0, P 1, …, P 0} 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, …, Pn– 1 is waiting for a resource that is held by Pn, and Pn is waiting for a resource that is held by P 0. 6

Resource-Allocation Graph § 8. 2. 2 A set of vertices V and a set of edges E. V is partitioned into two types of nodes: P = {P 1, P 2, …, Pn}, the set consisting of all the processes in the system. R = {R 1, R 2, …, Rm}, the set consisting of all resource types in the system. Request edge – directed edge Pi Rj Assignment edge – directed edge Rj Pi 7

Resource-Allocation Graph Process Resource Type with 4 instances P Pi requests instance of Rj Pi is holding an instance of Rj i Rj Pi A request edge points to only the square Rj, whereas an assignment edge must also designate one of the dots in the square. Rj 8

Resource Allocation Graph Example: 9

Resource Allocation Graph Deadlocked: 10

Resource Allocation Graph With Cycle but No Deadlock: 11

Resource Allocation Graph With Cycle but No Deadlock: Short Answer Question: Please add two edges to the resource allocation graph show below to make it deadlocked. 12

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. 13

Basic Facts If. True-False graph contains Question: no cycles no deadlock. ( ) In order to prevent deadlock, we must contains ensure that a nocycle can If graph happen in the resource allocation if only one instance per resource type, graph associated with this system. then deadlock. Answer: × if several instances per resource type, possibility of deadlock. 14

Methods for Handling Deadlocks § 8. 3 Use a protocol to ensure that the system will never enter a deadlock state. Allow the system to enter a deadlock state and Deadlock Prevention: (§ 8. 4) Deadlock avoidance: (§ 8. 5)an then recover. The system can provide a set of methods for. OS ensuring information algorithm that give examines the stateabout of the system to determine whether a deadlock has occurred, and that at least one of the processes and resources for annecessary algorithm to from the deadlock. conditions cannot therecover decision of satisfying or Ignore the problem and(§ 8. 5) pretend that deadlocks hold. delaying the requests. Deadlock Detection: never occur in the system. Recovery: (§ 8. 7) Deadlock 15

Methods for Handling Deadlocks § 8. 3 True-False Question: Use a protocol to ensure that the system will ( ) If a system has deadlock never enter a deadlock state. available, Allowprevention the system scheme to enter a deadlockno state and then recover. system or canavoidance provide an deadlock. The detection algorithm that examines the state of the system to are required. determine whether a deadlock has occurred, and Answer: ○ to recover from the deadlock. an algorithm Ignore the problem and pretend that deadlocks never occur in the system. 16

Methods for Handling Deadlocks § 8. 3 True-False Question: Use a protocol to ensure that the system will ( ( ) )If a system has deadlock never enter a deadlock state. Usually deadlock recovery scheme available, no Allowprevention the system to enter a deadlock state and method should be invoked after then recover. The system or canavoidance provide an deadlock detection. algorithm that examines the state of the system to are required. determine whether a deadlock has occurred, and an. Answer: algorithm ○ to recover from the deadlock. Ignore the problem and pretend that deadlocks never occur in the system. 17

Methods for Handling Deadlocks If no prevention, no avoidance, no detection and no recovery schemes, then the system may reach a deadlock state and eventually need to be restarted manually. Although does not seem to be viable, it is used in most OS, including UNIX. In many systems, deadlocks occur infrequently; thus it is cheaper to use this method. JVM does nothing to manage deadlocks, it is up to the application developer to write programs that are deadlock-free. 18

Methods for Handling Deadlocks Question: If. True-False no prevention, no avoidance, no detection and no schemes, then the systems system may reach ( recovery ) Although the operating a deadlock and sophisticated, eventually need to be today state are very most restarted manually. of them has no deadlock Although does not seem to beavailable. viable, it is used in manipulation schemes most OS, including UNIX. ○ In Answer: many systems, deadlocks occur infrequently; thus it is cheaper to use this method. JVM does nothing to manage deadlocks, it is up to the application developer to write programs that are deadlock-free. 19

Deadlock Prevention § 8. 4 By ensuring that at least one of the conditions (§ 8. 2) cannot hold, we can prevent the occurrence of a deadlock. 1. Mutual Exclusion – not required for sharable resources, but must hold for nonsharable resources (printers, synchronized method) 20

Deadlock Prevention § 8. 4 By ensuring that at least one of the True-False Question: conditions (§ 8. 2) cannot hold, we can ( ) Mutual exclusion is not required prevent the occurrence of a deadlock. for nonsharable resources in a 1. Mutual multi-threaded Exclusion – not required for environment. sharable resources, but must hold for nonsharable resources (printers, Answer: × synchronized method) 21

Deadlock Prevention 2. 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… release all the resources that is is currently allocated before requesting additional resources. Low resource utilization; starvation possible. Impractical in Java (difficult to implement these protocols with synchronized methods or blocks). 22

Deadlock Prevention 2. 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… release all the resources that is is currently allocated before requesting additional resources. Low resource utilization; starvation possible. Impractical in Java (difficult to implement these protocols with synchronized methods or blocks). 23

Deadlock Prevention 2. 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… release all the resources that is is currently allocated before requesting additional resources. Low resource utilization; starvation possible. Impractical in Java (difficult to implement these protocols with synchronized methods or blocks). 24

Deadlock Prevention 2. 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… release all the resources that is currently allocated before requesting additional resources. Low resource utilization; starvation possible. Impractical in Java (difficult to implement these protocols with synchronized THINK methods or blocks). 25

Deadlock Prevention 2. Hold and Wait – must guarantee that whenever process. Question: requests a Multiplea. Choices resource, it doesthe not“Hold hold any other ( ) Breaking and wait” resources. condition in a multi-threaded Require process request and be system maytocause allocated all its resources beforeexclusive it begins (a) deadlock (b) mutual execution, or allow process to request (c) starvation (d) implementation resources only when the process has problem none… release all the resources that is currently before requesting Answer: allocated c additional resources. Low resource utilization; starvation possible. 26

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. Often applied to resources whose state can be easily saved and restored later, such as CPU registers and memory space. It cannot be applied to resources such as printers and tape drives. 27

Deadlock Prevention (Cont. ) Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration. Let R={R 1, R 2, …, Rm} be the set of resource types. F: R→N Should be defined according to F(type drive) =1 the normal order of usage of the F(disk drive) = 5 resources in a system. F(printer) = 12 A process owning resource type Ri can continue to request type Rj iff F(Rj)>F(Ri) or Whenever a process requests an instance of resource type Rj, it has released any resources Ri such that F(Ri) >=F(Rj). 28

Proof by contradiction Assume circular wait exists in {P 0, P 1, . . . , Pn}, where Pi is waiting for a resource Ri, which is held by process Pi+1. F(Ri) < F(Ri+1) => F(R 0) < F(R 1) <. . . <F(Rn) <F(R 0) => F(R 0) < F(R 0) Impossible 29

Deadlock Avoidance § 8. 5 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. 30

Safe State § 8. 5. 1 A state is safe if the system can allocate resources to each process in some order and still avoid a deadlock. Safe sequence <P 1, P 2, . . . , Pn> : if, for each Pi, the resources that Pi can still request can be safisfied by the currently available resources plus the resources held by all the Pj with j < i. If no such sequence exists, then the system is said to be unsafe. 31

Safe State A safe state is not a deadlock state. A deadlock state is an unsafe state. Not all unsafe states are deadlocks. An unsafe state may lead to a deadlock. In unsafe state, OS cannot prevent processes from requesting resources and cause deadlock: the behavior of the processes controls unsafe state. 32

Safe State Example: 12 tape drives, 3 processes. max needs current holding P 0 10 5 P 1 4 2 P 2 9 2 Safe for <P 1, P 0, P 2> If P 2 request one more, system go into unsafe state. 33

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. 34

Basic Facts If a system is in safe state no. Question: deadlocks. True-False ) It is notisnecessary that a system If( a system in willstate become unsafe deadlocked even when the system reaching a possibility of deadlock. unsafe state. Avoidance ensure that a system will never enter an unsafe state. Answer: ○ 35

Resource-Allocation Graph Algorithm § 8. 5. 2 Claim edge Pi Rj indicated that process Pi may request resource Rj at some time in the future. It is represented by a dashed line. Claim edge converts to request edge when a process requests a resource. When the resource is allocated to the process, assignment edge is added. When a resource is released by a process, assignment edge reconverts to a claim edge. P 1 R 1 36

Safe State A request can be granted only if converting the request edge to assignment edge does not result in the formation of a cycle in the resourceallocation graph. If no cycle exists, then the allocation of the resource will leave the system in a safe state. Otherwise, process have to wait. 37

Banker’s Algorithm (From SHAY) § 8. 5. 3 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. 38

Example 1 Maximum Number Current Number of pages allocated of pages needed Process 1 Process 2 Process 3 Process 4 2 3 1 4 4 6 6 8 14 pages in the system 4 pages are unallocated • Safe • Process 1, 2, or 4 could request its maximum number of pages and finish • For example: process 1 finish 39

Example 1 Maximum Number Current Number of pages allocated of pages needed Process 1 Process 2 Process 3 Process 4 2 3 1 4 4 6 6 8 14 pages in the system 6 pages are unallocated • Now any process could request pages up to its maximum and finish • Suppose process 3 finish 40

Example 1 Maximum Number Current Number of pages allocated of pages needed Process 1 Process 2 Process 3 Process 4 2 3 1 4 4 6 6 8 14 pages in the system 7 pages are unallocated • Any one can finish. • However, a sequence of requests that allowed all processes to finish does not indicate a safe state. 41

Example 1 Maximum Number Current Number of pages allocated of pages needed Process 1 Process 2 Process 3 Process 4 2 3 1 4 5 4 6 6 8 14 pages in the system 0 pages are unallocated • If process 3 requests the 4 available pages, and each process subsequently requests 1 more page, the system is deadlocked • However, it doesn't necessarily occur. . . A unsafe state doesn't mean that the deadlock will occur inevitably. 42

Current Number of pages allocated Process 1 Process 2 Process 3 Process 4 1 2 2 1 Maximum Number of pages needed 2 4 7 6 Example 2 7 pages in the system 1 pages are unallocated 43

Current Number of pages allocated Maximum Number of pages needed Process 1 Process 2 Process 3 Process 4 1 2 2 1 2 4 7 6 Example 2 7 pages in the system 1 pages are unallocated 7 pages in the system 2 pages are unallocated 44

Current Number of pages allocated Maximum Number of pages needed Process 1 Process 2 Process 3 Process 4 1 2 2 1 2 4 7 6 Example 2 7 pages in the system 1 pages are unallocated 7 pages in the system 2 pages are unallocated 7 pages in the system 4 pages are unallocated If each process asserts its claim, deadlock will occur Unsafe ! 45

Banker’s Algorithm Allocation ABC P 0 P 1 P 2 P 3 P 4 0 2 3 2 0 1 0 0 0 2 1 2 Max ABC 7 3 9 2 4 5 2 0 2 3 3 2 2 2 3 § 8. 5. 3 Need ABC Available ABC 3 3 2 P 0 P 1 P 2 P 3 P 4 7 1 6 0 4 4 2 0 1 3 3 2 0 1 1 Safe state. <P 1, P 3, P 4, P 2, P 0> If request 1=(1, 0, 2) 46

Banker’s Algorithm Allocation ABC P 0 P 1 P 2 P 3 P 4 0 3 3 2 0 1 0 0 2 2 1 2 Max ABC 7 3 9 2 4 5 2 0 2 3 3 2 2 2 3 § 8. 5. 3 Need ABC Available ABC 2 3 0 P 1 P 2 P 3 P 4 7 0 6 0 4 4 2 0 1 3 3 0 0 1 1 Safe state. <P 1, P 3, P 4, P 2, P 0> If request 1=(1, 0, 2) Still safe with < P 1, P 3, P 4, P 0, P 2 > 47

Banker’s Algorithm Allocation ABC P 0 P 1 P 2 P 3 P 4 0 3 3 2 0 1 0 0 2 2 1 2 Max ABC 7 3 9 2 4 5 2 0 2 3 3 2 2 2 3 § 8. 5. 3 Need ABC Available ABC 2 3 0 P 1 P 2 P 3 P 4 7 0 6 0 4 4 2 0 1 3 3 0 0 1 1 Safe state. <P 1, P 3, P 4, P 2, P 0> Request 4=(3, 3, 0) Cannot be granted. No resources. 48

Banker’s Algorithm Allocation ABC P 0 P 1 P 2 P 3 P 4 0 3 3 2 0 1 0 0 2 2 1 2 Max ABC 7 3 9 2 4 5 2 0 2 3 3 2 2 2 3 § 8. 5. 3 Need ABC Available ABC 2 3 0 P 1 P 2 P 3 P 4 7 0 6 0 4 4 2 0 1 3 3 0 0 1 1 Safe state. <P 1, P 3, P 4, P 2, P 0> Request 0 =(0, 2, 0) Will result in unsafe state. 49

Deadlock Detection § 8. 6 Allow system to enter deadlock state System should provide: An algorithm that examines the state of the system to determine whether a deadlock has occurred. An algorithm to recover from the deadlock. It requires overhead that includes maintaining necessary information executing the detection algorithm potential losses when recovering 50

Deadlock Detection § 8. 6 Allow system to enter deadlock state True-False Question: System should provide: ( An) Deadlock prevention is the not state of the algorithm that examines system to determine whether required for a system that a deadlock has occurred. equipped with proper deadlock Andetection algorithmscheme. to recover from the deadlock. It requires overhead that includes maintaining necessary information executing Answer: × the detection algorithm potential losses when recovering 51

Single Instance of Each Resource Type § 8. 6. 1 Maintain wait-for graph: obtained from the resource-allocation graph by removing the nodes of type resource and collapsing the appropriate edges. Nodes are processes. Pi Pj if Pi is waiting for Pj. to release a resource that Pi needs. Needs to maintain the wait-for graph and 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 O(n 2) operations, where n is the number of vertices in the graph. 52

Resource-Allocation Graph And Wait-for Graph Resource-Allocation Graph Corresponding wait-for graph 53

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 deadlocks occur frequently, then the detection algorithm should be invoked frequently. In the extreme, we could invoke the deadlock-detection algorithm every time a request for allocation cannot be granted immediately. Expensive 54

Detection-Algorithm Usage When, and how often, to invoke depends on: True-False Question: often a deadlock likely to occur? ( How ) Deadlock detectionis algorithm How many processes will need to be rolled should be invoked only when the back? is deadlocked. • system one for each disjoint cycle It therefore examinesoccur the system allocation If deadlocks frequently, then the situation and prepare for be a roll detection algorithm should invoked back. frequently. In. Answer: the extreme, we could invoke the × deadlock-detection algorithm every time a request for allocation cannot be granted immediately. Expensive 55

Detection-Algorithm Usage Less expensive alternative: invoke the algorithm at less frequent intervals. 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. 56

Several Instances of a Resource § 8. 6. 2 Type Allocation ABC P 0 P 1 P 2 P 3 P 4 0 2 3 2 0 1 0 0 0 3 1 2 Request ABC 0 2 0 1 0 0 0 0 2 Available ABC 0 0 0 Not deadlocked. <P 0, P 2, P 3, P 1, P 4> If P 2 make request (0, 0, 1), Deadlocked between P 1, P 2, P 3, P 4. 57

Several Instances of a Resource § 8. 6. 2 Type Allocation ABC P 0 P 1 P 2 P 3 P 4 0 2 3 2 0 1 0 0 0 3 1 2 Request ABC 0 2 0 1 0 0 0 0 2 1 0 2 Available ABC 0 0 0 Not deadlocked. <P 0, P 2, P 3, P 1, P 4> If P 2 make request (0, 0, 1), Deadlocked between P 1, P 2, P 3, P 4. 58

Recovery from Deadlock: Process Termination § 8. 7 Abort all deadlocked processes. Expensive to discard and re-compute partial computation. Abort one process at a time until the deadlock cycle is eliminated. overhead for invoking deadlockdetection algorithm after each process is aborted. 59

Recovery from Deadlock: Process Termination We should abort those processes that will incur the minimum cost when terminated. 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? 60

Recovery from Deadlock: Resource Preemption Successively preempt some resources from processes and give these resources to other processes until the deadlock cycle is broken. Three matters addressed: Selecting a victim – Which resources and which processes are to be preempted? We must determine the order to minimize cost. Factors include: • Number of resources a deadlock process is holding • the amount of time a deadlocked process has consumed Rollback – return to some safe state, restart process fro that state. Requires the system to keep more information about the state of all the running processes. Starvation – same process may always be picked as victim. Should include number of rollback in cost factor. 61