chapter 8 deadlocks page 2 chapter 8: deadlocks system model deadlock characterization methods for...

Chapter 8 Deadlocks

Chapter 8: Deadlocks

System Model

Deadlock Characterization

Methods for Handling Deadlocks

Deadlock Prevention

Deadlock Avoidance

Deadlock Detection

Recovery from Deadlock

System Model

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

§8.1

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.


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, …, P0} 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.

Deadlock can arise if four conditions hold simultaneously.

§8.2


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, …, P0} 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.

Deadlock can arise if four conditions hold simultaneously.

§8.2

Multiple Choices Question:( ) If the processes in the system can be

preempted, no deadlock can happen.

Answer: ○

Resource-Allocation Graph

V is partitioned into two types of nodes: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

A set of vertices V and a set of edges E.

§8.2.2

Resource-Allocation Graph

Process

Resource Type with 4 instances

Pi requests instance of Rj

Pi is holding an instance of Rj

A request edge points to only the square Rj, whereas an assignment edge must also designate one of the dots in the square.

Pi

Pi

Rj

Rj

Resource Allocation Graph

Example:


Deadlocked:


With Cycle butNo Deadlock:


With Cycle butNo Deadlock:

Short Answer Question:Please add two edges to the resource allocation graph show below to make it deadlocked.

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.

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.

True-False Question:( ) In order to prevent deadlock, we

must ensure that no cycle can happen in the resource allocation graph associated with this system.

Answer: ×


Use a protocol to ensure that the system will never enter a deadlock state.

§8.3

Allow the system to enter a deadlock state and then recover. The system can provide an algorithm that examines the state of the system to determine whether a deadlock has occurred, and an algorithm to recover from the deadlock.

Deadlock Prevention: (§8.4) a set of methods for ensuring that at least one of

the necessary conditions cannot hold.

Deadlock avoidance: (§8.5) give OS information about processes and resources for the decision of satisfying or

delaying the requests.Deadlock Detection: (§8.5)Deadlock Recovery: (§8.7)

Ignore the problem and pretend that deadlocks never occur in the system.



§8.3

Allow the system to enter a deadlock state and then recover. The system can provide an algorithm that examines the state of the system to determine whether a deadlock has occurred, and an algorithm to recover from the deadlock.Ignore the problem and pretend that deadlocks never occur in the system.

True-False Question:( ) If a system has deadlock

prevention scheme available, no deadlock detection or avoidance are required.

Answer: ○



§8.3

Allow the system to enter a deadlock state and then recover. The system can provide an algorithm that examines the state of the system to determine whether a deadlock has occurred, and an algorithm to recover from the deadlock.Ignore the problem and pretend that deadlocks never occur in the system.

True-False Question:( ) If a system has deadlock

prevention scheme available, no deadlock detection or avoidance are required.

True-False Question:( ) Usually deadlock recovery

method should be invoked after deadlock detection.

Answer: ○


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.


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.

True-False Question:( ) Although the operating systems

today are very sophisticated, most of them has no deadlock manipulation schemes available.

Answer: ○

Deadlock Prevention

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)

§8.4

Deadlock Prevention

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)

§8.4

True-False Question:( ) Mutual exclusion is not required

for nonsharable resources in a multi-threaded environment.

Answer: ×

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).

Deadlock Prevention


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 methods or blocks).

THINKTHINK

Deadlock Prevention


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.

Multiple Choices Question:( ) Breaking the “Hold and wait”

condition in a multi-threaded system may cause(a) deadlock (b) mutual exclusive (c) starvation (d) implementation problem

Answer: c

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.

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={R1,R2, …, Rm} be the set of resource types.F:R→NF(type drive) =1F(disk drive) = 5F(printer) = 12A process owning resource type Ri can continue to request type Rj iff F(Rj)>F(Ri)orWhenever a process requests an instance of resource type Rj, it has released any resources Ri such that F(Ri) >=F(Rj).

Should be defined according to the normal order of usage of

the resources in a system.

Proof by contradiction

Assume circular wait exists in {P0, P1, ..., Pn}, where Pi is waiting for a resource Ri, which is held by process Pi+1.

F(Ri) < F(Ri+1)=>F(R0) < F(R1) < ... <F(Rn) <F(R0)=> F(R0) < F(R0)

Impossible

Deadlock Avoidance

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.

Requires that the system has some additional a priori information available.

§8.5

Safe State

A state is safe if the system can allocate resources to each process in some order and still avoid a deadlock.

Safe sequence <P1, P2, ..., 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.

§8.5.1

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.

Safe State

Example: 12 tape drives, 3 processes. max needs current holdingP0 10 5P1 4 2P2 9 2

Safe for <P1,P0,P2>

If P2 request one more, system go into unsafe state.

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.

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.

True-False Question:( ) It is not necessary that a system

will become deadlocked even when the system reaching a unsafe state.

Answer: ○

Resource-Allocation Graph Algorithm

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.

P1 R1

§8.5.2

Safe StateA request can be granted only if converting the request edge to assignment edge does not result in the formation of a cycle in the resource-allocation graph.

If no cycle exists, then the allocation of the resource will leave the system in a safe state. Otherwise, process have to wait.

Banker’s Algorithm (From SHAY)

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.

§8.5.3

• Safe• Process 1,2, or 4 could request its maximum

number of pages and finish• For example: process 1 finish

Current Number of pages allocated

Maximum Number of pages needed

Process 1 2 4 Process 2 3 6Process 3 1 6Process 4 4 8

14 pages in the system4 pages are unallocated

Example 1

• Now any process could request pages up to its maximum and finish

• Suppose process 3 finish





Example 1

• Any one can finish.• However, a sequence of requests that allowed

all processes to finish does not indicate a safe state.





Example 1

• 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.



Process 1 2 4 Process 2 3 6Process 3 1 5 6Process 4 4 8


Example 1





Example 2

If each process asserts its claim, deadlock will occur

Unsafe !









Example 2

Banker’s Algorithm

Safe state. <P1, P3, P4, P2, P0>

If request1=(1,0,2)

§8.5.3

P0 0 1 0 7 5 3 3 3 2P1 2 0 0 3 2 2 P2 3 0 2 9 0 2 P3 2 1 1 2 2 2P4 0 0 2 4 3 3

Allocation Max Available A B C A B C A B C

P0 7 4 3P1 1 2 2P2 6 0 0P3 0 1 1P4 4 3 1

Need A B C



If request1=(1,0,2)

Still safe with < P1, P3, P4, P0, P2 >

§8.5.3

P0 0 1 0 7 5 3 2 3 0P1 3 0 2 3 2 2 P2 3 0 2 9 0 2 P3 2 1 1 2 2 2P4 0 0 2 4 3 3


P0 7 4 3P1 0 2 0P2 6 0 0P3 0 1 1P4 4 3 1

Need A B C



Request4=(3,3,0)

Cannot be granted. No resources.

§8.5.3

P0 0 1 0 7 5 3 2 3 0P1 3 0 2 3 2 2 P2 3 0 2 9 0 2 P3 2 1 1 2 2 2P4 0 0 2 4 3 3


P0 7 4 3P1 0 2 0P2 6 0 0P3 0 1 1P4 4 3 1

Need A B C



Request0 =(0,2,0)

Will result in unsafe state.

§8.5.3

P0 0 1 0 7 5 3 2 3 0P1 3 0 2 3 2 2 P2 3 0 2 9 0 2 P3 2 1 1 2 2 2P4 0 0 2 4 3 3


P0 7 4 3P1 0 2 0P2 6 0 0P3 0 1 1P4 4 3 1

Need A B C

Deadlock Detection

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 includesmaintaining necessary information

executing the detection algorithm

potential losses when recovering

§8.6

Deadlock Detection

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 includesmaintaining necessary information

executing the detection algorithm

potential losses when recovering

§8.6

True-False Question:( ) Deadlock prevention is not

required for a system that equipped with proper deadlock detection scheme.

Answer: ×

Single Instance of Each Resource Type

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(n2) operations, where n is the number of vertices in the graph.

§8.6.1

Resource-Allocation Graph And Wait-for Graph

Resource-Allocation Graph Corresponding wait-for graph

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


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

True-False Question:( ) Deadlock detection algorithm

should be invoked only when the system is deadlocked. It therefore examines the system allocation situation and prepare for a roll back.

Answer: ×


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.

Several Instances of a Resource Type

Not deadlocked. <P0, P2, P3, P1, P4>

If P2 make request (0, 0, 1), Deadlocked between P1, P2, P3, P4.

§8.6.2

P0 0 1 0 0 0 0 0 0 0P1 2 0 0 2 0 2 P2 3 0 3 0 0 0 P3 2 1 1 1 0 0P4 0 0 2 0 0 2

Allocation Request Available A B C A B C A B C

Several Instances of a Resource Type

Not deadlocked. <P0, P2, P3, P1, P4>

If P2 make request (0, 0, 1), Deadlocked between P1, P2, P3, P4.

§8.6.2

P0 0 1 0 0 0 0 0 0 0P1 2 0 0 2 0 2 P2 3 0 3 0 0 1 P3 2 1 1 1 0 0P4 0 0 2 0 0 2

Allocation Request Available A B C A B C A B C

Recovery from Deadlock: Process Termination

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 deadlock-detection algorithm after each process is aborted.

§8.7

Recovery from Deadlock: Process TerminationWe 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?

Recovery from Deadlock: Resource PreemptionSuccessively 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.

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