Ch 7 Deadlock

8/4/2019 Ch 7 Deadlock

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Chapter 7

Deadlock


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Deadlock

A waiting process is never again able to

change state, because the resources it has

requested are held by other waiting processes

called as deadlock


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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

System has 2 tape drives.

P1 and P2 each hold one tape drive and eachneeds another one.


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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.


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System Model

Resource typesR1,R2, . . .,Rm

CPU cycles, memory space, I/O devices

Each resource typeRi has Wi instances.

Each process utilizes a resource as follows:

request

userelease


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System Model

RequestIf the request cannot be granted

immediately, then requesting process must wait

until it can acquire the resource.UseThe process can operate on resource

Release- The process releases the resource


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Deadlock Characterization

Deadlock can arise if four conditions hold

Simultaneously

Mutual exclusion: only one process at a time

can use a resource. If another process requeststhat resource, the requesting process must bedelayed until the resource has been released.

Hold and wait: a process must be holding atleast one resource and waiting to acquireadditional resources held by other processes.


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Deadlock Characterization Cont.

No preemption: Resources cant be preempted,i.e. 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, , Pn1 is waiting for

a resource that is held by Pn, and P0 is waiting

for a resource that is held by P0.


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Resource-Allocation Graph

A set of vertices Vand a set of edgesE.

V is partitioned into two types:

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 edgedirected edge P1Rj

assignment edgedirected edgeRjPi


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Resource-Allocation Graph (Cont.)

Process

Resource Type with 4 instances

Pi requests instance ofRj

Pi is holding an instance ofRj

Pi

Pi


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Resource-Allocation Graph


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Graph with a deadlock


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Resource allocation graph with a cycle

but no deadlock


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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.


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Methods for Handling Deadlocks

Ensure that the system will neverenter a

deadlock state.

Allow the system to enter a deadlock state and

then recover.

To ensure that deadlock never occur, the

system can use either a deadlock prevention or

deadlock avoidance scheme.


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Deadlock Prevention

Mutual Exclusionnot required for sharable

resources; must hold for non-sharable resources.

Hold and Waitmust 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 processto request resources only when the process has none.

-Low resource utilization; starvation possible.


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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 canregain its old resources, as well as the new ones

that it is requesting.


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Deadlock Prevention (Cont.)Circular Waitimpose a total ordering of all

resource types, and require that each process

requests resources in an increasing order of

enumeration


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Deadlock Avoidance

Requires that the system has some additional a

prioriinformation available.Simplest and most useful model requires that each

process declare the maximum numberof resourcesof each type that it may need.

The deadlock-avoidance algorithm dynamicallyexamines the resource-allocation state to ensurethat there can never be a circular-wait condition.

Resource-allocation state is defined by thenumber of available and allocated resources, andthe maximum demands of the processes.


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Safe State

A state is safe if the system can allocateresources to each process in some order and

still avoid a deadlock.When a process requests an available resource,

system must decide if immediate allocationleaves the system in a safe state.

System is in safe state if there exists a safesequence of all processes.


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Safe State Cont.Sequence is safe if for each Pi, the

resources that Pi can still request can be satisfied bycurrently available resources + resources held by all

the Pj If Pi resource needs are not immediately available,

then Pi can wait until all Pjhave finished.

When Pj is finished, Pi can obtain needed

resources, execute, return allocated resources, andterminate.

When Pi terminates, Pi+1 can obtain its neededresources, and so on.


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Basic FactsA safe state is not a deadlocked state.

A deadlocked state is an unsafe state.

Not all unsafe states are deadlock, however anunsafe state may lead to a deadlock.

To avoid deadlock ensure that a system will

never enter an unsafe state.


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Safe, Unsafe , Deadlock State


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Example

Consider system with 12 magnetic tape drives

3 processes P0 , P1 , P2

Sequence P1 ,P0 ,P2 satisfy the safety condition

Processes Maximum

Needs

Allocated

P0 10 5

P1 4 2

P2 9 2


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Resource-Allocation Graph Algorithm

Claim edgePiRj indicated that process Pjmay request resourceRj; represented by a dashed

line.

Claim edge converts to request edge when aprocess requests a resource.

When a resource is released by a process,

assignment edge reconverts to a claim edge.

Resources must be claimed a priori in the

system.


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Resource-Allocation Graph For

Deadlock Avoidance


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Unsafe State In Resource-Allocation

Graph


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Bankers 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.


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Data Structures for the Bankers

Algorithm

Available- the number of available resources of

each type.

Max- maximum demand of each process

Allocation- The number of resources of each

type currently allocated to each process

Need- the remaining resource need of each

type

Safet Algorithm


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Safety Algorithm

n-No. of processes, m- no of resource type

1. Let Workand Finish be vectors of length m and n,respectively. Initialize:

Work=Available

Finish [i] = false for i= 0,1,3, , n.

2. Find an i such that both:(a) Finish [i]= =false

(b)NeediWork

If no such i exists, go to step 4.

3. Work= Work+AllocationiFinish[i] = truego to step 2.

4. IfFinish [i] == true for all i, then the system is in a


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Resource-Request Algorithm for Process Pi

1. If RequestiNeedigo to step 2. Otherwise, raise

error condition, since process has exceeded itsmaximum claim.

2. If RequestiAvailable, go to step 3. Otherwise Pimust wait, since resources are not available.

3. Pretend to allocate requested resources to Pi bymodifying the state as follows:

Available =Available =Requesti;

Allocationi=Allocationi +Requesti;

Needi= NeediRequesti;;If safe the resources are allocated to Pi.If unsafe Pi must wait, and the old resource-

allocation state is restored


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Example of Bankers Algorithm5 processes P0 through P4; 3 resource typesA

(10 instances),B (5instances, and C(7 instances).

Snapshot at time T0:

Allocation Max Available

A B C A B C A B C

P0 0 1 0 7 5 3 3 3 2

P1 2 0 0 3 2 2

P2 3 0 2 9 0 2

P3 2 1 1 2 2 2

P4 0 0 2 4 3 3

E l (C t )


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Example (Cont.)

The content of the matrix. Need is defined to be

MaxAllocation.Need

A B C

P0 7 4 3P1 1 2 2

P2 6 0 0

P3 0 1 1P4 4 3 1

The system is in a safe state since the sequence satisfies safety criteria.


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Deadlock Detection

If a system does not employ either a deadlock-

prevention or a deadlock avoidance, then the

deadlock situation may occur. In this case

system must provide

Deadlock Detection algorithm

Recovery scheme


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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 muchlonger 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?


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Recovery from Deadlock: Resource

Preemption

Selecting a victimminimize cost.

Rollbackreturn to some safe state, restart

process from that state.

Starvation same process may always be

picked as victim, include number of rollback in

cost factor.

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Ch 7 Deadlock