deadlock condition system programming & operating system pdf

7/23/2019 Deadlock Condition System Programming & Operating system pdf

1/41

Silberschatz, Galvin and Gagne20028.1Operating System Concepts

Chapter 8: Deadlocks

System Model Deadlock Characterization

Methods for Handling Deadlocks

Deadlock Prevention

Deadlock Avoidance

Deadlock Detection

Recovery from Deadlock

Combined Approach to Deadlock Handling


2/41


The Deadlock Problem

A set of blocked processes each holding a resource andwaiting to acquire a resource held by another process in theset.

Example

System has 2 tape drives.

P1andP2each hold one tape drive and each needs another one. Example

semaphoresAandB, initialized to 1

P0 P1wait (A);wait(B)wait (B);wait(A)


3/41


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 backsup (preempt resources and rollback).

Several cars may have to be backed up if a deadlockoccurs.

Starvation is possible.


4/41


System Model

Resource typesR1,R2, . . .,Rm

CPU cycles, memory space, I/O devices

Each resource typeRihasWiinstances.

Each process utilizes a resource as follows:

request use

release


5/41


Deadlock Characterization

Mutual exclusion:only one process at a time can use aresource.

Hold and wait:a process holding at least one resourceis waiting to acquire additional resources held by otherprocesses.

No preemption:a resource can be released onlyvoluntarily by the process holding it, after that process hascompleted its task.

Circular wait:there exists a set {P0,P1, ,P0} of waitingprocesses such thatP0is waiting for a resource that is

held byP1,P1is waiting for a resource that is held byP2, ,

Pn1is waiting for a resource that is held byPn,

andP0is waiting for a resource that is held byP0.

Deadlock can arise if four conditions hold simultaneously.


6/41


Resource-Allocation Graph

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 edge directed edgeP1Rj

assignment edge directed edgeRjPi

A set of verticesVand a set of edgesE.


7/41Silberschatz, Galvin and Gagne20028.7Operating System Concepts

Resource-Allocation Graph (Cont.)

Process

Resource Type with 4 instances

Pirequests instance ofRj

Piis holding an instance ofRj

Pi

Pi

Rj

Rj



Example of a Resource Allocation Graph



Resource Allocation Graph With A Deadlock



Resource Allocation Graph With A Cycle But No Deadlock



Basic Facts

If graph contains no cyclesno deadlock.

If graph contains a cycle if only one instance per resource type, then deadlock.

if several instances per resource type, possibility of deadlock.



Methods for Handling Deadlocks

Ensure that the system willneverenter a deadlockstate.

Allow the system to enter a deadlock state and then

recover.

Ignore the problem and pretend that deadlocks neveroccur in the system; used by most operating systems,

including UNIX.



Deadlock Prevention

Mutual Exclusion not required for sharable resources;must hold for nonsharable resources.

Hold and Wait must guarantee that whenever aprocess requests a resource, it does not hold any other

resources.

Require process to request and be allocated all its resourcesbefore it begins execution, or allow process to request

resources only when the process has none. Low resource utilization; starvation possible.

Restrain the ways request can be made.



Deadlock Prevention (Cont.)

No Preemption If a process that is holding some resources requests anotherresource 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 oldresources, as well as the new ones that it is requesting.

Circular Wait impose a total ordering of all resource

types, and require that each process requests resourcesin an increasing order of enumeration.


15/41


Deadlock Avoidance

Simplest and most useful model requires that eachprocess declare themaximum numberof resources ofeach type that it may need.

The deadlock-avoidance algorithm dynamically examinesthe resource-allocation state to ensure that there can

never be a circular-wait condition.

Resource-allocationstateis defined by the number ofavailable and allocated resources, and the maximum

demands of the processes.

Requires that the system has some additionala prioriinformationavailable.


16/41


Safe State

When a process requests an available resource, systemmust decide if immediate allocation leaves the system in a

safe state.

System is in safe state if there exists a safe sequence of allprocesses.

Sequence is safe if for eachPi, theresources thatPican still request can be satisfied bycurrently available resources + resources held by all thePj,withj


17/41


Basic Facts

If a system is in safe stateno deadlocks.

If a system is in unsafe statepossibility of deadlock.

Avoidanceensure that a system will never enter anunsafe state.


18/41


Safe, Unsafe , Deadlock State


19/41


Resource-Allocation Graph Algorithm

Claim edgePiRjindicated that processPimay requestresourceRj; represented by a dashed line.

Claim edgeconverts torequest edgewhen a process

requests a resource.

When a resource is released by a process,assignmentedge reconverts to a claim edge.

Resources must be claimeda prioriin the system.


20/41


Resource-Allocation Graph For Deadlock Avoidance


21/41


Unsafe State In Resource-Allocation Graph


22/41


Bankers Algorithm

Multiple instances.

Each process must a priori claim maximum use.

When a process requests a resource it mayhave to wait.

When a process gets all its resources it must

return them in a finite amount of time.


23/41


Data Structures for the Bankers Algorithm

Available:Vector of lengthm. If available [j] =k, there arekinstances of resource typeRjavailable.

Max:n x mmatrix. IfMax[i,j] =k, then processPimay

request at mostkinstances of resource typeRj.

Allocation:nxmmatrix. If Allocation[i,j] =kthenPiis

currently allocatedkinstances ofRj.

Need:nxmmatrix. IfNeed[i,j] =k, thenPimay needk

more instances ofRjto complete its task.

Need[i,j]= Max[i,j] Allocation[i,j].

Letn= number of processes, and

m= number of resources types.


24/41


Safety Algorithm

1.LetWorkandFinishbe vectors of lengthmandn,

respectively. Initialize:Work=Available

Finish[i] =falsefori- 1,3, ,n.

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

(b)NeediWorkIf no suchiexists, go to step 4.

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

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


25/41


Resource-Request Algorithm for ProcessPi

Request= request vector for processPi. IfRequesti[j] =kthen

processPiwantskinstances of resource typeRj.

1.IfRequestiNeedigo to step 2. Otherwise, raise errorcondition, since process has exceeded its maximum claim.

2.IfRequestiAvailable, go to step 3. OtherwisePimust

wait, since resources are not available.3.Pretend to allocate requested resources toPiby modifying thestate as follows:

Available=Available-Requesti;

Allocationi=Allocationi+Requesti;

Needi=NeediRequesti;;

If safe the resources are allocated to Pi.

If unsafe Pimust wait, and the old resource-allocationstate is restored


26/41


Example of Bankers Algorithm

5 processesP0throughP4;3 resource typesA(10 instances),

B(5instances, andC(7 instances).

Snapshot at timeT0:

Allocation Max Available

A B C A B C A B C

P0 0 1 07 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 24 3 3

Need

A B CP0 7 4 3

P1 1 2 2

P2 6 0 0

P3 0 1 1

P4 4 3 1


27/41


Example (Cont.)

The content of the matrix. Need is defined to be Max Allocation.

Need

A B C

P0 7 4 3

P1 1 2 2

P2 6 0 0

P3 0 1 1

P4 4 3 1

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


28/41


ExampleP1Request (1,0,2) (Cont.)

Check that RequestAvailable (that is, (1,0,2)(3,3,2)true.AllocationNeedAvailable

A B C A B CA B C

P0 0 1 0 7 4 3 2 3 0

P1 3 0 20 2 0

P2 3 0 1 6 0 0

P3 2 1 1 0 1 1

P4 0 0 2 4 3 1

Executing safety algorithm shows that sequence

satisfies safety requirement.

Can request for (3,3,0) byP4be granted?

Can request for (0,2,0) byP0be granted?


29/41


Deadlock Detection

Allow system to enter deadlock state

Detection algorithm

Recovery scheme


30/41


Single Instance of Each Resource Type

Maintainwait-forgraph Nodes are processes.

PiPjifPiis waiting forPj.

Periodically invoke an algorithm that searches for a cycle

in the graph.

An algorithm to detect a cycle in a graph requires anorder ofn2operations, wherenis the number of verticesin the graph.


31/41


Resource-Allocation Graph and Wait-for Graph

Resource-Allocation Graph Corresponding wait-for graph

S lI t f R T


32/41


Several Instances of a Resource Type

Available:A vector of lengthmindicates the number ofavailable resources of each type.

Allocation:Annxmmatrix defines the number ofresources of each type currently allocated to eachprocess.

Request:Annxmmatrix indicates the current requestof each process. IfRequest[ij] =k, then processPiis

requestingkmore instances of resource type.Rj

.


33/41


Detection Algorithm

1.LetWorkandFinishbe vectors of lengthmandn,respectively Initialize:

(a)Work=Available

(b)Fori= 1,2, ,n, ifAllocationi0, then

Finish[i] = false;otherwise,Finish[i] =true.

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

(b)RequestiWork

If no suchiexists, go to step 4.


34/41


Detection Algorithm (Cont.)

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

4.IfFinish[i] == false, for somei, 1in, then the system is indeadlock state. Moreover, ifFinish[i] ==false, thenPiis deadlocked.

Algorithm requires an order of O(mxn2)operations to detectwhether the system is in deadlocked state.


35/41


Example of Detection Algorithm

Five processesP0throughP4;three resource typesA (7 instances),B(2 instances), andC(6 instances).

Snapshot at timeT0:

Allocation Request Available

A B C A B C A B C

P0 0 1 0 0 0 0 0 0 0

P1 2 0 0 2 0 2

P2 3 0 30 0 0

P3 2 1 1 1 0 0

P4 0 0 2 0 0 2

Sequence will result inFinish[i] = true for all

i.

C


36/41


Example (Cont.)

P2requests an additional instance of typeC.Request

A B C

P0 0 0 0

P1 2 0 1

P2 0 0 1

P3 1 0 0

P4 0 0 2

State of system?

Can reclaim resources held by processP0, but insufficientresources to fulfill other processes; requests.

Deadlock exists, consisting of processesP1,P2,P3, andP4.

D i Al ih U


37/41


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 bemany cycles in the resource graph and so we would not

be able to tell which of the many deadlocked processes

caused the deadlock.


38/41


Recovery from Deadlock: Process Termination

Abort all deadlocked processes.

Abort one process at a time until the deadlockcycle 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?


39/41


Recovery from Deadlock: Resource Preemption

Selecting a victim minimize cost.

Rollback return to some safe state, restartprocess for that state.

Starvation same process may always bepicked as victim, include number of rollback incost factor.

CombinedApproachtoDeadlockHandling


40/41


Combined Approach to Deadlock Handling

Combine the three basic approaches

prevention avoidance

detection

allowing the use of the optimal approach for each of

resources in the system.

Partition resources into hierarchically orderedclasses.

Use most appropriate technique for handlingdeadlocks within each class.

T ffi D dl kf E i 84


41/41

Traffic Deadlock for Exercise 8.4

deadlock condition system programming & operating system pdf

Documents