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1IT105Operating SystemsProcess Management: DeadlocksHarris Chikunya

2Objectives• By the end of this unit, you should be able to: Define a deadlocks; Discuss four conditions for a deadlock; Explain the ways of avoiding deadlocks; and Describe two ways to recover from the deadlock situation.

3Resources• There are two types of resources namely: Pre-emptable and Non-pre-emptable Resources.a) Pre-emptable Resources• This resource can be taken away from the process with no ill effects.Memory is an example of a pre-emptable resource.b) Non-preemptable Resource• This resource cannot be taken away from the process (without causing illeffect). For example, CD resources are not pre-emptable at any arbitrarymoment.• Under normal mode of operation, a process may utilize a resource inonly the following sequence: Request: If the request cannot be granted immediately, then therequesting process must wait until it can acquire the resource. Use: The process can operate on the resource. Release: The process releases the resource.

4Deadlocks• A process requests resources; if the resources are not available atthat time, the process enters a wait state.• It may happen that waiting processes will never again change state,because the resources they have requested are held by otherwaiting processes.• This situation is called a deadlock.

5Characterisation of a Deadlock1. Mutual Exclusion condition2. Hold and Wait condition3. No Preemption4. Circular Wait

6Mutual Exclusion Condition• The resources involved are non-shareable.• At least one resource must be held in a non-shareable mode, that is,only one process at a time claims exclusive control of the resource.• If another process requests that resource, the requesting must bedelayed until the resource has been released.

7Hold and Wait Condition• In this condition, a requesting process already holds resources andwaiting for the requested resources.• There must exist a process that is holding a resource already allocatedto it while waiting for additional resource that are currently beingheld by other processes.

8No-Preemption Condition• Resources already allocated to a process cannot be preempted.• Resources cannot be removed forcibly from the processes.• After completion, they will be released voluntarily by the processholding it.

9Circular Wait Condition• The processes in the system form a circular list or chain where eachprocess in the list is waiting for a resource held by the next processin the list.• There exists a set {P0, P1, …, P0} of waiting processes such that P0 iswaiting for a resource that is held by P1, P1 is waiting for a resourcethat is held by P2, …, Pn–1 is waiting for a resource that is held byPn, and P0 is waiting for a resource that is held by P0.

11Resource allocation Graph• This graph consists of a set of vertices V and a set of edges E.• The set of vertices V is partitioned into two different types of nodes P = {PI,P2, ..., Pn}, the set consisting of all the active processes in the system, andR = {R1, R2, ..., Rm}, the set consisting of all resource types in the system.• A directed edge from process Pi to resource type Rj is denoted by Pi Rj; itsignifies that process Pi requested an instance of resource type Rj and is currentlywaiting for that resource.• A directed edge from resource type Rj to process Pi is denoted by Rj Pi; itsignifies that an instance of resource type Rj has been allocated to process Pi.

12Resource allocation Graph illustration

13Resource Allocation GraphThe following are the ways which will help you to check the grapheasily to predict the presence of a deadlock:(a) If no cycle exists in the resource allocation graph, there is nodeadlock.(b) If there is a cycle in the graph and each resource has only oneinstance, then there is a deadlock. In this case, a cycle is a necessaryand sufficient condition for deadlock.(c) If there is a cycle in the graph, and each resource has more thanone instance, there may or may not be a deadlock.

14Example• Suppose that process P3 requests an instance of resource type R2.Since no resource instance is currently available, a request edgeP3→ R2 is added to the graph.• At this point, two minimal cycles exist in the system:P1→R1→ P2→ R3→ P3→ R2→ P1P2→ R3 → P3→ R2→ P2

15Dealing with Deadlocks• In general, there are four strategies of dealing with deadlockproblem:1. Deadlock Prevention: Prevent deadlock by resource scheduling so asto negate at least one of the four conditions.2. Deadlock Avoidance: Avoid deadlock by careful resource scheduling.3. Deadlock Detection and Recovery: Detect deadlock and when itoccurs, take steps to recover.4. The Ostrich Approach: Just ignore the deadlock problem altogether.

16Deadlock Prevention1. Elimination of “Mutual Exclusion” Condition•The mutual exclusion condition must hold for non-shareableresources.•That is, several processes cannot simultaneously share a singleresource.•This condition is difficult to eliminate because some resources, suchas the disc drive and printer, are inherently non-shareable.•Note that shareable resources like read-only-file do not requiremutually exclusive access and thus cannot be involved in deadlock.

17Deadlock Prevention2. Elimination of “Hold and Wait” Condition• The first alternative is that a process request be granted all the resources itneeds at once, prior to execution.• The second alternative is to disallow a process from requesting resourceswhenever it has previously allocated resources.• This strategy requires that all the resources a process will need must berequested at once.• The system must grant resources on “all or none” basis.• If the complete set of resources needed by a process is not currentlyavailable, then the process must wait until the complete set is available.• While the process waits, however, it may not hold any resources.• Thus the “wait for” condition is denied and deadlocks simply cannot occur.• This strategy can lead to serious waste of resources.

18Deadlock Prevention3. Elimination of “Non-preemption” Condition• The non-preemption condition can be alleviated by forcing a processwaiting for a resource that cannot immediately be allocated to relinquishall of its currently held resources, so that other processes may use themto finish.• This strategy requires that when a process that is holding some resourcesis denied a request for additional resources.• The process must release its held resources and, if necessary, requestthem again together with additional resources.• Implementation of this strategy denies the “no-preemptive” conditioneffectively.

19Deadlock Prevention4. Elimination of “Circular Wait” Condition•The last condition, the circular wait, can be denied by imposing atotal ordering on all of the resource types and then forcing, allprocesses to request the resources in order (increasing ordecreasing).•This strategy impose a total ordering of all resources types, and torequire that each process requests resources in a numerical order(increasing or decreasing) of enumeration.•With this rule, the resource allocation graph can never have a cycle.

20Deadlock Avoidance• This approach to the deadlock problem anticipates deadlock beforeit actually occurs.• This approach employs an algorithm to access the possibility thatdeadlock could occur and acting accordingly.• If the necessary conditions for a deadlock are in place, it is stillpossible to avoid deadlock by being careful when resources areallocated.• It employs the most famous deadlock avoidance algorithm that isthe Banker’s algorithm.

21Deadlock Detection and RecoveryDeadlock Detection• Detection of deadlocks is the most practical policy, which being bothliberal and cost efficient most operating systems deploy.• To detect a deadlock, we must go about in a recursive manner andsimulate the most favoured execution of each unblocked process:(a) An unblocked process may acquire all the needed resources and willexecute;(b) It will then release all the acquired resources and remain dormantthereafter;(c) The now released resources may wake up some previously blockedprocess;(d) Continue the above steps as long as possible; and(e) If any blocked processes remain, they are deadlocked.

22Deadlock Detection and RecoveryRECOVERY FROM DEADLOCK• There are various options for breaking a deadlock.1. Recovery by Process Termination• Abort all deadlocked processes: This method will break the deadlockcycle clearly by terminating all process.• This method is cost effective.• Abort one process at a time until the deadlock cycle is eliminated: Thismethod terminates one process at a time, and invokes a deadlock-detection algorithm to determine whether any processes are stilldeadlocked.

23Deadlock Detection and RecoveryRECOVERY FROM DEADLOCK2. Recovery by Resource Pre-emption the operator or system pre-empts some resources from processes andgive these resources to other processes until the deadlock cycle isbroken. If pre-emption is required to deal with deadlocks, then three issuesneed to be addressed:• Selecting a victim: The system or operator selects which resources andwhich processes are to be pre-empted based on cost factor.• Rollback: The system or operator must roll back the process to some safestate and restart it from that state.• Starvation: The system or operator should ensure that resources will notalways be pre-empted from the same process?

24Ostrich Algorithm• It is hoped that deadlock doesn’t happen• In general, this is a reasonable strategy.• Deadlock is unlikely to occur very often; a system can run for yearswithout deadlock occurring.

25ACTIVITY1. What is the purpose of deadlock?2. What is the different between process nodes and resource nodes ?3. What is a deadlock and what are the four conditions that will createthe deadlock situation?4. How can deadlock be avoided? Explain with the help of an example.

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