5TH OPERATING SYSTEM NOTES

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DEADLOCKS 1). What is a deadlock? Ans:-

A deadlock is a situation wherein two or more competing actions are waiting for t he other to finish, and thus neither ever does.

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In computer science, deadlock refers to a specific condition when two or more processes are each waiting for each other to release a resource, or more than two processes are waiting for resources in a circular chain.

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Deadlock is a common problem in multiprocessing where many processes share a specific type of resource.

2).What are the necessary and sufficient conditions for a deadlock to occur? Ans:-

Mutual exclusion condition :- a resource that cannot be used by more than one process at a time

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Hold and wait condition:- processes already holding resources may request new resources

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No preemption condition:- No resource can be forcibly removed from a process holding it, resources can be released only by t he explicit action of the process

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Circular wait condition:-two or more processes form a circular chain where each process waits for a resource that the next process in the chain holds.

3).How can we prevent the occuring of deadlocks? Ans:-Elimination of “Mutual Exclusion” Condition:- The mutual exclusion condition must hold for non-sharable resources. That is, several processes cannot simultaneously share a single

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2 www.bookspar.com | Website for students | VTU NOTES resource. Removing the mutual exclusion condition means that no process may have exclusive access to a resource. -Elimination of “Hold and Wait” Condition:- The "hold and wait" conditions may be removed by requiring processes to request all the resources they will need before starting up. Another way is to require processes to release all their resources before requesting all the resources they will need. The system must grant resources on “all or none” basis. If the complete set of  resources needed by a process is not currently available, 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.

-Elimination of “No-preemption” Condition:- A "no preemption" (lockout) condition may also be difficult or impossible to avoid as a process has to be able to have a resource for a certain amount of time, or the processing outcome may be inconsistent or thrashing may occur. This strategy require that when a process that i s holding some resources is denied a request for additional resources. The process must release its held resources and, if necessary, request them again together with additional resources. Implementation of this strategy denies the “nopreemptive” condition effectively.

-Elimination of “Circular Wait” Condition:- The last condition, the circular wait, can be denied by imposing a total ordering on all of the resource types and than forcing, all processes to request the resources in order (increasing or decreasing). This strategy impose a total or dering of all resources types, and to require 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.

4).What is deadlock detection and how do you recover from it? Ans:Deadlock detection is the process of actually determining that a deadlock exists and identifying the processes and resources involved in the deadlock. The basic idea is to check allocation against resource availability for all possible a llocation sequences to determine if the system is in deadlocked state a. Of course, the deadlock detection algorithm is only half of this strategy. Once a deadlock is detected, there needs to be a way to recover several alternatives exists: Temporarily prevent resources from deadlocked processes. Back off a process to some check point al lowing preemption of a needed resource and restarting the process at the checkpoint later.

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3 www.bookspar.com | Website for students | VTU NOTES Successively kill processes until the system is deadlock free.

These methods are expensive in the sense that each iteration calls the detection algorithm until the system proves to be deadlock free.

5).what are the two options of breaking a dead lock?xplain. Ans:-

-Prevention : Set of methods for ensuring that at least o ne of the condition cannot hold.

-Avoidance : OS be given information about the resources request used in advance. rd

For preventionif you have to explain then you can write the whole 3 answer........ For avoidance you can write:-Requires additional information about how resources are to be used. -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 allo cated resources, and the maximum demands of the processes.

6).what is wait-for-graph?xplain. Ans:- A Wait-For Graph in computer science is a directed graph used for deadlock detection in operating systems and relational database systems.

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4 www.bookspar.com | Website for students | VTU NOTES In Computer Science, a system that allows concurrent operation of multiple processes and locking of  resources and which does not provide mechanisms to avoid o r prevent deadlock must support a mechanism to detect deadlocks and an algorithm for recovering from them.

One such deadlock detection algorithm makes use of a Wait-For Graph to track which other processes a process is currently blocking on. In a Wait-for Graph, processes are represented as nodes, and an edge from process Pi to Pj implies Pj is holding a resource that Pi needs and thus Pi is waiting for Pj to release its lock on that resource.

7).what is resource request algorithm? Ans:-

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