Explain different methods to handle deadlock in details.

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Bipin Tiwari · Accepted Answer

Methods of handling deadlocks: There are three approaches to dealing with deadlocks. 1. Deadlock Prevention : The strategy of deadlock prevention is to design the system in such a way that the possibility of deadlock is excluded. Indirect method prevent the occurrence of one of three necessary condition of deadlock i.e., mutual exclusion, no pre-emption and hold and wait. The direct method prevents the occurrence of circular wait. Prevention techniques – Mutual exclusion – are supported by the OS. Hold and Wait – condition can be prevented by requiring that a process requests all its required resources at one time and blocking the process until all of its requests can be granted at a same time simultaneously. But this prevention does not yield good results because : long waiting time required in efficient use of allocated resource A process may not know all the required resources in advance No pre-emption – techniques for ‘no pre-emption are’ If a process that is holding some resource, requests another resource that can not be immediately allocated to it, the all resources currently being held are released and if necessary, requested again together with the additional resource. If a process requests a resource that is currently held by another process, the OS may pre-empt the second process and require it to release its resources. This works only if both processes do not have the same priority. Circular wait One way to ensure that this condition never holds is to impose a total ordering of all resource types and to require that each process requests resources in increasing order of enumeration, i.e., if a process has been allocated resources of type R. It may subsequently request only those resources of types following R in ordering. 2. Deadlock Avoidance : This approach allows the three necessary conditions of deadlock but makes judicious choices to assure that deadlock point is never reached. It allows more concurrency than avoidance detection A decision is made dynamically whether the current resource allocation request will, if granted, potentially lead to deadlock. It requires the knowledge of future process requests. Two techniques to avoid deadlock : Process initiation denial Resource allocation denial Advantages of deadlock avoidance techniques : Not necessary to pre-empt and rollback processes Less restrictive than deadlock prevention Disadvantages : Future resource requirements must be known in advance Processes can be blocked for long periods Exists fixed number of resources for allocation 3. Deadlock Detection : Deadlock detection is used by employing an algorithm that tracks the circular waiting and killing one or more processes so that deadlock is removed. The system state is examined periodically to determine if a set of processes is deadlocked. A deadlock is resolved by aborting and restarting a process, relinquishing all the resources that the process holds. This technique does not limit resource access or restrict process action. Requested resources are granted to processes whenever possible. It never delays the process initiation and facilitates online handling. The disadvantage is the inherent pre-emption losses.

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