OS K.Nishitha presentation on dead locks .pptx

Deadlocks presentation

1. DEAD LOCKS S.SaiNishitha 22011A0527

2. what is a DEADLOCK? Deadlock is a situation in which group of processes are blocked. Each process acquires a resource needed for its completion Simultaneously the processes waits for the resources to be released by others – thus making impossible for any process to proceed.

3. CONDITIONS FOR DEADLOCK TO OCCUR Mutual Exclusion :  At least one resource must be held in a non-sharable mode, meaning only one process can use it at a time. Hold and Wait :  A process must be holding at least one resource is waiting to acquire additional resources held by another processes. No Preemption :  Resources cannot be forcibly taken away from a process; they must be released voluntarily. Circular Wait :   A circular chain of two or more processes exists, where each process is waiting for a resource held by the next process in the chain.

4. METHODS FOR HANDLING DEADLOCKS Ensure that the system will never enter a deadlock state Allow the system to enter a deadlock and then recover Ignore the problem and pretend that deadlock never occurred in the system.This is the simplest and most used in OS. The first and second methods need techniques to be developed and implemented.These methods are Deadlock Prevention and Deadlock Avoidance.

5. DEADLOCK PREVENTION INDIRECT :- prevent the occurrence of Mutual Exclusion, Hold and Wait, No Preemption DIRECT : - prevent the occurrence of Circular Wait

6. Mutual exclusion – not required for sharable resources,must hold for non sharable resources. Hold and wait – must guarantee that whenever a process request a resource,it does not hold any other resources. 3 .No Preemption – can be prevented in many ways. If a process that is holding some resources requests another resources that cannot be immediately allocated to it,then all resources currently being held are released

7. 4.Circular wait -- impose a total ordering of resources types and require that each process requests resources in an increasing order of enumeration. 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 old resources,as well as the new resources that it is requesting

8. RECOVERY FROM DEADLOCK 1.PROCESS TERMINATION -> - Abort all deadlock processes -Abort one process at a time until deadlock cycle is eliminated -In partial termination process choosen to abort Priority of process How long process has completed and how much longer to completion Resources process has used How many resources process need to complete

9. 2 . RESOURCE PREEMPTION -> - successively preempt resources from processes and give it to other processes until deadlock is broken - issues to be considered SELECTING A VICTIM : minimize cost ROLL BACK : return to some safe state,restart process for that state STARVATION : same process may always be picked as victim,include a number of rollback in cost factor

10. THANK YOU!!