Unit 3 part 2(DEADLOCK)
- 1. Unit 3 – Part 2 Deadlock
- 2. Methods for handling deadlock There are three ways to handle deadlock 1) Deadlock prevention or avoidance: The idea is to not let the system into deadlock state. One can zoom into each category individually, Prevention is done by negating one of above mentioned necessary conditions for deadlock. Avoidance is kind of futuristic in nature. By using strategy of “Avoidance”, we have to make an assumption. We need to ensure that all information about resources which process WILL need are known to us prior to execution of the process. 2) Deadlock detection and recovery: Let deadlock occur, then do preemption to handle it once occurred. 3) Ignore the problem all together: If deadlock is very rare, then let it happen and reboot the system. This is the approach that both Windows and UNIX take.
- 3. Deadlock Detection A deadlock can be detected by a resource scheduler as it keeps track of all the resources that are allocated to different processes. After a deadlock is detected, it can be resolved using the following methods. All the processes that are involved in the deadlock are terminated. This is not a good approach as all the progress made by the processes is destroyed. Resources can be preempted from some processes and given to others till the deadlock is resolved. If resources have single instance: In this case for Deadlock detection we can run an algorithm to check for cycle in the Resource Allocation Graph. Presence of cycle in the graph is the sufficient condition for deadlock. In the diagram, resource 1 and resource 2 have single instances. There is a cycle R1 → P1 → R2 → P2. So, Deadlock is Confirmed.
- 4. Deadlock Detection If there are multiple instances of resources: Detection of the cycle is necessary but not sufficient condition for deadlock detection, in this case, the system may or may not be in deadlock varies according to different situations.
- 5. Deadlock Recovery A traditional operating system such as Windows doesn’t deal with deadlock recovery as it is time and space consuming process. Real-time operating systems use Deadlock recovery. Recovery method Killing the process: killing all the process involved in the deadlock. Killing process one by one. After killing each process check for deadlock again keep repeating the process till system recover from deadlock. Resource Preemption: Resources are preempted from the processes involved in the deadlock, preempted resources are allocated to other processes so that there is a possibility of recovering the system from deadlock. In this case, the system goes into starvation.
- 6. Deadlock Avoidance Deadlock avoidance merely works to avoid deadlock; it does not totally prevent it. The basic idea here is to allocate resources only if the resulting global state is a safe state. In other words, unsafe states are avoided, meaning that deadlock is avoided as well. • When a process requests a resource, even if it is available, it is not immediately allocated to the process. Instead the systems assumes (pretends) it is so allocated. • With this assumption, along with advance knowledge of all the resources needed for all the processes, the system performs some analysis to decide whether or not granting the request is safe. • If the request is safe, the resource is granted to the requesting process. Otherwise, the resource is not given to the process at this time. This type of algorithm requires each site to have access to a global state (requiring too much storage and communication). Also the checking of the involved data structures must be done in a mutually exclusive fashion. With many sites and resources, it just takes too long.
- 7. Deadlock avoidance can be done with Banker’s Algorithm. Banker’s Algorithm Bankers’s Algorithm is resource allocation and deadlock avoidance algorithm which test all the request made by processes for resources, it checks for the safe state, if after granting request system remains in the safe state it allows the request and if there is no safe state it doesn’t allow the request made by the process. Inputs to Banker’s Algorithm: • Max need of resources by each process. • Currently allocated resources by each process. • Max free available resources in the system. The request will only be granted under the below condition: • If the request made by the process is less than equal to max need to that process. • If the request made by the process is less than equal to the freely available resource in the system.
- 8. Deadlock Prevention It is very important to prevent a deadlock before it can occur. So, the system checks each transaction before it is executed to make sure it does not lead to deadlock. If there is even a slight chance that a transaction may lead to deadlock in the future, it is never allowed to execute
- 9. Deadlock Prevention This strategy involves designing a system that violates one of the four necessary conditions required for the occurrence of deadlock. This ensures that the system remains free from the deadlock. The various conditions of deadlock occurrence may be violated as- • Eliminate Mutual Exclusion • Eliminate Hold and Wait • Eliminate No Pre-emption • Eliminate Circular Wait
- 10. Deadlock Prevention 1- Eliminate Mutual Exclusion To violate this condition, all the system resources must be such that they can be used in a shareable mode. In a system, there are always some resources which are mutually exclusive by nature. So, this condition can not be violated.
- 11. Deadlock Prevention 2- Eliminate Hold and Wait This condition can be violated in the following ways- Approach-01: In this approach, • A process has to first request for all the resources it requires for execution. • Once it has acquired all the resources, only then it can start its execution. • This approach ensures that the process does not hold some resources and wait for other resources. Drawbacks- The drawbacks of this approach are- • It is less efficient. • It is not implementable since it is not possible to predict in advance which resources will be required during execution.
- 12. Deadlock Prevention Approach-02: In this approach, • A process is allowed to acquire the resources it desires at the current moment. • After acquiring the resources, it start its execution. • Now before making any new request, it has to compulsorily release all the resources that it holds currently. • This approach is efficient and implementable. Approach-03: In this approach, • A timer is set after the process acquires any resource. • After the timer expires, a process has to compulsorily release the resource.
- 13. Deadlock Prevention 3-Eliminate No Pre-emption This condition can by violated by forceful pre-emption. • Consider a process is holding some resources and request other resources that can not be immediately allocated to it. • Then, by forcefully pre-empting the currently held resources, the condition can be violated. • A process is allowed to forcefully pre-empt the resources possessed by some other process only if It is a high priority process or a system process. The victim process is in the waiting state.
- 14. Deadlock Prevention 4-Eliminate Circular Wait This condition can be violated by not allowing the processes to wait for resources in a cyclic manner. To violate this condition, the following approach is followed- Approach A numerical number is assigned to every resource. Each process is allowed to request for the resources either in only increasing or only decreasing order of the resource number. In case increasing order is followed, if a process requires a lesser number resource, then it must release all the resources having larger number and vice versa. This approach is the most practical approach and implementable. However, this approach may cause starvation but will never lead to deadlock.