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Unit-3 Deadlock-Operating System | BCA 4th Sem

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deadlock

Unit-3

Deadlock

Meaning of deadlock

  • In computer science, deadlock refers to a situation where multiple processes in a system are unable to proceed because each process is waiting for a resource that is being held by another process. This creates a circular dependency, causing the system to come to a halt.
  • To understand deadlock, let’s imagine a scenario. Picture a dining room with a limited number of forks and knives. There are multiple people sitting at the table, and each person needs both a fork and a knife to eat. However, if everyone picks up a fork and is waiting for a knife, and no one is willing to release their fork until they have a knife, a deadlock occurs. No one can proceed with their meal because they are all waiting for a resource that is being held by someone else.
  • In computer systems, deadlock occurs when processes are stuck in a state where they are waiting for resources that are being held by other processes, leading to a standstill. This can happen due to various reasons, such as resource allocation issues, improper synchronization, or programming errors.
  • To handle deadlocks, different techniques can be employed, such as resource scheduling algorithms, deadlock prevention, and deadlock detection. These methods help identify and resolve deadlocks, ensuring the smooth operation of the system.

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Coffman conditions

  • The Coffman conditions, also known as the necessary conditions for deadlock, are a set of conditions that must be present for a deadlock to occur in a computer system. These conditions were proposed by Edward G. Coffman Jr. and include four main conditions: mutual exclusion, hold and wait, no preemption, and circular wait.
  1. Mutual Exclusion: This condition states that at least one resource must be held in a non-sharable mode, meaning that only one process can access the resource at a time. For example, if a printer is being used by one process, it cannot be used by any other process simultaneously.
  2. Hold and Wait: This condition states that a process must be holding at least one resource while waiting to acquire additional resources. In other words, a process that has already acquired a resource can request additional resources and will not release the held resources until it acquires all the requested resources. This can lead to resource wastage and potential deadlock situations.
  3. No Preemption: This condition states that resources cannot be forcibly taken away from a process. Once a process is allocated a resource, it will hold onto that resource until it voluntarily releases it. This condition can contribute to deadlock situations because if a process is holding a resource and not releasing it, other processes waiting for that resource may be unable to proceed.
  4. Circular Wait: This condition states that there must be a circular chain of two or more processes, where each process is waiting for a resource held by the next process in the chain. For example, Process A is waiting for a resource held by Process B, Process B is waiting for a resource held by Process C, and so on, until Process N is waiting for a resource held by Process A. This circular dependency creates a deadlock situation.
  • To prevent deadlocks, it is necessary to break at least one of these Coffman conditions. Various techniques can be used, such as resource allocation strategies, deadlock avoidance algorithms, and deadlock detection and recovery mechanisms. These techniques aim to ensure that the Coffman conditions are not satisfied, thus preventing the occurrence of deadlocks.

Wikipedia- https://en.wikipedia.org/wiki/Deadlock


Method of handling deadlock

  1. Deadlock Prevention:
  • One approach to handling deadlock is to prevent it from occurring in the first place. This involves designing systems and implementing strategies that ensure the Coffman conditions for deadlock are not satisfied. Some techniques for deadlock prevention include:
  • Mutual Exclusion: Ensure that resources can be accessed by multiple processes simultaneously, if possible.
  • Hold and Wait: Implement a policy where a process must request and acquire all required resources before starting execution, or release held resources if additional resources cannot be acquired.
  • No Preemption: Allow resources to be preempted from processes if necessary. This means that if a process is holding a resource and requests additional resources that cannot be granted immediately, it may be forced to release its held resources.
  • Circular Wait: Enforce a total ordering of resources and require processes to request resources in a specific order, eliminating the possibility of a circular wait.
  • By carefully designing systems and implementing these prevention techniques, deadlock situations can be minimized or eliminated.
  1. Deadlock Avoidance:
  • Another method of handling deadlock is through deadlock avoidance. This approach involves dynamically analyzing the resource allocation state and making decisions to avoid potential deadlocks. It requires a system to have additional information about the resource requirements of processes, as well as the available resources. Some techniques for deadlock avoidance include:
  • Resource Allocation Graph: Use a resource allocation graph to model the resource allocation state and determine if a request for resources will result in a deadlock. If a deadlock is predicted, the request can be denied.
  • Banker’s Algorithm: This algorithm is a popular method for deadlock avoidance. It considers the available resources, the maximum resource requirements of processes, and the current allocation state to determine if granting a resource request will lead to a safe state or a deadlock.
  • Deadlock avoidance requires careful resource management and decision-making to ensure that resource requests are granted in a way that avoids potential deadlocks.
  1. Deadlock Detection and Recovery:
  • If prevention and avoidance techniques are not feasible or practical, another method is to detect and recover from deadlocks. Deadlock detection involves periodically examining the resource allocation state to identify if a deadlock has occurred. Once a deadlock is detected, recovery strategies can be implemented to resolve the deadlock. Some techniques for deadlock detection and recovery include:

Deadlock detection

  • Deadlock detection is a method used in computer systems to identify whether a deadlock has occurred. A deadlock is a situation where two or more processes are unable to proceed because each is waiting for a resource held by another process. Detecting deadlocks is crucial for maintaining the stability and efficiency of a system.
  • There are several algorithms and techniques used for deadlock detection. One commonly used algorithm is the Resource Allocation Graph (RAG) algorithm. The RAG algorithm represents the resource allocation state and checks for the presence of cycles in the graph to identify deadlocks. Here’s how it works:
  1. Resource Allocation Graph:
  • The RAG algorithm utilizes a graph representation to track the allocation of resources to processes. The graph consists of two types of nodes: process nodes and resource nodes. Process nodes represent the processes in the system, and resource nodes represent the resources available.
  1. Resource Request and Allocation:
  • Each process can request and hold multiple resources. When a process requests a resource, it creates an edge from the process node to the resource node. When a resource is allocated to a process, an edge is created from the resource node to the process node. This graph dynamically represents the current resource allocation state.
  1. Cycle Detection:
  • To detect deadlocks, the RAG algorithm checks for the presence of cycles in the graph. If a cycle is found, it indicates a potential deadlock. The algorithm performs a depth-first search (DFS) or breadth-first search (BFS) traversal of the graph to identify cycles.
  1. Deadlock Identification:
  • During the traversal, the algorithm marks the visited nodes to avoid revisiting them. If a cycle is encountered, the algorithm checks if any of the processes in the cycle are waiting for a resource. If all processes in the cycle are waiting, a deadlock is detected.
  1. Deadlock Resolution:
  • Once a deadlock is identified, the system can take appropriate actions to resolve it. Deadlock resolution strategies include:
  • Process Termination: Terminate one or more processes involved in the deadlock to break the circular wait.
  • Resource Preemption: Preempt resources from one or more processes involved in the deadlock and allocate them to other processes.
  • Rollback: Roll back the progress of one or more processes involved in the deadlock to a safe state and re-execute them.
  1. Periodic Deadlock Detection:
  • Deadlock detection is typically performed periodically to identify new deadlocks that may have occurred. This ensures that the system remains responsive and can take appropriate actions in a timely manner.

Deadlock prevention

  • Deadlock prevention is a proactive approach to avoid the occurrence of deadlocks in computer systems. It focuses on structuring the system and its resource allocation in a way that eliminates the possibility of deadlocks. By carefully designing the system and its resource allocation policies, we can prevent deadlocks from happening. Here’s an overview of some common deadlock prevention techniques:
  1. Mutual Exclusion:
  • One of the necessary conditions for a deadlock to occur is the mutual exclusion of resources, meaning that only one process can access a resource at a time. To prevent deadlocks, we can ensure that resources are not mutually exclusive. However, this may not always be feasible, especially for certain types of resources.
  1. Hold and Wait:
  • Another condition for deadlock is the hold and wait condition, where a process holds a resource while waiting for another resource. To prevent deadlocks, we can adopt a strategy where a process must request and acquire all the necessary resources it needs before execution. This way, the process will not hold any resource while waiting for others, reducing the likelihood of deadlocks.
  1. No Preemption:
  • Preemption refers to forcibly taking resources from a process. If a resource is preemptable, it can be taken away from a process and allocated to another. To prevent deadlocks, we can make resources non-preemptable, meaning that once a process holds a resource, it cannot be taken away until the process voluntarily releases it. This ensures that a process will not be left waiting indefinitely for a resource that has been preempted.
  1. Circular Wait:
  • Circular wait occurs when a set of processes are waiting for resources in a circular chain. To prevent deadlocks, we can impose a total ordering of resources and require that processes request resources in a specific order. This breaks the possibility of circular wait and ensures that resources are allocated in a consistent manner.
  1. Resource Allocation Policies:
  • Effective resource allocation policies can also help prevent deadlocks. Some common policies include:
  • Resource Hierarchies: Establishing a hierarchy among resources and requiring processes to acquire resources in a top-down manner can prevent circular wait.
  • Resource Request Timeout: Setting a timeout for resource requests can prevent a process from waiting indefinitely and help identify potential deadlocks.
  • Resource Reclamation: Reclaiming resources from processes that have been idle for a long time can free up resources and reduce the chances of deadlocks.

Deadlock Avoidance

  • Deadlock avoidance is a strategy used to dynamically allocate resources in a way that avoids the possibility of deadlocks. It involves carefully analyzing the resource allocation requests made by processes and making decisions based on the current state of the system to ensure that deadlocks do not occur. Here’s an overview of how deadlock avoidance works:
  1. Resource Allocation Graph:
  • One commonly used technique in deadlock avoidance is the resource allocation graph. This graph represents the current state of the system, with processes as nodes and resources as edges. The graph helps identify potential deadlocks by detecting cycles in the graph.
  1. Safe State:
  • A system is said to be in a safe state if there is a sequence of resource allocations that allows all processes to complete their execution without encountering a deadlock. To ensure a safe state, the resource allocation graph is analyzed to check for the presence of cycles. If no cycles are found, the system is in a safe state.
  1. Banker’s Algorithm:
  • The Banker’s algorithm is a popular deadlock avoidance algorithm. It works by considering the available resources, the maximum resource requirements of each process, and the resources currently allocated to each process. The algorithm simulates the allocation of resources to see if it can reach a safe state. If a safe state is possible, the resources are allocated; otherwise, the process is made to wait.
  1. Resource Request Evaluation:
  • When a process requests additional resources, the system evaluates the request to determine if granting the resources will lead to a potential deadlock. This evaluation is done by considering the current state of the system, the available resources, and the potential future resource requests of other processes. If granting the request will not result in a deadlock, the resources are allocated; otherwise, the process is made to wait.
  1. Resource Release:
  • To avoid deadlocks, it is important to ensure that processes release resources when they are no longer needed. By releasing resources promptly, other processes can utilize them, reducing the chances of resource contention and potential deadlocks.
  1. Resource Allocation Policies:
  • Effective resource allocation policies are crucial for deadlock avoidance. These policies govern how resources are allocated to processes and play a significant role in preventing deadlocks. Some common policies include:
  • Resource Priority: Assigning priorities to processes and allocating resources based on priority can help avoid deadlocks.
  • Resource Preemption: Preempting resources from lower-priority processes and allocating them to higher-priority processes can prevent deadlocks.

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