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Deadlock Detection and Recovery

Introduction to Deadlocks:

Deadlock is a situation in operating systems where two or more processes are unable to proceed because each is waiting for the other to release resources.
It can occur in a system with shared resources and multiple processes.
Deadlock detection and recovery are crucial to ensure system stability and resource availability.

Key Concepts of Deadlock Detection:

Resource Allocation Graph (RAG) is used to represent the allocation of resources and requests by processes.
Cycle detection algorithms are applied to RAG to identify deadlocks.
Detection algorithms help in identifying deadlocks after they have occurred.

Recovery from Deadlock:

Recovery involves breaking the deadlock by preempting resources or terminating processes.
Process termination can be done selectively to minimize disruption.
Resource preemption requires careful handling to avoid data inconsistency.

Resource Allocation Graph Example

Understanding RAG:

A Resource Allocation Graph (RAG) is a directed graph that represents the state of resource allocation in a system. Nodes represent processes and resources, while edges represent allocation and request relationships.


    // Example of a simple RAG
    P1 -> R1
    R1 -> P2
    P2 -> R2

Cycle Detection in RAG:

If a cycle is detected in the RAG, it indicates a potential deadlock involving the processes and resources in the cycle.

Cycle Detection Algorithm

Algorithm Overview:

Cycle detection algorithms, such as Depth First Search (DFS), are used to identify cycles in the Resource Allocation Graph.


    // Pseudocode for cycle detection using DFS
    function detectCycle(graph):
      visited = set()
      for each node in graph:
        if node not in visited:
          if dfs(node, visited, stack):
            return True
      return False

Handling Detected Cycles:

Once a cycle is detected, the system can initiate recovery mechanisms to resolve the deadlock.

Deadlock Recovery Techniques

Process Termination:

Terminate one or more processes involved in the deadlock to break the cycle and release resources.


    // Example of process termination
    terminateProcess(P1);

Resource Preemption:

Preempt resources from some processes and allocate them to others to resolve the deadlock.

Preemption Strategy

Resource Preemption:

Identify resources that can be safely preempted from processes and reallocated to resolve deadlocks.


    // Example of resource preemption
    preemptResource(R1, P2);

Challenges in Preemption:

Preemption must be handled carefully to avoid data inconsistency and ensure system stability.

Example: Deadlock Detection in Banking System

Banker's Algorithm:

The Banker's Algorithm is used to allocate resources safely and avoid deadlocks in systems like banking.


    // Example of Banker's Algorithm for deadlock avoidance
    if (request <= available && request <= need):
      allocateResources(request);

Ensuring Safety:

The algorithm ensures that resources are allocated only if it leads to a safe state, preventing deadlocks.

Example: Deadlock Detection in File Systems

File Locking Mechanism:

In file systems, deadlocks can occur due to improper file locking mechanisms where multiple processes request exclusive access to files.


    // Example of file locking
    lock(file1);
    lock(file2);

Avoiding Deadlocks:

Implementing proper file locking orders and timeouts can help in avoiding deadlocks in file systems.

Example: Deadlock Detection in Database Systems

Transaction Locking:

Deadlocks can occur in databases when transactions lock resources and wait for each other to release locks.


    // Example of transaction locking
    beginTransaction();
    lock(table1);
    lock(table2);

Resolving Deadlocks:

Databases use techniques like timeouts and deadlock detection mechanisms to resolve deadlocks.

Example: Deadlock Detection in Network Systems

Network Resource Allocation:

Deadlocks can occur in network systems when multiple nodes compete for limited communication channels.


    // Example of network resource allocation
    requestChannel(node1, node2);

Preventing Network Deadlocks:

Implementing priority-based channel allocation and timeout mechanisms can prevent deadlocks in network systems.