Deadlock Detection using Resource Allocation Graphs (RAGs): Identifying Deadlock Situations in Operating Systems

Learn how resource allocation graphs (RAGs) are used to detect deadlocks in operating systems. This guide explains how to construct RAGs, interpret them to identify deadlock cycles (in systems with single and multiple resource instances), and use RAGs for deadlock detection.

Deadlock Detection using Resource Allocation Graphs (RAG)

Deadlock Detection in RAGs

A Resource Allocation Graph (RAG) is a visual tool used to detect deadlocks in a system. In a system with single instances of each resource, a cycle in the RAG is a definitive sign of a deadlock. This means that processes are waiting for each other to release resources, creating a circular dependency.

However, things get a bit more complex when dealing with multiple instances of a resource. In such cases, a cycle in the RAG is a necessary but not a sufficient condition for a deadlock. A cycle might exist, but the system might still not be deadlocked if there are enough available resources to satisfy the waiting processes.

Example: Deadlock Detection

Let's consider an example with three processes (P1, P2, P3) and three resources (R1, R2, R3), each resource having only one instance. If a cycle is found in the RAG, it indicates a deadlock.

Allocation Matrix

The allocation matrix shows which resources are currently held by each process.

Request Matrix

The request matrix shows which resources each process is currently requesting.

Available Resources: (0, 0, 0)

Since there are no available resources and each process is holding a resource and requesting another, the system is deadlocked. This is confirmed by the cycle detected earlier in the RAG.

C Code Example: Declaring a Character Variable

char ch = 'a';

Example Output

She said "Hello!" to me.

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