Resource Allocation Graph (RAG)

A Resource Allocation Graph (RAG) is a directed graph used in operating systems to visualize how processes are using resources and which resources they are waiting for. It helps in detecting and preventing deadlocks.

1. Components of a RAG

  1. Nodes

    • Process Nodes (Circles P1, P2, …): Represent active processes in the system.

    • Resource Nodes (Rectangles R1, R2, …): Represent resource types. Dots inside the rectangle indicate instances of that resource type. For example, if a resource type has 3 instances, there will be 3 dots inside its rectangle.

  2. Edges

    • Request Edge (Process to Resource): A directed edge from a process P to a resource R (P \to R). This means process P has requested an instance of resource type R and is currently waiting for it.

    • Assignment Edge (Resource to Process): A directed edge from a resource R to a process P (R \to P). This means an instance of resource type R has been allocated (assigned) to process P.

2. Deadlock Detection Using RAG

  1. Cycle Detection: A crucial aspect of RAGs is detecting cycles.

    • If the RAG contains a cycle, a deadlock may exist.

    • Important Conditions:

      • If each resource type in the system has only one instance, then a cycle in the RAG implies that a deadlock has occurred.

      • If a resource type has multiple instances, then a cycle in the RAG does not necessarily imply a deadlock, but it indicates the possibility of a deadlock. Further analysis (like using a Banker's algorithm) might be needed in such cases to confirm a deadlock.