Resource Allocation Graph | Operating System

Resource Allocation Graph-

 

Resource Allocation Graph (RAG) is a graph that represents the state of a system pictorially.

 

It gives complete information about the state of a system such as-

• How many processes exist in the system?
• How many instances of each resource type exist?
• How many instances of each resource type are allocated?
• How many instances of each resource type are still available?
• How many instances of each resource type are held by each process?
• How many instances of each resource type does each process need for execution?

 

Components Of RAG-

 

There are two major components of a Resource Allocation Graph-

1. Vertices
2. Edges

 

Vertices-

 

There are following types of vertices in a Resource Allocation Graph-

 

 

1. Process Vertices
2. Resource Vertices

 

Process Vertices-

 

• Process vertices represent the processes.
• They are drawn as a circle by mentioning the name of process inside the circle.

 

Resource Vertices-

 

• Resource vertices represent the resources.
• Depending on the number of instances that exists in the system, resource vertices may be single instance or multiple instance.
• They are drawn as a rectangle by mentioning the dots inside the rectangle.
• The number of dots inside the rectangle indicates the number of instances of that resource existing in the system.

 

Edges-

 

There are two types of edges in a Resource Allocation Graph-

 

 

1. Assign Edges
2. Request Edges

 

Assign Edges-

 

• Assign edges represent the assignment of resources to the processes.
• They are drawn as an arrow where the head of the arrow points to the process and tail of the process points to the instance of the resource.

 

Request Edges-

 

• Request edges represent the waiting state of processes for the resources.
• They are drawn as an arrow where the head of the arrow points to the instance of the resource and tail of the process points to the process.
• If a process requires ‘n’ instances of a resource type, then ‘n’ assign edges will be drawn.

 

Example Of RAG-

 

The following diagram represents a Resource Allocation Graph-

 

 

It gives the following information-

• There exist three processes in the system namely P1, P2 and P3.
• There exist two resources in the system namely R1 and R2.
• There exists a single instance of resource R1 and two instances of resource R2.
• Process P1 holds one instance of resource R1 and is waiting for an instance of resource R2.
• Process P2 holds one instance of resource R2 and is waiting for an instance of resource R1.
• Process P3 holds one instance of resource R2 and is not waiting for anything.

 

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Next Article- Practice Problems On Resource Allocation Graph

 

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