Example of a Resource Allocation Graph


Resource allocation graphs are drawn in order to see the allocatioand resources easily. In these graphs, processes are represented are represented by boxes. Resource boxes have some number available number of that resource, that is number of instances.

• If the resource allocation graph contains no cycles then there is no deadlock in the system at that instance.
• If the resource allocation graph contains a cycle then a deadlock may exist.
• If there is a cycle, and the cycle involves only resources which have a single instance, then a deadlock has occurred.






Resource Allocation Graph With A Deadlock


*In addition to the request and assignment edges, a claim edge is also introduced.
*Claim edge Pi ® Rj indicated that process Pj may request resource Rj in future; represented by a dashed line.
*Claim edge converts to request edge when a process requests a resource.
*When a resource is released by a process, assignment edge reconverts to a claim edge.
*Resources must be claimed a priori in the system. That is, before a process starts executing, all of its claim edges must already appear in the resource-allocation graph.
*Suppose that process Pi requests resource Rj. The request can be granted only if converting the request edge if converting the request edge Pi®Rj to an assignment edge does not result in a cycle in the resource-allocation graph. That is we use a cycle detection algorithm is used. If no cycle exits, the process Pi will have to wait.

Resource Allocation Graph With A Cycle But No Deadlock

*2 resources "R1, R2"
*4 processes "P1, P2, P3, P4"
*P1 and P3 is requesting a process to R1 and R2 respectively.
*P2 and P3 holds an instance of R1
*P1 and P4 holds an instance of R2
*There is no DeadLock













Resource-Allocation Graph For Deadlock Avoidance


The Resource Allocation Graph (RAG) is composed of 2 processes and 2 resources.
· P1 holds an instance of R1
· P2 requests an instance of R1
· P1 and P2 may request an instance of R2

Unsafe State In Resource-Allocation Graph


*the graph shows that R1 is holding the instances of P1, then P1 is requesting for a resource in R2, then R2 is holding the instances of P2 and P2 is requesting instance of R1.