Page 135 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes edge Rj + Pi is called an assignment edge. Pictorially, we represent each process Pi as
a circle, and each resource type Ri as a square. Since resource type Ri may have more
than one instance, we represent each such instance as a dot within the square. Note
that a request edge points to only the square Xi, whereas an assignment edge must also
designate one of the dots in the square. When process Pi requests an instance of resource
type Rj, a request edge is inserted in the resource-allocation graph. When this request
can be fulfilled, the request edge is instantaneously transformed to an assignment edge.
When the process no longer needs access to the resource it releases the resource, and as
a result the assignment edge is deleted. The resource-allocation graph shown in Figure
depicts the following situation.
The sets P, R, and E:

• P = {P , P , P }
2
1
3
• R = {R , R , R , R }
3
2
1
4
• E = {P → R , P → R , R → P , R → P , R → P , R → P }
1
2
2
1
3
2
2
3
2
1
1
3
Resource instances:
• One instance of resource type R1
• Two instances of resource type R2
• One instance of resource type R3
• Three instances of resource type R4
Figure 4.10: Resource-Allocation Graphs

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