Question

Consider the following snapshot of a system that has four resource types: A, B,
C, and D and five processes, P0, P1, P2, P3, and P4.

Allocation Max
A B C D A B C D
P0 3 0 1 5 5 1 1 7
P1 2 2 1 0 3 2 1 1
P2 3 1 2 1 3 3 2 1
P3 0 5 1 0 4 6 1 2
P4 4 2 1 3 6 3 2 5

Consider the situation in which Available = (0, 2, 1, 2). Give an example of a request
from P0 that can be served from the available resources, does not violate the maximum resources declared by the process but causes the system to be in a non-safe state as defined by the banker’s algorithm. Show all work (how the Available array changes after each step of the algorithm).

Answer 1

Greetings!!

At present, the allocation,max,need and available resouces are as shown below:

• At this state, the available resouces are 0,2,1,2 and the Need of P2 can be satisfied because Need<=Available

ie 0,2,0,0 <= 0,2,1,2

• Then process P2 continues and after the execution it releases all the resources which were allocated to it. ie 3,1,2,1 are added back to the Available as shown in red colour.

• With the Available resources of 3,3,3,3 process P0 can start execution. After the execution of P0,3,0,1,5 resources will be added back to the Available

Now P1 can proceed followed by P3 and P4

Hence all the processes completed their execution and the system is in safe state and the safe sequence is P2,P0,P1,P3 and P4.

No other request only by P0 will not make the system unsafe state. If more than one process change their request an unsafe state can occur.

Hope this helps