Consider a system with five processes P₀, through P₄, and three resource types A, B and C. Resource type A has seven instances, resource type B has two instances and resource type C has six instances suppose at time T₀ we Have the following allocation.

Process	Allocation			Request			Available
	A	B	C	A	B	C	A	B	C
P0	0	1	0	0	0	0	0	0	0
P1	2	0	0	2	0	2
P2	3	0	3	0	0	0
P3	2	1	1	1	0	0
P4	0	2	2	0	0	2

 

If we implement Deadlock detection algorithm we claim that system is ______.

The correct answer is "option 4".

CONCEPT: 

Deadlock is a condition in which a process P1 wants a resource that is held by any other process P2 & process P2 wants a resource that is held by process P1.

No processes that are in deadlock can further proceed and get blocked.

EXPLANATION: 

Deadlock detection algorithm is used to detect any chance of deadlock while allocating resources to the processes.

If any process will cause or has a chance of creating a deadlock, then this algorithm will not give resources to that particular process causing deadlock.

Steps of deadlock detection algorithm:

1.Deadlock detection algorithm will check that process requesting resources are available.

Request <= Available

2. If the above condition gets satisfied then the algorithm will allocate the resources demanded by the process.

Also, after fulfilling the request, availability gets added with the allocation of that process.

available = available + allocation

3. Else, the algorithm will keep the process till its next turn.

CALCULATION: 

Step 1.

Process P0 is requesting instances (0,0,0) and it is available

(0,0,0) <= (0,0,0)

so, P0 will get the resources:

available = (0,0,0) + (0,1,0) = (0,1,0)                { A - 0 instance, B - 1 instance , C - 0 instance  }

Step 2.

Process P1 is requesting instances (2,0,2) it is not available 

(2,0,2) > (0,1,0)                                       { request < available }

so, P2 will wait.

Step 3.

Process P2 is requesting instances (0,0,0) and it is available

(0,0,0) < (0,1,0)

so, P2 will get the resources:

available = (0,1,0) + (3,0,3) = (3,1,3)                { A - 3 instance, B - 1 instance , C - 3 instance  }

Step 4.

Process P3 is requesting instances (1,0,0) and it is available 

(1,0,0) < (3,1,3)

so, P3 will get the resources:

available = (3,1,3) + (2,1,1) = (5,2,4)                { A - 5 instance, B - 2 instance , C - 4 instance  }

Step 5.

Process P4 is requesting instances (0,0,2) and it is available 

(0,0,2) < (5,2,4)

so, P4 will get the resources:

available = (5,2,4) + (0,0,2) = (5,2,6)                { A - 5 instance, B - 2 instance , C - 6 instance  }

Step 6.

Process P1 is requesting instances (2,0,2) and it is available 

(2,0,2) < (5,2,6)

so, P1 will get the resources:

available = (5,2,6) + (2,0,0) = (7,2,6)                { A - 7 instance, B - 2 instance , C - 6 instance  }

Also, detection algorithm create safe sequence i.e. sequence of resource allocated processes without any deadlock.

Here, safe sequence is <P0,P2,P3,P4,P1>

Hence, the system is not in a deadlock state.

Important Points

If a system is in the safe state then the system is not in a deadlock state but if the system is not in the safe state then the system may or may not be in a deadlock state.

Mistake Points

Always compare as follows:

(1,2,3) < (1,0,0) means 1<1, 2<0 & 3<0 

Never check as :123<100