If a system does not employ either a deadlock prevention or deadlock avoidance algorithm then a deadlock situation may occur. In this case-
- Apply an algorithm to examine state of system to determine whether deadlock has has occurred or not.
- Apply an algorithm to recover from the deadlock. For more refer- Deadlock Recovery
Deadlock Detection Algorithm:
The algorithm employs several time varying data structures:
- Available- A vector of length m indicates the number of available resources of each type.
- Allocation- An n*m matrix defines the number of resources of each type currently allocated to a process. Column represents resource and resource represent process.
- Request- An n*m matrix indicates the current request of each process. If request[i][j] equals k then process Pi is requesting k more instances of resource type Rj.
We treat rows in the matrices Allocation and Request as vectors, we refer them as Allocationi and Requesti.
Steps of Algorithm:
- Let Work and Finish be vectors of length m and n respectively. Initialize Work= Available. For i=0, 1, …., n-1, if Allocationi = 0, then Finish[i] = true; otherwise, Finish[i]= false.
- Find an index i such that both
a) Finish[i] == false
b) Requesti <= Work
If no such i exists go to step 4.
- Work= Work+ Allocationi
Go to Step 2.
- If Finish[i]== false for some i, 0<=i<n, then the system is in a deadlocked state. Moreover, if Finish[i]==false the process Pi is deadlocked.
In this, Work = [0, 0, 0] &
Finish = [false, false, false, false, false]
- i=0 is selected as both Finish = false and [0, 0, 0]<=[0, 0, 0].
- Work =[0, 0, 0]+[0, 1, 0] =>[0, 1, 0] &
Finish = [true, false, false, false, false].
- i=2 is selected as both Finish = false and [0, 0, 0]<=[0, 1, 0].
- Work =[0, 1, 0]+[3, 0, 3] =>[3, 1, 3] &
Finish = [true, false, true, false, false].
- i=1 is selected as both Finish = false and [2, 0, 2]<=[3, 1, 3].
- Work =[3, 1, 3]+[2, 0, 0] =>[5, 1, 3] &
Finish = [true, true, true, false, false].
- i=3 is selected as both Finish = false and [1, 0, 0]<=[5, 1, 3].
- Work =[5, 1, 3]+[2, 1, 1] =>[7, 2, 4] &
Finish = [true, true, true, true, false].
- i=4 is selected as both Finish = false and [0, 0, 2]<=[7, 2, 4].
- Work =[7, 2, 4]+[0, 0, 2] =>[7, 2, 6] &
Finish = [true, true, true, true, true].
- Since Finish is a vector of all true it means there is no deadlock in this example.
- Operating System | Deadlock detection in Distributed systems
- Program for Deadlock free condition in Operating System
- Operating System | Process Management | Deadlock Introduction
- Deadlock Detection And Recovery
- Techniques used in centralized approach of deadlock detection in distributed systems
- Operating System | Dekker's algorithm
- Operating System | Banker's Algorithm
- Operating System | Bakery Algorithm
- Operating System | Program for Next Fit algorithm in Memory Management
- Operating System | Peterson's Algorithm (Using processes and shared memory)
- Operating System | Banker's Algorithm : Print all the safe state (or safe sequences)
- Operating System | Buddy System - Memory allocation technique
- Operating System | Starvation and Aging in Operating Systems
- Operating System | Semaphores in operating system
- Operating System | Introduction of Operating System - Set 1
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