Deadlock Detection Algorithm

Introduction

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 occurred or not.
• Apply an algorithm to recover from the deadlock. For more refer- Deadlock Recovery

Deadlock Avoidance Algorithm/ Bankers Algorithm 

The algorithm employs several times 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.

Now, Bankers algorithm includes a Safety Algorithm / Deadlock Detection Algorithm 

The algorithm for finding out whether a system is in a safe state can be described as follows:

Steps of Algorithm: 

1. Let Work and Finish be vectors of length m and n respectively. Initialize Work= Available. For i=0, 1, ...., n-1, if Request_i = 0, then Finish[i] = true; otherwise, Finish[i]= false.
2. 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+ Allocation_i
   Finish[i]= true
   Go to Step 2.
3. 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.

For example, 

1. In this, Work = [0, 0, 0] &
   Finish = [false, false, false, false, false]
2. i = 0 is selected as both Finish[0] = false and [0, 0, 0] <= [0, 0, 0].
3. Work =[0, 0, 0] + [0, 1, 0] => [0, 1, 0] &
   Finish = [true, false, false, false, false].
4. i = 2 is selected as both Finish[2] = false and [0, 0, 0] <= [0, 1, 0].
5. Work = [0, 1, 0] + [3, 0, 3] => [3, 1, 3] &
   Finish = [true, false, true, false, false].
6. i = 1 is selected as both Finish[1] = false and [2, 0, 2] <= [3, 1, 3].
7. Work = [3, 1, 3] + [2, 0, 0] => [5, 1, 3] &
   Finish = [true, true, true, false, false].
8. i = 3 is selected as both Finish[3] = false and [1, 0, 0] <= [5, 1, 3].
9. Work =[5, 1, 3] + [2, 1, 1] => [7, 2, 4] &
   Finish = [true, true, true, true, false].
10. i = 4 is selected as both Finish[4] = false and [0, 0, 2] <= [7, 2, 4].
11. 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.