Banker's Algorithm in Operating System

Banker's Algorithm is a resource allocation and deadlock avoidance algorithm used in operating systems. It ensures that a system remains in a safe state by carefully allocating resources to processes while avoiding unsafe states that could lead to deadlocks.

• The Banker's Algorithm is a smart way for computer systems to manage how programs use resources, like memory or CPU time.

• It helps prevent situations where programs get stuck and can not finish their tasks. This condition is known as deadlock.

• By keeping track of what resources each program needs and what's available, the banker algorithm makes sure that programs only get what they need in a safe order.

Why Banker's Algorithm is Named So? 

The banker's algorithm is named so because it is used in the banking system to check whether a loan can be sanctioned to a person or not. Suppose there are n number of account holders in a bank and the total sum of their money is S. Let us assume that the bank has a certain amount of money Y. If a person applies for a loan then the bank first subtracts the loan amount from the total money that the bank has (Y) and if the remaining amount is greater than S then only the loan is sanctioned. It is done because if all the account holders come to withdraw their money then the bank can easily do it.

It also helps the OS to successfully share the resources between all the processes. The banker's algorithm uses the notation of a safe allocation state to ensure that granting a resource request cannot lead to a deadlock either immediately or in the future. In other words, the bank would never allocate its money in such a way that it can no longer satisfy the needs of all its customers. The bank would try to be in a safe state always.

Components of the Banker's Algorithm

The following Data structures are used to implement the Banker’s Algorithm:

Let ‘n’ be the number of processes in the system and ‘m’ be the number of resource types.

1. Available

• It is a 1-D array of size ‘m’ indicating the number of available resources of each type.
• Available[ j ] = k means there are ‘k’ instances of resource type R_j

2. Max

• It is a 2-d array of size ‘n*m’ that defines the maximum demand of each process in a system.
• Max[ i, j ] = k means process P_i may request at most ‘k’ instances of resource type R_j.

3. Allocation

• It is a 2-d array of size ‘n*m’ that defines the number of resources of each type currently allocated to each process.
• Allocation[ i, j ] = k means process P_i is currently allocated ‘k’ instances of resource type R_j

4. Need

• It is a 2-d array of size ‘n*m’ that indicates the remaining resource need of each process.
• Need [ i,   j ] = k means process P_i currently needs ‘k’ instances of resource type R_j
• Need [ i,   j ] = Max [ i,   j ] – Allocation [ i,   j ]

Allocation specifies the resources currently allocated to process P_i and Need specifies the additional resources that process P_i may still request to complete its task.

Banker's algorithm consists of a Safety algorithm and a Resource request algorithm.

Banker's Algorithm

1. Active := Running U Blocked
for k = 1 to r:
New_request[k] := Requested_resources[requesting_process, k]

2. Simulated_allocation := Allocated_resources
for k = 1 to r: // Compute projected allocation state
Simulated_allocation[requesting_process, k] :=
Simulated_allocation[requesting_process, k] + New_request[k]

3. Feasible := true
for k = 1 to r: // Check if projected allocation state is feasible
if Total_resources[k] < Simulated_total_alloc[k]:
Feasible := false

4. if Feasible = true:

while set Active contains a process P1 such that:
for all k, Total_resources[k] - Simulated_total_alloc[k] >=
Max_need[P1, k] - Simulated_allocation[P1, k]:
Delete P1 from Active
for k = 1 to r:
Simulated_total_alloc[k] :=
Simulated_total_alloc[k] - Simulated_allocation[P1, k]

5. if set Active is empty:

for k = 1 to r: // Delete the request from pending requests
Requested_resources[requesting_process, k] := 0
for k = 1 to r: // Grant the request
Allocated_resources[requesting_process, k] :=
Allocated_resources[requesting_process, k] + New_request[k]
Total_alloc[k] := Total_alloc[k] + New_request[k]

Safety Algorithm

The algorithm for finding out whether or not a system is in a safe state can be described as follows: 

1. Let Work and Finish be vectors of length 'm' and 'n' respectively. 
Initialize: Work = Available 
Finish[i] = false; for i=1, 2, 3, 4....n
2. Find an i such that both 
2.1 Finish[i] = false 
2.2 Need_i <= Work 
if no such i exists goto step (4)
3. Work = Work + Allocation[i] 
Finish[i] = true 
goto step (2)
4. if Finish [i] = true for all i 
then the system is in a safe state 

Resource-Request Algorithm

Let Request_i be the request array for process P_i. Request_i [j] = k means process P_i wants k instances of resource type R_j. When a request for resources is made by process P_i, the following actions are taken:

1. If Request_i <= Need_i 
Goto step (2) ; otherwise, raise an error condition, since the process has exceeded its maximum claim.
2. If Request_i <= Available 
Goto step (3); otherwise, P_i must wait, since the resources are not available.
3. Have the system pretend to have allocated the requested resources to process Pi by modifying the state as 
follows: 
Available = Available - Requesti 
Allocation_i = Allocation_i + Request_i 
Need_i = Need_i- Request_i

Example: Considering a system with five processes P₀ through P₄ and three resources of type A, B, C. Resource type A has 10 instances, B has 5 instances and type C has 7 instances. Suppose at time t₀ following snapshot of the system has been taken:

Q.1 What will be the content of the Need matrix?

Need [i, j] = Max [i, j] – Allocation [i, j]
So, the content of Need Matrix is:

Q.2 Is the system in a safe state? If Yes, then what is the safe sequence?

Applying the Safety algorithm on the given system,

Q.3 What will happen if process P1 requests one additional instance of resource type A and two instances of resource type C?

We must determine whether this new system state is safe. To do so, we again execute Safety algorithm on the above data structures. 

Hence the new system state is safe, so we can immediately grant the request for process  P_{1 .}

Unsafe State in The Bankers Algorithm

In the context of the Banker's Algorithm, an unsafe state refers to a situation in which, all processes in a system currently hold resources within their maximum needs, there is no guarantee that the system can avoid a deadlock in the future. This state is contrasted with a safe state, where there exists a sequence of resource allocation and deallocation that guarantees that all processes can complete without leading to a deadlock.

Key Concepts in Banker's Algorithm

• Safe State: There exists at least one sequence of processes such that each process can obtain the needed resources, complete its execution, release its resources, and thus allow other processes to eventually complete without entering a deadlock.
• Unsafe State: Even though the system can still allocate resources to some processes, there is no guarantee that all processes can finish without potentially causing a deadlock.

Example of Unsafe State

Consider a system with three processes (P1, P2, P3) and 6 instances of a resource. Let's say:

• The available instances of the resource are: 1
• The current allocation of resources to processes is:
  • P1: 2
  • P2: 3
  • P3: 1
• The maximum demand (maximum resources each process may eventually request) is:
  • P1: 4
  • P2: 5
  • P3: 3

In this situation:

Need = Maximum Demand - Allocation

• P1 may need 2 more resources to complete.
• P2 may need 2 more resource to complete.
• P3 may need 2 more resources to complete.

However, there is only 1 resource available. Even though none of the processes are currently deadlocked, the system is in an unsafe state because there is no sequence of resource allocation that guarantees all processes can complete.

Code Implementation

• Program for Banker’s Algorithm | Set 1 (Safety Algorithm)