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Banker’s Algorithm in Operating System

  • Difficulty Level : Medium
  • Last Updated : 08 Oct, 2021
 

The banker’s algorithm is a resource allocation and deadlock avoidance algorithm that tests for safety by simulating the allocation for predetermined maximum possible amounts of all resources, then makes an “s-state” check to test for possible activities, before deciding whether allocation should be allowed to continue.
Why Banker’s algorithm is named so? 
Banker’s algorithm is named so because it is used in banking system to check whether 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. If a person applies for a loan then the bank first subtracts the loan amount from the total money that bank has and if the remaining amount is greater than S then only the loan is sanctioned. It is done because if all the account holders comes to withdraw their money then the bank can easily do it.
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 safe state always.
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 resources types.
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 Rj

Max : 
 

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  • 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 Pi may request at most ‘k’ instances of resource type Rj.

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 Pi is currently allocated ‘k’ instances of resource type Rj

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 Pi currently need ‘k’ instances of resource type Rj
  • Need [ i,   j ] = Max [ i,   j ] – Allocation [ i,   j ]

 

 

Allocationi specifies the resources currently allocated to process Pi and Needi specifies the additional resources that process Pi may still request to complete its task.
Banker’s algorithm consists of Safety algorithm and Resource request algorithm
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 
a) Finish[i] = false 
b) Needi <= 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 Requesti be the request array for process Pi. Requesti [j] = k means process Pi wants k instances of resource type Rj. When a request for resources is made by process Pi, the following actions are taken:
 

1) If Requesti <= Needi 
Goto step (2) ; otherwise, raise an error condition, since the process has exceeded its maximum claim.
2) If Requesti <= Available 
Goto step (3); otherwise, Pi 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 
Allocationi = Allocationi + Requesti 
Needi = Needi– Requesti
 

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:
 

safety



Question1. 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:
 

unnamed

Question2.  Is the system in a safe state? If Yes, then what is the safe sequence?
Applying the Safety algorithm on the given system,
 

questionsolved

Question3. What will happen if process Prequests one additional instance of resource type A and two instances of resource type C?
 

allocation

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

Q31

Hence the new system state is safe, so we can immediately grant the request for process  P1 .
Code for Banker’s Algorithm
 

C++




// Banker's Algorithm
#include <iostream>
using namespace std;
  
int main()
{
    // P0, P1, P2, P3, P4 are the Process names here
  
  int n, m, i, j, k;
  n = 5; // Number of processes
  m = 3; // Number of resources
  int alloc[5][3] = { { 0, 1, 0 }, // P0 // Allocation Matrix
                     { 2, 0, 0 }, // P1
                     { 3, 0, 2 }, // P2
                     { 2, 1, 1 }, // P3
                     { 0, 0, 2 } }; // P4
  
  int max[5][3] = { { 7, 5, 3 }, // P0 // MAX Matrix
                   { 3, 2, 2 }, // P1
                   { 9, 0, 2 }, // P2
                   { 2, 2, 2 }, // P3
                   { 4, 3, 3 } }; // P4
  
  int avail[3] = { 3, 3, 2 }; // Available Resources
  
  int f[n], ans[n], ind = 0;
  for (k = 0; k < n; k++) {
    f[k] = 0;
  }
  int need[n][m];
  for (i = 0; i < n; i++) {
    for (j = 0; j < m; j++)
      need[i][j] = max[i][j] - alloc[i][j];
  }
  int y = 0;
  for (k = 0; k < 5; k++) {
    for (i = 0; i < n; i++) {
      if (f[i] == 0) {
  
        int flag = 0;
        for (j = 0; j < m; j++) {
          if (need[i][j] > avail[j]){
            flag = 1;
            break;
          }
        }
  
        if (flag == 0) {
          ans[ind++] = i;
          for (y = 0; y < m; y++)
            avail[y] += alloc[i][y];
          f[i] = 1;
        }
      }
    }
  }
  
  cout << "Following is the SAFE Sequence" << endl;
  for (i = 0; i < n - 1; i++)
    cout << " P" << ans[i] << " ->";
  cout << " P" << ans[n - 1] <<endl;
  
    return (0);
}
  
// This code is contributed by SHUBHAMSINGH10

C




// Banker's Algorithm
#include <stdio.h>
int main()
{
    // P0, P1, P2, P3, P4 are the Process names here
  
    int n, m, i, j, k;
    n = 5; // Number of processes
    m = 3; // Number of resources
    int alloc[5][3] = { { 0, 1, 0 }, // P0    // Allocation Matrix
                        { 2, 0, 0 }, // P1
                        { 3, 0, 2 }, // P2
                        { 2, 1, 1 }, // P3
                        { 0, 0, 2 } }; // P4
  
    int max[5][3] = { { 7, 5, 3 }, // P0    // MAX Matrix
                      { 3, 2, 2 }, // P1
                      { 9, 0, 2 }, // P2
                      { 2, 2, 2 }, // P3
                      { 4, 3, 3 } }; // P4
  
    int avail[3] = { 3, 3, 2 }; // Available Resources
  
    int f[n], ans[n], ind = 0;
    for (k = 0; k < n; k++) {
        f[k] = 0;
    }
    int need[n][m];
    for (i = 0; i < n; i++) {
        for (j = 0; j < m; j++)
            need[i][j] = max[i][j] - alloc[i][j];
    }
    int y = 0;
    for (k = 0; k < 5; k++) {
        for (i = 0; i < n; i++) {
            if (f[i] == 0) {
  
                int flag = 0;
                for (j = 0; j < m; j++) {
                    if (need[i][j] > avail[j]){
                        flag = 1;
                         break;
                    }
                }
  
                if (flag == 0) {
                    ans[ind++] = i;
                    for (y = 0; y < m; y++)
                        avail[y] += alloc[i][y];
                    f[i] = 1;
                }
            }
        }
    }
  
    printf("Following is the SAFE Sequence\n");
    for (i = 0; i < n - 1; i++)
        printf(" P%d ->", ans[i]);
    printf(" P%d", ans[n - 1]);
  
    return (0);
  
    // This code is contributed by Deep Baldha (CandyZack)
}

Java




//Java Program for Bankers Algorithm
public class GfGBankers
{
    int n = 5; // Number of processes 
    int m = 3; // Number of resources
    int need[][] = new int[n][m];
    int [][]max;
    int [][]alloc;
    int []avail;
    int safeSequence[] = new int[n];
  
    void initializeValues()
    {
    // P0, P1, P2, P3, P4 are the Process names here 
    // Allocation Matrix 
    alloc = new int[][] { { 0, 1, 0 }, //P0   
                  { 2, 0, 0 }, //P1
                  { 3, 0, 2 }, //P2
                  { 2, 1, 1 }, //P3
                  { 0, 0, 2 } }; //P4
            
    // MAX Matrix
    max = new int[][] { { 7, 5, 3 }, //P0
             { 3, 2, 2 }, //P1
             { 9, 0, 2 }, //P2
             { 2, 2, 2 }, //P3 
             { 4, 3, 3 } }; //P4
      
    // Available Resources  
     avail = new int[] { 3, 3, 2 }; 
    }
  
    void isSafe()
    {
    int count=0;
      
    //visited array to find the already allocated process
    boolean visited[] = new boolean[n]; 
    for (int i = 0;i < n; i++)
    {
        visited[i] = false;
    }
          
    //work array to store the copy of available resources
    int work[] = new int[m];    
    for (int i = 0;i < m; i++)
    {
        work[i] = avail[i];
    }
  
    while (count<n)
    {
        boolean flag = false;
        for (int i = 0;i < n; i++)
        {
            if (visited[i] == false)
             {
            int j;
            for (j = 0;j < m; j++)
            {        
                if (need[i][j] > work[j])
                break;
            }
            if (j == m)
            {
               safeSequence[count++]=i;
               visited[i]=true;
               flag=true;
                          
                for (j = 0;j < m; j++)
                {
                work[j] = work[j]+alloc[i][j];
                }
            }
             }
        }
        if (flag == false)
        {
            break;
        }
    }
    if (count < n)
    {
        System.out.println("The System is UnSafe!");
    }
    else
    {
        //System.out.println("The given System is Safe");
        System.out.println("Following is the SAFE Sequence");
                for (int i = 0;i < n; i++)
        {
            System.out.print("P" + safeSequence[i]);
            if (i != n-1)
            System.out.print(" -> ");
        }
    }
    }
      
    void calculateNeed()
    {
    for (int i = 0;i < n; i++)
    {
        for (int j = 0;j < m; j++)
         {
        need[i][j] = max[i][j]-alloc[i][j];
         }
    }        
    }
  
    public static void main(String[] args)
    {  
      int i, j, k; 
      GfGBankers gfg = new GfGBankers();
           
      gfg.initializeValues();    
      //Calculate the Need Matrix    
      gfg.calculateNeed();            
              
       // Check whether system is in safe state or not 
      gfg.isSafe();        
    }
}

Python3




# Banker's Algorithm
  
# Driver code:
if __name__=="__main__":
      
    # P0, P1, P2, P3, P4 are the Process names here
    n = 5 # Number of processes
    m = 3 # Number of resources
      
    # Allocation Matrix
    alloc = [[0, 1, 0 ],[ 2, 0, 0 ],
            [3, 0, 2 ],[2, 1, 1] ,[ 0, 0, 2]]
      
    # MAX Matrix 
    max = [[7, 5, 3 ],[3, 2, 2 ],
            [ 9, 0, 2 ],[2, 2, 2],[4, 3, 3]]
      
    avail = [3, 3, 2] # Available Resources
      
    f = [0]*n
    ans = [0]*n
    ind = 0
    for k in range(n):
        f[k] = 0
          
    need = [[ 0 for i in range(m)]for i in range(n)]
    for i in range(n):
        for j in range(m):
            need[i][j] = max[i][j] - alloc[i][j]
    y = 0
    for k in range(5):
        for i in range(n):
            if (f[i] == 0):
                flag = 0
                for j in range(m):
                    if (need[i][j] > avail[j]):
                        flag = 1
                        break
                  
                if (flag == 0):
                    ans[ind] = i
                    ind += 1
                    for y in range(m):
                        avail[y] += alloc[i][y]
                    f[i] = 1
                      
    print("Following is the SAFE Sequence")
      
    for i in range(n - 1):
        print(" P", ans[i], " ->", sep="", end="")
    print(" P", ans[n - 1], sep="")
  
# This code is contributed by SHUBHAMSINGH10

C#




// C# Program for Bankers Algorithm
using System;
using System.Collections.Generic;
      
class GFG
{
static int n = 5; // Number of processes 
static int m = 3; // Number of resources
int [,]need = new int[n, m];
int [,]max;
int [,]alloc;
int []avail;
int []safeSequence = new int[n];
  
void initializeValues()
{
    // P0, P1, P2, P3, P4 are the Process 
    // names here Allocation Matrix 
    alloc = new int[,] {{ 0, 1, 0 }, //P0 
                        { 2, 0, 0 }, //P1
                        { 3, 0, 2 }, //P2
                        { 2, 1, 1 }, //P3
                        { 0, 0, 2 }};//P4
              
    // MAX Matrix
    max = new int[,] {{ 7, 5, 3 }, //P0
                        { 3, 2, 2 }, //P1
                      { 9, 0, 2 }, //P2
                      { 2, 2, 2 }, //P3 
                      { 4, 3, 3 }};//P4
      
    // Available Resources 
    avail = new int[] { 3, 3, 2 }; 
}
  
void isSafe()
{
    int count = 0;
      
    // visited array to find the 
    // already allocated process
    Boolean []visited = new Boolean[n]; 
    for (int i = 0; i < n; i++)
    {
        visited[i] = false;
    }
          
    // work array to store the copy of 
    // available resources
    int []work = new int[m]; 
    for (int i = 0; i < m; i++)
    {
        work[i] = avail[i];
    }
      
    while (count<n)
    {
        Boolean flag = false;
        for (int i = 0; i < n; i++)
        {
            if (visited[i] == false)
            {
                int j;
                for (j = 0; j < m; j++)
                {     
                    if (need[i, j] > work[j])
                    break;
                }
                if (j == m)
                {
                    safeSequence[count++] = i;
                    visited[i] = true;
                    flag = true;
                    for (j = 0; j < m; j++)
                    {
                        work[j] = work[j] + alloc[i, j];
                    }
                }
            }
        }
        if (flag == false)
        {
            break;
        }
    }
    if (count < n)
    {
        Console.WriteLine("The System is UnSafe!");
    }
    else
    {
        //System.out.println("The given System is Safe");
        Console.WriteLine("Following is the SAFE Sequence");
        for (int i = 0; i < n; i++)
        {
            Console.Write("P" + safeSequence[i]);
            if (i != n - 1)
            Console.Write(" -> ");
        }
    }
}
  
void calculateNeed()
{
    for (int i = 0;i < n; i++)
    {
        for (int j = 0;j < m; j++)
        {
            need[i, j] = max[i, j] - alloc[i, j];
        }
    }     
}
  
// Driver Code
public static void Main(String[] args)
    GFG gfg = new GFG();
          
    gfg.initializeValues(); 
      
    // Calculate the Need Matrix 
    gfg.calculateNeed();         
              
    // Check whether system is in
    // safe state or not 
    gfg.isSafe();     
}
}
  
// This code is contributed by Rajput-Ji

Javascript

    Limitations of Banker Algorithm:

  • As the processes enter the system, they must predict the maximum number of resources needed which is not impractical to determine.
  • In this algorithm, the number of processes remain fixed which is not possible in interactive systems.
  • This algorithm requires that there should be a fixed number of resources to allocate. If a device breaks and becomes suddenly unavailable the algorithm would not work.
  • Overhead cost incurred by the algorithm can be high when there are many processes and resources because it has to be invoked for every processes.



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