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Program for Best Fit algorithm in Memory Management

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Prerequisite : Partition allocation methods
Best fit allocates the process to a partition which is the smallest sufficient partition among the free available partitions. 
Example: 
 

Input : blockSize[]   = {100, 500, 200, 300, 600};
        processSize[] = {212, 417, 112, 426};
Output:
Process No.    Process Size    Block no.
 1        212        4
 2        417        2
 3        112        3
 4        426        5


 


 

first-fit


 

Implementation:
1- Input memory blocks and processes with sizes.
2- Initialize all memory blocks as free.
3- Start by picking each process and find the
   minimum block size that can be assigned to
   current process i.e., find min(bockSize[1], 
   blockSize[2],.....blockSize[n]) > 
   processSize[current], if found then assign 
   it to the current process.
5- If not then leave that process and keep checking
   the further processes.


Below is implementation. 

C++

// C++ implementation of Best - Fit algorithm
#include<iostream> 
using namespace std;
  
// Method to allocate memory to blocks as per Best fit algorithm 
void bestFit(int blockSize[], int m, int processSize[], int n) 
    // Stores block id of the block allocated to a process 
    int allocation[n]; 
  
    // Initially no block is assigned to any process 
    for (int i = 0; i < n; i++) 
        allocation[i] = -1; 
  
    // pick each process and find suitable blocks 
    // according to its size ad assign to it 
    for (int i = 0; i < n; i++) 
    
        // Find the best fit block for current process 
        int bestIdx = -1; 
        for (int j = 0; j < m; j++) 
        
            if (blockSize[j] >= processSize[i]) 
            
                if (bestIdx == -1) 
                    bestIdx = j; 
                else if (blockSize[bestIdx] > blockSize[j]) 
                    bestIdx = j; 
            
        
  
        // If we could find a block for current process 
        if (bestIdx != -1) 
        
            // allocate block j to p[i] process 
            allocation[i] = bestIdx; 
  
            // Reduce available memory in this block. 
            blockSize[bestIdx] -= processSize[i]; 
        
    
  
    cout << "\nProcess No.\tProcess Size\tBlock no.\n"
    for (int i = 0; i < n; i++) 
    
        cout << " " << i+1 << "\t\t" << processSize[i] << "\t\t"
        if (allocation[i] != -1) 
            cout << allocation[i] + 1; 
        else
            cout << "Not Allocated"
        cout << endl; 
    
  
// Driver Method 
int main() 
    int blockSize[] = {100, 500, 200, 300, 600}; 
    int processSize[] = {212, 417, 112, 426}; 
    int m = sizeof(blockSize) / sizeof(blockSize[0]); 
    int n = sizeof(processSize) / sizeof(processSize[0]); 
  
    bestFit(blockSize, m, processSize, n); 
  
    return 0 ; 
}

                    

Java

// Java implementation of Best - Fit algorithm
  
public class GFG 
{
    // Method to allocate memory to blocks as per Best fit
    // algorithm
    static void bestFit(int blockSize[], int m, int processSize[], 
                                                     int n)
    {
        // Stores block id of the block allocated to a
        // process
        int allocation[] = new int[n];
       
        // Initially no block is assigned to any process
        for (int i = 0; i < allocation.length; i++)
            allocation[i] = -1;
       
     // pick each process and find suitable blocks
        // according to its size ad assign to it
        for (int i=0; i<n; i++)
        {
            // Find the best fit block for current process
            int bestIdx = -1;
            for (int j=0; j<m; j++)
            {
                if (blockSize[j] >= processSize[i])
                {
                    if (bestIdx == -1)
                        bestIdx = j;
                    else if (blockSize[bestIdx] > blockSize[j])
                        bestIdx = j;
                }
            }
       
            // If we could find a block for current process
            if (bestIdx != -1)
            {
                // allocate block j to p[i] process
                allocation[i] = bestIdx;
       
                // Reduce available memory in this block.
                blockSize[bestIdx] -= processSize[i];
            }
        }
       
        System.out.println("\nProcess No.\tProcess Size\tBlock no.");
        for (int i = 0; i < n; i++)
        {
            System.out.print("   " + (i+1) + "\t\t" + processSize[i] + "\t\t");
            if (allocation[i] != -1)
                System.out.print(allocation[i] + 1);
            else
                System.out.print("Not Allocated");
            System.out.println();
        }
    }
      
    // Driver Method
    public static void main(String[] args)
    {
         int blockSize[] = {100, 500, 200, 300, 600};
         int processSize[] = {212, 417, 112, 426};
         int m = blockSize.length;
         int n = processSize.length;
           
         bestFit(blockSize, m, processSize, n);
    }
}

                    

Python3

# Python3 implementation of Best - Fit algorithm 
  
# Function to allocate memory to blocks 
# as per Best fit algorithm 
def bestFit(blockSize, m, processSize, n):
      
    # Stores block id of the block 
    # allocated to a process 
    allocation = [-1] *
      
    # pick each process and find suitable 
    # blocks according to its size ad 
    # assign to it
    for i in range(n):
          
        # Find the best fit block for
        # current process 
        bestIdx = -1
        for j in range(m):
            if blockSize[j] >= processSize[i]:
                if bestIdx == -1
                    bestIdx =
                elif blockSize[bestIdx] > blockSize[j]: 
                    bestIdx = j
  
        # If we could find a block for 
        # current process 
        if bestIdx != -1:
              
            # allocate block j to p[i] process 
            allocation[i] = bestIdx 
  
            # Reduce available memory in this block. 
            blockSize[bestIdx] -= processSize[i]
  
    print("Process No. Process Size     Block no.")
    for i in range(n):
        print(i + 1, "         ", processSize[i], 
                                end = "         "
        if allocation[i] != -1
            print(allocation[i] + 1
        else:
            print("Not Allocated")
  
# Driver code 
if __name__ == '__main__'
    blockSize = [100, 500, 200, 300, 600
    processSize = [212, 417, 112, 426
    m = len(blockSize) 
    n = len(processSize) 
  
    bestFit(blockSize, m, processSize, n)
      
# This code is contributed by PranchalK

                    

C#

// C# implementation of Best - Fit algorithm
using System;
  
public class GFG {
      
    // Method to allocate memory to blocks
    // as per Best fit
    // algorithm
    static void bestFit(int []blockSize, int m,
                      int []processSize, int n)
    {
          
        // Stores block id of the block 
        // allocated to a process
        int []allocation = new int[n];
      
        // Initially no block is assigned to
        // any process
        for (int i = 0; i < allocation.Length; i++)
            allocation[i] = -1;
      
        // pick each process and find suitable
        // blocks according to its size ad
        // assign to it
        for (int i = 0; i < n; i++)
        {
              
            // Find the best fit block for
            // current process
            int bestIdx = -1;
            for (int j = 0; j < m; j++)
            {
                if (blockSize[j] >= processSize[i])
                {
                    if (bestIdx == -1)
                        bestIdx = j;
                    else if (blockSize[bestIdx]
                                   > blockSize[j])
                        bestIdx = j;
                }
            }
      
            // If we could find a block for
            // current process
            if (bestIdx != -1)
            {
                  
                // allocate block j to p[i] 
                // process
                allocation[i] = bestIdx;
      
                // Reduce available memory in
                // this block.
                blockSize[bestIdx] -= processSize[i];
            }
        }
      
        Console.WriteLine("\nProcess No.\tProcess"
                            + " Size\tBlock no.");
        for (int i = 0; i < n; i++)
        {
            Console.Write(" " + (i+1) + "\t\t" 
                        + processSize[i] + "\t\t");
              
            if (allocation[i] != -1)
                Console.Write(allocation[i] + 1);
            else
                Console.Write("Not Allocated");
                  
            Console.WriteLine();
        }
    }
      
    // Driver Method
    public static void Main()
    {
        int []blockSize = {100, 500, 200, 300, 600};
        int []processSize = {212, 417, 112, 426};
        int m = blockSize.Length;
        int n = processSize.Length;
          
        bestFit(blockSize, m, processSize, n);
    }
}
  
// This code is contributed by nitin mittal.

                    

Javascript

function bestFit(blockSize, m, processSize, n) {
  // Stores block id of the block allocated to a process
  let allocation = new Array(n).fill(-1);
  
  // Pick each process and find suitable blocks according to its size and assign to it
  for (let i = 0; i < n; i++) {
    // Find the best fit block for current process
    let bestIdx = -1;
    for (let j = 0; j < m; j++) {
      if (blockSize[j] >= processSize[i]) {
        if (bestIdx === -1) {
          bestIdx = j;
        } else if (blockSize[bestIdx] > blockSize[j]) {
          bestIdx = j;
        }
      }
    }
  
    // If we could find a block for current process
    if (bestIdx !== -1) {
      // Allocate block j to p[i] process
      allocation[i] = bestIdx;
  
      // Reduce available memory in this block.
      blockSize[bestIdx] -= processSize[i];
    }
  }
  
  console.log("Process No. Process Size     Block no.");
  for (let i = 0; i < n; i++) {
    console.log(`${i + 1}         ${processSize[i]}         ${allocation[i] !== -1 ? allocation[i] + 1 : "Not Allocated"}`);
  }
}
  
// Driver code
let blockSize = [100, 500, 200, 300, 600];
let processSize = [212, 417, 112, 426];
let m = blockSize.length;
let n = processSize.length;
bestFit(blockSize, m, processSize, n);

                    

Output: 
 

Process No.    Process Size    Block no.
 1        212        4
 2        417        2
 3        112        3
 4        426        5

The time complexity of Best-Fit algorithm is O(n2) as it requires two loops to process the memory blocks and processes. The outer loop is used to iterate through the processes and the inner loop is used to iterate through the blocks.
The space complexity of Best-Fit algorithm is O(n) as it requires an array of size n to store the block allocation for each process.


Is Best-Fit really best? 
Although, best fit minimizes the wastage space, it consumes a lot of processor time for searching the block which is close to required size. Also, Best-fit may perform poorer than other algorithms in some cases. For example, see below exercise.
Example: Consider the requests from processes in given order 300K, 25K, 125K and 50K. Let there be two blocks of memory available of size 150K followed by a block size 350K.
Best Fit: 
300K is allocated from block of size 350K. 50 is left in the block. 
25K is allocated from the remaining 50K block. 25K is left in the block. 
125K is allocated from 150 K block. 25K is left in this block also. 
50K can’t be allocated even if there is 25K + 25K space available.
First Fit: 
300K request is allocated from 350K block, 50K is left out. 
25K is be allocated from 150K block, 125K is left out. 
Then 125K and 50K are allocated to remaining left out partitions. 
So, first fit can handle requests.
 



 



Last Updated : 13 Sep, 2023
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