Program for Best Fit algorithm in Memory Management

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 C++ implementation.

// C++ implementation of Best - Fit algorithm
#include<bits/stdc++.h>
using namespace std;

// Function 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
    memset(allocation, -1, sizeof(allocation));

    // 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 code
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 ;
}

Output:

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

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.

This article is contributed by Sahil Chhabra (akku). If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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