Best Fit Allocation Strategy
What is Best Fit Algorithm?
First Fit and Next Fit are fast because they grab the first available block that works. However, they can be highly inefficient with space, often placing small processes into massive blocks and wasting the leftover space.
The Best Fit Algorithm takes a more careful approach. When a new process arrives, Best Fit scans the ENTIRE list of available memory holes to find the tightest possible fit. It allocates the process to the SMALLEST hole that is still large enough to hold it.
The Core Philosophy
Best Fit aims to save the largest memory blocks for the processes that truly need them. By squeezing smaller processes into the tightest available spaces, it attempts to leave the massive holes completely untouched for the future.
Steps to Implement Best Fit Algorithm
The logic requires tracking the "best" candidate during a full scan of the memory blocks:
- Take the memory blocks and the incoming processes as input.
- Initialize an allocation array to keep track of assignments (set all to -1).
- Pick the first process from the process queue.
- Initialize a variable `bestIdx` to -1. This will track the index of the best candidate block found so far.
- Scan through EVERY single memory block.
- If the current block is large enough for the process AND it is smaller than the block currently at `bestIdx` (or if `bestIdx` is still -1), update `bestIdx` to this current block's index.
- After checking all blocks, if `bestIdx` is not -1, allocate the process to that block and reduce its available size.
- Repeat steps 3-7 for all remaining processes.
C++ Code for Best Fit Algorithm
Here is the programmatic implementation of the full-scan Best Fit logic in C++:
cpp
#include <iostream>
using namespace std;
void bestFit(int blockSize[], int m, int processSize[], int n) {
int allocation[n];
// Initially, no block is assigned
for (int i = 0; i < n; i++) {
allocation[i] = -1;
}
// Pick each process and find the best fit block
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 the block is large enough
if (blockSize[j] >= processSize[i]) {
// If it's the first one found, or if it's tighter than the current best
if (bestIdx == -1 || blockSize[j] < blockSize[bestIdx]) {
bestIdx = j;
}
}
}
// If we found a block, allocate it
if (bestIdx != -1) {
// Allocate block to process
allocation[i] = bestIdx;
// Reduce available memory in this block
blockSize[bestIdx] -= processSize[i];
}
}
// Print the results
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;
}
}
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;
}Advantages of Best Fit Algorithm
- Memory Conservation: By placing processes into the tightest possible holes, it leaves the largest blocks of memory completely intact for future, massive processes that might desperately need them.
- Reduced Large Fragmentation: It prevents small processes from indiscriminately ruining large, valuable blocks of memory.
Disadvantages of Best Fit Algorithm
- Slow Execution Speed: It is significantly slower than First Fit. Because it must guarantee it has found the absolute tightest fit, it is forced to scan the ENTIRE list of memory blocks every single time a process arrives.
- Tiny Unusable Splinters: While it saves large blocks, squeezing a process into a tight hole inevitably leaves behind a microscopically small fragment of memory. These tiny "splinters" are completely useless to future processes and clutter the system with severe external fragmentation.
Tracing Best Fit vs First Fit
Question 1 of 1Test your understanding of how Best Fit makes different choices than First Fit.