Optimal Page replacement Memory Management Operating Systems OS

Optimal Page replacement Memory Management Operating Systems OS

According to Optimal Page replacement OS replaces the page from the memory that is not used for a long period of time.

PF stands for a page fault  Read More  . FH is used in a table to represent the page hit.  Read More 

1^st column: First of all, all memory is free.

2^nd column: Page 4 is loaded on memory. A page fault  Read More   occurs because page 4 is already not on memory.

3^rd column: Page 6 is loaded on memory. A page fault occurs.

4^th column: Page 8 is loaded on memory. A page fault occurs.

5^th column: Page 4 is already loaded on memory. No page fault.

6^th column: Page 2 is loaded on memory. A page fault occurs.

7th column: Page 6 is already loaded on memory. No page fault.  Read More 

C++ program to demonstrate the Optimal Page Replacement Algorithm

// CPP program for optimal page page replacement algorithm in operating systems
#include <bits/stdc++.h>
using namespace std;
 
// Page exists in memory or not?
bool PageSearch(int number, vector<int>& Frames)
{
    for (int loop = 0; loop < Frames.size();loop ++)
        if (Frames[loop] == number)
            return true;
    return false;
}
 
// Fins unused Frames
int predict(int page[], vector<int>& Frames, int pn, int index)
{
    int res = -1, farthest = index;
    for (int loop = 0; loop < Frames.size();loop ++) {
        int loop2;
        for (loop2 = index; loop2 < pn; loop2++) {
            if (Frames[loop] == page[loop2]) {
                if (loop2 > farthest) {
                    farthest = loop2;
                    res = loop;
                }
                break;
            }
        }
 
        // Suppose the page is never referenced in future,
        if (loop2 == pn)
            return loop;
    }
 
    // If all of the Frames  were not in future,
    // return any of them, we return 0. Otherwise
    // we return res.
    return (res == -1) ? 0 : res;
}
 
void optimalAlgorithm(int page[], int pn, int TotalFrames)
{
    vector<int> Frames;
    // Traverse array with page reference array to check for the PageHit and miss.
    int PageHit = 0;
    for (int loop = 0; loop < pn; loop++) {
        // Page detected in memory
        if (PageSearch(page[loop], Frames)) {
            PageHit++;
            continue;
        }
 
        // Page not detected in memory

        if (Frames.size() < TotalFrames)
            Frames.push_back(page[loop]);
 
        // Find the page to be replaced.
        else {
            int loop2 = predict(page, Frames, pn, loop + 1);
            Frames[loop2] = page[loop];
        }
    }
   cout << " Total  Page misses = " << pn - PageHit << endl;
 cout << "Total  Page Hits = " << PageHit << endl;
    }
 int main()
{
    int page[] = { 4, 6, 8, 4, 2, 6 };
    int pn = sizeof(page) / sizeof(page[0]);
    int TotalFrames = 3;
    optimalAlgorithm(page, pn, TotalFrames);
    return 0;
}

Output

MCQs on Page Replacement Algorithm