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Implementation of Least Recently Used (LRU) page replacement algorithm using Counters
  • Last Updated : 02 Dec, 2019

Prerequisite – Least Recently Used (LRU) Page Replacement algorithm
Least Recently Used page replacement algorithm replaces the page which is not used recently.

Implementation:
In this article, LRU is implemented using counters, a ctime (i.e., counter) variable is used to represent the current time, it is incremented for every page of the reference array. A vector of pair is used to represent the page frames, the 1’st variable of the pair is the page number and the second variable is the current time. When a new page is to be inserted and the frames are full, the page with minimum ctime is deleted (as it is the least recently used page). If the page is accessed again the value of ctime is updated.

Note:
Least ctime (counter / current time) value represents the least recently used page.

Examples:

Reference array is : 0, 0, 0, 2, 3, 0, 5, 7, 1, 2, 0, 8
Output :
When the number of frames is : 3
The number of page faults are : 9

Reference array is : 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 0, 0
Output :
When the number of frames is : 3
The number of page faults are : 15 

Code:




#include <bits/stdc++.h>
  
using namespace std;
  
// To calculate the number of page faults
void pageFaults(int frame_size, int* ref, int len, int ctime)
{
    // Count variable to count the
    // number of page faults
    int cnt = 0;
    // Arr to simulate frames
    vector<pair<int, int> > arr;
  
    // To initialise the array
    for (int i = 0; i < frame_size; i++) {
        arr.push_back(make_pair(-1, ctime));
    }
  
    int page;
  
    for (int i = 0; i < len; i++) {
        page = ref[i];
        auto it = arr.begin();
  
        for (it = arr.begin(); it != arr.end(); it++) {
            if (it->first == page) {
                break;
            }
        }
  
        // If page is found
        if (it != arr.end()) {
            // update the value of
            // current time
            it->second = ctime;
        }
  
        // If page is not found
        else {
            // Find the page with min value of ctime,
            // as it will be the least recently used
            vector<pair<int, int> >::iterator pos;
            pos = arr.begin();
            int min = pos->second;
            for (auto itr = arr.begin(); itr != arr.end(); itr++) {
                if (itr->second < min) {
                    pos = itr;
                    min = pos->second;
                }
            }
  
            // Erase this page from the frame vector
            arr.erase(pos);
  
            // Insert the new page
            arr.push_back(make_pair(page, ctime));
            cnt++;
        }
        ctime++;
    }
    cout << "The number of page faults is : " << cnt << endl;
}
  
int main()
{
    // This is the reference array
    int ref[] = { 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 };
    int len = sizeof(ref) / sizeof(ref[0]);
    int frame_size = 3;
    // Ctime represents current time,
    // it is incremented for every page
    int ctime = 0;
    pageFaults(frame_size, ref, len, ctime);
}
Output:
The number of page faults is : 10

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