Prerequisite: Page Replacement Algorithms

In operating systems that use paging for memory management, page replacement algorithm are needed to decide which page needed to be replaced when new page comes in. Whenever a new page is referred and not present in memory, page fault occurs and Operating System replaces one of the existing pages with newly needed page. Different page replacement algorithms suggest different ways to decide which page to replace. The target for all algorithms is to reduce number of page faults.

In **L**east **R**ecently **U**sed (LRU) algorithm is a Greedy algorithm where the page to be replaced is least recently used. The idea is based on locality of reference, the least recently used page is not likely

Let say the page reference string 7 0 1 2 0 3 0 4 2 3 0 3 2 . Initially we have 4 page slots empty.

Initially all slots are empty, so when 7 0 1 2 are allocated to the empty slots —>** 4 Page faults**

0 is already their so —> **0 Page fault.**

when 3 came it will take the place of 7 because it is least recently used —>**1 Page fault**

0 is already in memory so —>** 0 Page fault**.

4 will takes place of 1 —>** 1 Page Fault**

Now for the further page reference string —>** 0 Page fault** because they are already available in the memory.

**Given memory capacity (as number of pages it can hold) and a string representing pages to be referred, write a function to find number of page faults.**

Letcapacitybe the number of pages that memory can hold. Letsetbe the current set of pages in memory. 1- Start traversing the pages. i)If set holds less pages than capacity.a) Insert page into the set one by one until the size ofsetreachescapacityor all page requests are processed. b) Simultaneously maintain the recent occurred index of each page in a map calledindexes. c) Increment page fault ii)ElseIfcurrent page is present inset, do nothing.Elsea) Find the page in the set that was least recently used. We find it using index array. We basically need to replace the page with minimum index. b) Replace the found page with current page. c) Increment page faults. d) Update index of current page. 2. Return page faults.

Below is implementation of above steps.

## C++

//C++ implementation of above algorithm #include<bits/stdc++.h> using namespace std; // Function to find page faults using indexes int pageFaults(int pages[], int n, int capacity) { // To represent set of current pages. We use // an unordered_set so that we quickly check // if a page is present in set or not unordered_set<int> s; // To store least recently used indexes // of pages. unordered_map<int, int> indexes; // Start from initial page int page_faults = 0; for (int i=0; i<n; i++) { // Check if the set can hold more pages if (s.size() < capacity) { // Insert it into set if not present // already which represents page fault if (s.find(pages[i])==s.end()) { s.insert(pages[i]); // increment page fault page_faults++; } // Store the recently used index of // each page indexes[pages[i]] = i; } // If the set is full then need to perform lru // i.e. remove the least recently used page // and insert the current page else { // Check if current page is not already // present in the set if (s.find(pages[i]) == s.end()) { // Find the least recently used pages // that is present in the set int lru = INT_MAX, val; for (auto it=s.begin(); it!=s.end(); it++) { if (indexes[*it] < lru) { lru = indexes[*it]; val = *it; } } // Remove the indexes page s.erase(val); // insert the current page s.insert(pages[i]); // Increment page faults page_faults++; } // Update the current page index indexes[pages[i]] = i; } } return page_faults; } // Driver code int main() { int pages[] = {7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2}; int n = sizeof(pages)/sizeof(pages[0]); int capacity = 4; cout << pageFaults(pages, n, capacity); return 0; }

## Java

// Java implementation of above algorithm import java.util.HashMap; import java.util.HashSet; import java.util.Iterator; class Test { // Method to find page faults using indexes static int pageFaults(int pages[], int n, int capacity) { // To represent set of current pages. We use // an unordered_set so that we quickly check // if a page is present in set or not HashSet<Integer> s = new HashSet<>(capacity); // To store least recently used indexes // of pages. HashMap<Integer, Integer> indexes = new HashMap<>(); // Start from initial page int page_faults = 0; for (int i=0; i<n; i++) { // Check if the set can hold more pages if (s.size() < capacity) { // Insert it into set if not present // already which represents page fault if (!s.contains(pages[i])) { s.add(pages[i]); // increment page fault page_faults++; } // Store the recently used index of // each page indexes.put(pages[i], i); } // If the set is full then need to perform lru // i.e. remove the least recently used page // and insert the current page else { // Check if current page is not already // present in the set if (!s.contains(pages[i])) { // Find the least recently used pages // that is present in the set int lru = Integer.MAX_VALUE, val=Integer.MIN_VALUE; Iterator<Integer> itr = s.iterator(); while (itr.hasNext()) { int temp = itr.next(); if (indexes.get(temp) < lru) { lru = indexes.get(temp); val = temp; } } // Remove the indexes page s.remove(val); // insert the current page s.add(pages[i]); // Increment page faults page_faults++; } // Update the current page index indexes.put(pages[i], i); } } return page_faults; } // Driver method public static void main(String args[]) { int pages[] = {7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2}; int capacity = 4; System.out.println(pageFaults(pages, pages.length, capacity)); } } // This code is contributed by Gaurav Miglani

Output:

6

Note : We can also find the number of page hits. Just have to maintain a separate count.

If the current page is already in the memory then that must be count as Page-hit.

We will discuss other Page-replacement Algorithms in further sets.

This article is contributed by **Sahil Chhabra**. 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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