FIFO Page Replacement Algorithm

Page Replacement Algorithms 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. 

First In First Out (FIFO) page replacement algorithm - 

 This is the simplest page replacement algorithm. In this algorithm, operating system keeps track of all pages in the memory in a queue, oldest page is in the front of the queue. When a page needs to be replaced page in the front of the queue is selected for removal. 

Example -1. Consider page reference string 1, 3, 0, 3, 5, 6 and 3 page slots. Initially all slots are empty, so when 1, 3, 0 came they are allocated to the empty slots —> 3 Page Faults. 

when 3 comes, it is already in memory so —> 0 Page Faults. Then 5 comes, it is not available in memory so it replaces the oldest page slot i.e 1. —>1Page Fault. 

Finally 6 comes, it is also not available in memory so it replaces the oldest page slot i.e 3 —>6 Page Fault. 

So total page faults = 5. 

Example -2. Consider the following reference string: 0, 2, 1, 6, 4, 0, 1, 0, 3, 1, 2, 1. Using FIFO page replacement algorithm - 

 So, total number of page faults = 9. 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. 

Implementation - Let capacity be the number of pages that memory can hold. Let set be 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 of set reaches capacity or all
 page requests are processed.
 b) Simultaneously maintain the pages in the
 queue to perform FIFO.
 c) Increment page fault
 ii) Else 
 If current page is present in set, do nothing.
 Else 
 a) Remove the first page from the queue
 as it was the first to be entered in
 the memory
 b) Replace the first page in the queue with 
 the current page in the string.
 c) Store current page in the queue.
 d) Increment page faults.

2. Return page faults.

Implementation:

// C++ implementation of FIFO page replacement // in Operating Systems. #include<bits/stdc++.h> using namespace std; // Function to find page faults using FIFO 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 the pages in FIFO manner  queue<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())  {  // Insert the current page into the set  s.insert(pages[i]);  // increment page fault  page_faults++;  // Push the current page into the queue  indexes.push(pages[i]);  }  }  // If the set is full then need to perform FIFO  // i.e. remove the first page of the queue from  // set and queue both and insert the current page  else  {  // Check if current page is not already  // present in the set  if (s.find(pages[i]) == s.end())  {  // Store the first page in the   // queue to be used to find and  // erase the page from the set  int val = indexes.front();    // Pop the first page from the queue  indexes.pop();  // Remove the indexes page from the set  s.erase(val);  // insert the current page in the set  s.insert(pages[i]);  // push the current page into  // the queue  indexes.push(pages[i]);  // Increment page faults  page_faults++;  }  }  }  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 implementation of FIFO page replacement // in Operating Systems. import java.util.HashSet; import java.util.LinkedList; import java.util.Queue; class Test {  // Method to find page faults using FIFO  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 the pages in FIFO manner  Queue<Integer> indexes = new LinkedList<>() ;    // 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++;    // Push the current page into the queue  indexes.add(pages[i]);  }  }    // If the set is full then need to perform FIFO  // i.e. remove the first page of the queue from  // set and queue both and insert the current page  else  {  // Check if current page is not already  // present in the set  if (!s.contains(pages[i]))  {  //Pop the first page from the queue  int val = indexes.peek();    indexes.poll();    // Remove the indexes page  s.remove(val);    // insert the current page  s.add(pages[i]);    // push the current page into  // the queue  indexes.add(pages[i]);    // Increment page faults  page_faults++;  }  }  }    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 

# Python3 implementation of FIFO page # replacement in Operating Systems. from queue import Queue # Function to find page faults using FIFO  def pageFaults(pages, n, 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  s = set() # To store the pages in FIFO manner  indexes = Queue() # Start from initial page  page_faults = 0 for i in range(n): # Check if the set can hold  # more pages  if (len(s) < capacity): # Insert it into set if not present  # already which represents page fault  if (pages[i] not in s): s.add(pages[i]) # increment page fault  page_faults += 1 # Push the current page into # the queue  indexes.put(pages[i]) # If the set is full then need to perform FIFO  # i.e. remove the first page of the queue from  # set and queue both and insert the current page  else: # Check if current page is not  # already present in the set  if (pages[i] not in s): # Pop the first page from the queue  val = indexes.queue[0] indexes.get() # Remove the indexes page  s.remove(val) # insert the current page  s.add(pages[i]) # push the current page into  # the queue  indexes.put(pages[i]) # Increment page faults  page_faults += 1 return page_faults # Driver code  if __name__ == '__main__': pages = [7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2] n = len(pages) capacity = 4 print(pageFaults(pages, n, capacity)) # This code is contributed by PranchalK 

// C# implementation of FIFO page replacement  // in Operating Systems.  using System; using System.Collections; using System.Collections.Generic;  class Test  {   // Method to find page faults using FIFO   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<int> s = new HashSet<int>(capacity);     // To store the pages in FIFO manner   Queue indexes = new Queue() ;     // 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.Count < 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++;     // Push the current page into the queue   indexes.Enqueue(pages[i]);   }   }     // If the set is full then need to perform FIFO   // i.e. Remove the first page of the queue from   // set and queue both and insert the current page   else  {   // Check if current page is not already   // present in the set   if (!s.Contains(pages[i]))   {   //Pop the first page from the queue   int val = (int)indexes.Peek();     indexes.Dequeue();     // Remove the indexes page   s.Remove(val);     // insert the current page   s.Add(pages[i]);     // push the current page into   // the queue   indexes.Enqueue(pages[i]);     // Increment page faults   page_faults++;   }   }   }     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;   Console.Write(pageFaults(pages, pages.Length, capacity));   }  }  // This code is contributed by Arnab Kundu 

<script>  // JavaScript code for the above approach  // Method to find page faults using FIFO  function pageFaults(pages, n, 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  let s = new Set();    // To store the pages in FIFO manner  var indexes = [];    // Start from initial page  let page_faults = 0;  for (let 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.has(pages[i]))  {  s.add(pages[i]);    // increment page fault  page_faults++;    // Push the current page into the queue  indexes.push(pages[i]);  }  }    // If the set is full then need to perform FIFO  // i.e. remove the first page of the queue from  // set and queue both and insert the current page  else  {  // Check if current page is not already  // present in the set  if (!s.has(pages[i]))  {  //Pop the first page from the queue  let val = indexes[0];    indexes.shift();    // Remove the indexes page  s.delete(val);    // insert the current page  s.add(pages[i]);    // push the current page into  // the queue  indexes.push(pages[i]);    // Increment page faults  page_faults++;  }  }  }    return page_faults;  } // Driver Code    let pages = [7, 0, 1, 2, 0, 3, 0, 4,  2, 3, 0, 3, 2];    let capacity = 4;  document.write(pageFaults(pages, pages.length, capacity));    // This code is contributed by sanjoy_62.  </script> 

7

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. 

Time Complexity: O(n), where n is the number of pages.
Space Complexity: O(capacity)

Belady’s anomaly:

Belady’s anomaly proves that it is possible to have more page faults when increasing the number of page frames while using the First in First Out (FIFO) page replacement algorithm. For example, if we consider reference string 3, 2, 1, 0, 3, 2, 4, 3, 2, 1, 0, 4 and 3 slots, we get 9 total page faults, but if we increase slots to 4, we get 10 page faults. 

FIFO Page Replacement Algorithm

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