L-5.22: Page Replacement Introduction | FIFO Page Replacement algorithm | Operating System

Page Replacement Algorithm and Virtual Memory

This section discusses the concept of virtual memory and the need for page replacement algorithms to manage limited main memory.

Introduction to Virtual Memory

• Virtual memory allows processes to be divided into pages and only brings a portion of the process into main memory at a time.

• The goal is to accommodate as many processes as possible in limited main memory.

Page Replacement in Virtual Memory

• When main memory is full, a page needs to be replaced with a new one.

• Three common page replacement algorithms are First-In-First-Out (FIFO), Optimal, and Least Recently Used (LRU).

First-In-First-Out (FIFO) Algorithm

• FIFO replaces the oldest page in main memory when a new page needs to be brought in.

• Pages are stored in frames in the main memory based on their arrival order.

Example: FIFO Algorithm

• A reference string represents the sequence of pages demanded by the CPU.

• For example: 7, 0, 1, 2, 0

• When a page fault occurs (a demanded page is not present in main memory), it is serviced by bringing it from the hard disk into main memory.

• Minimizing page faults is important to improve performance.

Optimal Page Replacement Algorithm

This section explains how the optimal page replacement algorithm works.

Introduction to Optimal Page Replacement

• The optimal algorithm replaces the page that will not be used for the longest period of time.

• It requires knowledge of future references, which is not practical in real-time scenarios.

Example: Optimal Algorithm

• Given a reference string and three frames in main memory:

• Reference string: 7, 0, 1, 2, 0

• Frames: Frame 1, Frame 2, Frame 3

• The optimal algorithm replaces the page that will be referenced furthest in the future.

Least Recently Used (LRU) Page Replacement Algorithm

This section discusses the least recently used (LRU) page replacement algorithm.

Introduction to LRU Page Replacement

• The LRU algorithm replaces the page that has not been used for the longest period of time.

• It is based on the principle of locality, where recently used pages are more likely to be used again.

Example: LRU Algorithm

• Using the same reference string and three frames in main memory:

• Reference string: 7, 0, 1, 2, 0

• Frames: Frame 1, Frame 2, Frame 3

• The LRU algorithm replaces the page that has not been referenced for the longest time.

Conclusion

In this transcript, we learned about virtual memory and the need for page replacement algorithms. We explored three common algorithms: First-In-First-Out (FIFO), Optimal, and Least Recently Used (LRU). These algorithms help manage limited main memory by replacing pages when necessary. FIFO replaces the oldest page, Optimal replaces the page with the longest future reference distance, and LRU replaces the least recently used page. Minimizing page faults is crucial for improving performance in virtual memory systems.

Understanding Page Faults and Replacement

In this section, the speaker explains the concept of page faults and replacement in computer memory.

Page Faults and Bringing Pages from Hard Disk

• A page fault occurs when a requested page is not present in main memory.

• When a page fault happens, the operating system retrieves the requested page from the hard disk.

• The retrieved page is then placed in an available frame in main memory.

• This process is known as bringing pages from the hard disk.

FIFO (First In First Out) Replacement Algorithm

• If there are no available frames in main memory, an old page needs to be replaced.

• The FIFO algorithm replaces the oldest frame that was filled first.

• The replaced frame is then used to store the new requested page.

Example Scenario

• Suppose there are three frames available in main memory: Frame 1, Frame 2, and Frame 3.

• Initially, Frame 1 contains Page 7, Frame 2 contains Page 0, and Frame 3 contains Page 1.

• When a request for Page 2 comes in, it results in a page fault because Page 2 is not present in any of the frames.

• Using FIFO replacement, Frame 1 (containing Page 7) is replaced with Page 2. The other frames remain unchanged.

Impact of Replacement on Performance

• Each time a replacement occurs due to a page fault, it adds overhead to the system's performance.

• It takes time to retrieve pages from the hard disk and replace them in main memory.

Analyzing Hits and Misses

In this section, the speaker discusses hits and misses when accessing pages from main memory.

Hits vs. Misses

• A hit occurs when a requested page is already present in main memory.

• A miss (or page fault) occurs when a requested page is not present in main memory and needs to be retrieved from the hard disk.

Example Scenario

• Suppose there are three frames available in main memory: Frame 1, Frame 2, and Frame 3.

• Initially, Frame 1 contains Page 0, Frame 2 contains Page 1, and Frame 3 contains Page 2.

• When a request for Page 3 comes in, it results in a miss because Page 3 is not present in any of the frames.

• Using FIFO replacement, Frame 1 (containing Page 0) is replaced with Page 3. The other frames remain unchanged.

Impact on Performance

• Hits improve performance as the requested pages are readily available in main memory.

• Misses introduce overhead due to the need to retrieve pages from the hard disk.

Continued Analysis of Hits and Misses

In this section, the speaker continues discussing hits and misses when accessing pages from main memory.

Example Scenario

• Suppose there are three frames available in main memory: Frame 1, Frame 2, and Frame 3.

• Initially, Frame 1 contains Page 0, Frame 2 contains Page 1, and Frame 3 contains Page 2.

• When a request for Page 4 comes in, it results in a miss because Page 4 is not present in any of the frames.

• Using FIFO replacement, Frame 1 (containing Page ) is replaced with Page . The other frames remain unchanged.

Impact on Performance

• Hits improve performance as the requested pages are readily available in main memory.

• Misses introduce overhead due to the need to retrieve pages from the hard disk.

Conclusion

In this section, the speaker concludes by summarizing the concepts of page faults, replacement algorithms, hits, and misses.

Key Takeaways

• Page faults occur when a requested page is not present in main memory.

• Replacement algorithms like FIFO are used to replace old pages when there are no available frames.

• Hits occur when a requested page is already present in main memory.

• Misses (or page faults) occur when a requested page needs to be retrieved from the hard disk.

Importance of Efficient Memory Management

• Efficient memory management is crucial for optimizing system performance.

• Minimizing the number of page faults and maximizing hits can significantly improve overall efficiency.

FIFO Page Replacement Algorithm Overview

In this section, we will learn about the FIFO (First-In-First-Out) page replacement algorithm. We will understand how it works and how to calculate hit ratio and miss ratio.

Introduction to FIFO Page Replacement Algorithm

• The FIFO page replacement algorithm replaces the oldest page in memory when a new page needs to be brought in.

• It follows a simple method of replacing frames one by one.

Steps of FIFO Page Replacement Algorithm

• When a new page is requested:

• Check if the page is already present in memory.

• If yes, it's a hit. Copy the previous frame as it is and mark it as a hit.

• If no, it's a miss or page fault. Replace the oldest frame with the new page and mark it as a miss.

Calculation of Hit Ratio and Miss Ratio

• To calculate the hit ratio:

• Count the number of hits.

• Divide the number of hits by the total number of references.

• Multiply by 100 to get the percentage.

• To calculate the miss ratio:

• Count the number of misses or page faults.

• Divide the number of misses by the total number of references.

• Multiply by 100 to get the percentage.

Example Calculation

• Given:

• Number of hits = 3

• Total number of references = 15

• Hit Ratio:

Hit Ratio = (Number of Hits / Total Number of References) * 100

Hit Ratio = (3 / 15) * 100

Hit Ratio = 20%

• Miss Ratio:

Miss Ratio = (Number of Misses / Total Number of References) * 100

Miss Ratio = (12 /15) *100

Miss Ratio = 80%

Conclusion

• The FIFO page replacement algorithm replaces the oldest page in memory when a new page needs to be brought in.

• It is a simple and easy-to-understand algorithm.

• Hit ratio and miss ratio can be calculated to evaluate the performance of the algorithm.