Os Memory Management Questions Medium
The LRU-K-2 (Least Recently Used with K and 2nd Chance) page replacement algorithm is a variation of the LRU (Least Recently Used) algorithm that takes into account both the recency and frequency of page accesses. It aims to improve the efficiency of page replacement by considering the history of page references.
The working of the LRU-K-2 algorithm can be described as follows:
1. Initialization: Initially, all page frames are empty. The algorithm maintains a data structure, such as a doubly linked list or a queue, to keep track of the page frames and their order.
2. Page Access: When a page is accessed, the algorithm checks if it is present in any of the page frames. If it is found, the algorithm updates the recency and frequency information of the page accordingly.
3. Page Fault: If the accessed page is not present in any of the page frames, a page fault occurs. In this case, the algorithm selects a victim page for replacement based on the following criteria:
a. Least Recently Used (LRU): The algorithm identifies the page frame that has not been accessed for the longest time. This page is considered the least recently used and is a potential candidate for replacement.
b. Kth Most Recently Used (KMRU): In addition to the LRU criterion, the algorithm considers the frequency of page accesses. It maintains a counter for each page frame, which is incremented whenever the corresponding page is accessed. The algorithm selects the page frame with the lowest counter value among the potential LRU candidates.
c. 2nd Chance: The algorithm provides a second chance to pages that have been accessed recently. It uses a reference bit associated with each page frame. When a page is accessed, its reference bit is set to 1. During page replacement, the algorithm scans the page frames in a circular manner. If a page frame with a reference bit of 0 is encountered, it is selected as the victim. Otherwise, the reference bit is set to 0, and the algorithm continues the scan until a suitable victim is found.
4. Page Replacement: Once the victim page is selected, it is replaced with the new page. The algorithm updates the page frame data structure to reflect the new page placement.
5. Repeat: Steps 2 to 4 are repeated for each page access until the end of the execution.
The LRU-K-2 algorithm combines the benefits of both recency and frequency information to make more informed decisions regarding page replacement. By considering the recency of page accesses through the LRU criterion and the frequency of page accesses through the KMRU criterion, it aims to minimize the number of page faults and improve overall system performance.