Os Memory Management Questions Medium
The LRU-2 (Least Recently Used with 2nd Chance) page replacement algorithm is a variation of the LRU algorithm that aims to reduce the overhead of constantly updating the access time of each page. It combines the concepts of both the LRU and the Second Chance algorithms.
In the LRU-2 algorithm, each page in memory is assigned a reference bit, which is initially set to 0. When a page is accessed, its reference bit is set to 1. The algorithm maintains a circular queue of pages, where the page at the front of the queue is the least recently used page.
When a page fault occurs, the algorithm examines the page at the front of the queue. If its reference bit is 0, indicating that it has not been accessed since the last time it was examined, it is selected for replacement. The selected page is removed from the queue, and the new page is inserted at the rear of the queue.
However, if the reference bit of the page at the front of the queue is 1, indicating that it has been accessed recently, it is given a second chance. The reference bit is set back to 0, and the page is moved to the rear of the queue. This gives the page another chance to be accessed before it is considered for replacement again.
This second chance mechanism allows the algorithm to prioritize pages that have been accessed more recently, while still considering pages that have not been accessed for a longer time. It provides a balance between the LRU algorithm's accuracy and the Second Chance algorithm's simplicity.
Overall, the LRU-2 page replacement algorithm ensures that the pages that are least recently used and have not been accessed recently are replaced first, while still giving a second chance to pages that have been accessed recently. This helps in improving the efficiency of memory management by reducing the number of unnecessary page replacements.