Os Memory Management Questions Medium
The clock page replacement algorithm, also known as the second-chance algorithm, is a commonly used page replacement algorithm in operating systems. It is based on the idea of a circular list or clock, where each page in memory is represented by a pointer on the clock.
The algorithm works as follows:
1. Each page in memory is assigned a reference bit, which is initially set to 0. This bit is used to determine whether a page has been accessed recently or not.
2. When a page fault occurs and a new page needs to be brought into memory, the clock algorithm starts scanning the pages in a circular manner, starting from the current position of the clock hand.
3. If the reference bit of the current page is 0, indicating that it has not been accessed recently, the page is selected for replacement. The new page is then brought into memory and the clock hand is moved to the next page.
4. If the reference bit of the current page is 1, indicating that it has been accessed recently, the reference bit is set to 0 and the clock hand moves to the next page. This gives the page a second chance to be selected for replacement.
5. Steps 3 and 4 are repeated until a page with a reference bit of 0 is found, which is then replaced with the new page.
6. If all pages have their reference bits set to 1, indicating that they have all been accessed recently, the clock algorithm starts again from the beginning of the circular list, giving each page a second chance.
The clock page replacement algorithm is efficient because it avoids unnecessary page replacements for pages that have been accessed recently. It provides a good balance between performance and simplicity, making it a popular choice for memory management in operating systems.