Os Memory Management Questions Medium
The CLOCK-Pro page replacement algorithm is an enhancement of the original CLOCK algorithm used for page replacement in operating systems. It aims to improve the efficiency and accuracy of page replacement decisions by incorporating additional information about page usage.
The working of the CLOCK-Pro algorithm can be described as follows:
1. Each page in the memory is represented by a circular list, known as the clock hand. The clock hand maintains a reference bit for each page, indicating whether the page has been accessed recently or not.
2. When a page fault occurs, the operating system checks the reference bit of the page pointed by the clock hand. If the reference bit is set (indicating that the page has been accessed recently), it is cleared, and the clock hand moves to the next page in the circular list.
3. If the reference bit is not set (indicating that the page has not been accessed recently), the operating system checks the modified bit of the page. If the modified bit is set (indicating that the page has been modified since it was last brought into memory), the page is written back to the disk (if necessary) and replaced with the new page.
4. If the modified bit is not set (indicating that the page has not been modified), the page is directly replaced with the new page.
5. After replacing a page, the clock hand moves to the next page in the circular list, and the process continues until a suitable page for replacement is found.
6. Additionally, the CLOCK-Pro algorithm introduces a second chance list, which contains pages that have been accessed recently but not yet modified. When the clock hand encounters a page with the reference bit set, it is moved to the second chance list instead of being replaced immediately.
7. The clock hand continues to move through the circular list, giving each page a chance to be accessed and modified. If a page in the second chance list is accessed again before the clock hand completes a full rotation, its reference bit is cleared, and it remains in the second chance list. However, if the clock hand completes a full rotation without any page being accessed again, the page at the clock hand position is replaced.
By incorporating the second chance list, the CLOCK-Pro algorithm provides a better balance between the need to replace pages that have not been accessed recently and the desire to retain pages that have been accessed recently but not modified. This helps in improving the overall performance and efficiency of memory management in operating systems.