Optimal Page Replacement algorithm is the best page replacement algorithm as it gives the least number of page faults. It is also known as OPT, clairvoyant replacement algorithm, or Belady’s optimal page replacement policy.
Herein, what are the different page replacement algorithms?
Types of Page Replacement Algorithms
- Optimal Page Replacement algorithm ? this algorithms replaces the page which will not be referred for so long in future. …
- Least recent used (LRU) page replacement algorithm ? this algorithm replaces the page which has not been referred for a long time.
Also, which is better FIFO or LRU?
FIFO keeps the things that were most recently added. LRU is, in general, more efficient, because there are generally memory items that are added once and never used again, and there are items that are added and used frequently. LRU is much more likely to keep the frequently-used items in memory.
Why do we use page replacement algorithms?
Whenever a process refers to a page that is not present in memory, a page fault occurs. … Page replacement algorithms are an important part of virtual memory management and it helps the OS to decide which memory page can be moved out, making space for the currently needed page.
A simple modification to FIFO that avoids the problem of throwing out a heavily used page is to inspect the R bit of the oldest page. If it is 0, the page is both old and unused, so it is replaced immediately.
Explanation: Optimal page replacement algorithm has the lowest fault rate as it has the knowledge of all the pages beforehand. Explanation: Optimal page replacement algorithm is also called a Clairvoyant replacement algorithm or Belady’s optimal replacement algorithm.
The advantage of LRU page replacement algorithm is that it does not suffer from Belady’s anomaly and the disadvantage is that it needs expensive hardware support or additional data structure to implement.
First In First Out (FIFO) –
This is the simplest page replacement algorithm. In this algorithm, the operating system keeps track of all pages in the memory in a queue, the oldest page is in the front of the queue. When a page needs to be replaced page in the front of the queue is selected for removal.
How does the second chance algorithm for page replacement differ from the FIFO page replacement algorithm?
How does the second–chance algorithm for page replacement differ from the FIFO page replacement algorithm? The second–chance algorithm is based on the FIFO replacement algorithm and even degenerates to FIFO in its worst-case scenario.
Although LRU is theoretically realizable, it is not cheap. To fully implement LRU, it is necessary to maintain a linked list of all pages in memory, with the most recently used page at the front and the least recently used page at the rear. The difficulty is that the list must be updated on every memory reference.
Any time a page is requested that isn’t in memory, a page fault occurs. The LRU algorithm determines which page to throw out when memory is full. That’s the only way that it influences when page faults occur – if it throws out a page that is later requested then that’s going to be another page fault. –
The Least Recently Used (LRU) page replacement policy replaces the page that has not been used for the longest period of time. … The optimal algorithm assumes the entire reference string to be present at the time of allocation and replaces the page that will not be used for the longest period of time.