What is lru page replacement

Before we talk about what page replacement is, you should know about paging. Different algorithms are used to perform page replacement. We will learn about some of the most common algorithms in this lesson. This is the simplest page replacement algorithm. In this algorithm, the operating system keeps a track of all the pages present in the memory in a queue fashion. When a page needs to be replaced, the oldest page in the queue is selected and replaced with the new page.

Initially, all the slots are empty so the first three reference strings 1, 3, 0 will be allocated to the empty slots with 3 page-faults. Next in the reference string is 3 which is already in the memory, so no page fault. Then comes 5.

Since there is no available memory space the newly arrived page will replace the oldest residing page in the memory, i. Likewise, now 6 will arrive and will replace 3 with another page fault.

And then 3 will arrive and will replace 0 with another page fault. Initially all the memory slots will be empty so 7, 0, 1, 2 will be allocated to the memory with 4 page-faults. Again, 0 is already there, so no page fault. And for the rest of the string, there will be no page fault as all the arriving pages are already there in the memory.

Now, as we understand this algorithm, we tend to realize that this algorithm can be implemented in practice as an operating system cannot know the future page requests in advance. So, this algorithm is just an instance of what can be the optimal solution. In this algorithm, the new page is replaced with the existing page that has been used the least recently. In other words, the page which has not been referred for a long time will be replaced with the newly arrived page.

So, if we have a page reference string 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2 with four page frames, then the page replacement will take place as shown below:.

In this algorithm, the operating system uses R and M bits to distinguish between pages. When a process starts, both page bits R and M are set to 0 by the operating system.

If a page has been referenced, a page fault will occur and the R bit is set by the OS. If a page is modified, then another page fault will occur and the OS will set the M bit. Periodically the R bit is cleared by a clock interrupt. When a page fault occurs and there are no empty frames , the operating system inspects all the pages and divides them into 4 categories:. Looking at it like this, it feels class 1 is not possible.

How can a page be modified but not referenced? Skip to content. Change Language. Related Articles. Save Article. Improve Article. Like Article. We use. Contains pages[i]. Add pages[i] ;. ContainsKey pages[i]. Add pages[i], i ;. Remove val ;. WriteLine pageFaults pages,. Length, capacity ;. Python3 program for page replacement algorithm. List of current pages in Main Memory.

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Remove i ;. Count, i ;. Next Optimal Page Replacement Algorithm. Recommended Articles. Minimize replacement of characters to its nearest alphabet to make a string palindromic. Check if a string can be made equal to another string by swapping or replacement of characters. Article Contributed By :.