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This article is to share with you about what the page replacement algorithm is. The editor thought it was very practical, so I shared it with you as a reference. Let's follow the editor and have a look.

Understand the page replacement algorithm: when a page fault occurs, if there are no free pages in the operating system's memory, the operating system must select a page in memory to move it out of memory in order to make room for the page to be transferred in. the rule used to choose which page is eliminated is called the page replacement algorithm.

In the process of address mapping, if it is found in the page that the page to be accessed is not in memory, a missing page interrupt will occur. When a page fault occurs, if there are no free pages in the operating system's memory, the operating system must select a page in memory to move it out of memory to make room for the pages to be called in. The rule used to choose which page to eliminate is called the page replacement algorithm.

Optimal permutation algorithm (OPT)

This is an ideal page replacement algorithm, but it is actually impossible to achieve. The basic idea of the algorithm is that when a page fault occurs, some pages are in memory, and one of them will soon be accessed (including the page that follows the next instruction). Other pages may not be visited until 10, 100 or 1000 instructions, and each page can be marked with the number of instructions to be executed before the page is accessed for the first time. The best page replacement algorithm simply states that the page with the largest tag should be replaced. The only problem with this algorithm is that it cannot be implemented. When a page fault occurs, the operating system does not know when each page will be accessed next. Although this algorithm is impossible, the best page replacement algorithm can be used to measure and compare the performance of achievable algorithms.

FIFO replacement algorithm (FIFO)

The simplest page replacement algorithm is the first-in first-out (FIFO) method. The essence of this algorithm is to always choose the page replacement that stays in the main memory for the longest time (that is, the oldest), that is, the page that enters the memory first and exits the memory first. The reason is that the first page called into memory is more likely to be no longer used than just called into memory. Set up a FIFO queue to hold all pages in memory. The replaced page is always at the head of the queue. When a page is placed in memory, it is inserted at the end of the line.

This algorithm is ideal only when accessing the address space in a linear order, otherwise it is inefficient. Because those pages that are often visited tend to stay in main memory for the longest time, as a result, they have to be replaced because they become "old".

Another disadvantage of FIFO is that it has an exception that increases the rate of missing pages in the case of increasing the block of storage. Of course, the page direction that causes this anomaly is actually very rare.

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