Detailed explanation of MySQL's FreeList mechanism

1. Introduction

After MySQL is started, BufferPool will be initialized. Before you perform any query operations, the cache pages in BufferPool are blocks of empty memory, which have not been used and no data is stored in them.

And you also know that the cache page it points to can be directly and uniquely found through the description information of the cache page.

Have you ever thought about which buffer page the data page we read from the disk should be placed in?

This problem brings up the Free List.

2. Free List

In fact, Free List is a bidirectional linked list organized in Buffer Pool based on cache page description information. In other words, each node in the Free List is the description information corresponding to the cache page. And the specified cache page (cache page) can be found through the description information

The original intention of InnoDB designing Free List is to solve the above problems.

If no data is stored in this cache page, its corresponding description information will be maintained in the Free List. At this time, when you want to put a data page read from the disk into the cache page, you must first find a node from the Free List (all nodes in the Free List will point to a cache page that has never been used), and then you can put the data page you read into the cache page pointed to by the node.

Corresponding: After data is put into the data page. Its corresponding description information block will be removed from the Free List.

3. How to determine whether the data page is in the cache?

Do you wonder how MySQL knows whether the data page just read is in the cache page?

The implementation of this function relies on another data structure: hash table

key = tablespace number + data page number

value = cache page address

If it exists in the hash table, it means that the data page already exists in the Buffer Pool, and the cache page in the Buffer Pool is used first. I believe you can definitely guess why the cache pages in the Buffer Pool are used first! First, random disk IO is avoided. Second, the data in the cache page may be dirty data that has been modified.

The above is a detailed explanation of MySQL's FreeList mechanism. For more information about MySQL's FreeList mechanism, please pay attention to other related articles on 123WORDPRESS.COM!