Page Cache is a caching mechanism implemented in the operating system kernel to cache data blocks in the file system. When a process requests to read a file, the operating system will first look for the data block in the Page Cache, and if the corresponding data block is found, it will be returned to the process directly; if not found, the data block will be read from the disk, and Cache it into the Page Cache, and then return the data block to the process. When a process requests to write a file, the operating system also caches the data block in the Page Cache and delays writing the data block to the disk, thereby improving the efficiency of disk I/O.
Page Cache is usually implemented based on the LRU (Least Recently Used) algorithm, that is, the least recently used algorithm. When the Page Cache has cached a certain number of data blocks, if there are new data blocks to be cached, it is necessary to select some infrequently used data blocks and delete them from the Page Cache to make room for new data blocks.
Page Cache is a cache at the kernel level of the operating system, which can significantly improve the performance and response speed of the file system, especially when dealing with a large number of random reads and writes. The benefits of Page Cache mainly include the following points:
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Improve disk I/O efficiency: Page Cache can cache data on disk into memory, thereby reducing disk I/O operations and improving data access speed and response performance.
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Reduce the number of system calls: When an application needs to read or write a file, if the corresponding data block has been cached in the Page Cache, it can be directly returned to the application, avoiding additional system calls, thereby improving the performance of the application .
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Reduce disk fragmentation: When using Page Cache, data blocks can be sorted and cached according to certain rules, thereby reducing disk fragmentation and improving disk read and write performance.
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Improve data reliability: When using Page Cache, the operating system will cache data blocks in memory and synchronize them to disk, thereby improving data reliability and avoiding the risk of data loss or damage.
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Reduce wear and tear on the disk: When using Page Cache, it can reduce the read and write operations on the disk, thereby prolonging the life of the disk.
Although the operating system provides the functionality of a page cache, in some cases it may still make sense to add caching at the application level.
On the one hand, application-level caching can provide more fine-grained control and can be optimized for specific applications and usage scenarios for better performance and higher throughput. The page cache of the operating system is shared globally and cannot be optimized for specific applications.
On the other hand, application-level caching can alleviate some operating system page cache limitations. For example, the page cache of the operating system is based on physical memory, and the data that the application needs to use may far exceed the size of the physical memory. In this case, the application can use its own caching mechanism to manage the data, avoiding the limitations of the operating system page cache.
In addition, caching at the application level can also provide some additional functions, such as data preprocessing, data filtering, and data format conversion. These features can reduce the amount of communication between the application and the database, thereby improving the performance and responsiveness of the application.
In summary, application-level caching can provide finer-grained control, ease the limitations of the operating system's page cache, and provide some additional functionality. So, in some cases, it still makes sense to add caching at the application level.