A memory-mapped file is a method of mapping the contents of a file in memory, and is an efficient file access method commonly used in modern operating systems.
The principle of memory-mapped files is to map part or all of the contents of the file into the virtual address space of the process. When using a memory-mapped file, the operating system will map the file data into the virtual address space of the process, so that the process can directly access the file content through memory without using system calls such as read and write for IO operations. This method can improve the efficiency of accessing files, reduce the overhead of IO operations, and allow multiple processes to share the contents of files.
Specifically, the process of memory-mapping files is to first create a virtual area and complete address mapping. At this time, no file data has been copied to the main memory. When a process initiates a read or write operation, it will access the virtual address space. By querying the page table, it is found that this address is not on the physical page. Because only the address mapping has been established, the real data has not been copied to the memory, so a page fault exception occurs. After a series of judgments are made on page fault exceptions, and no illegal operations are confirmed, the kernel initiates a request paging process.
During the paging process, the operating system will read the file data from the disk to the physical memory, and establish the mapping relationship between the physical page and the virtual page. Afterwards, the process can read and write to this piece of main memory. If the write operation modifies the content, the system will automatically write back the dirty page to the corresponding disk address after a certain period of time, completing the process of writing to the file. Alternatively, you can call msync() to force a sync, so that memory written to it is immediately saved to the file.
The advantage of memory-mapped files is that it can improve the efficiency of IO operations, because the operating system can optimize file reading and writing through pre-reading and pre-writing technologies. In addition, memory-mapped files can also allow multiple processes to share the contents of the same file, thereby improving system performance.
But memory-mapped files also have some disadvantages. First of all, memory-mapped files need to consume a large amount of virtual address space, so generally only files smaller than 4GB can be mapped in a 32-bit system. Second, memory-mapped files may increase the load on the system, because the operating system needs to read file data from disk into physical memory, which may take up a lot of CPU and IO resources. Finally, the security of memory-mapped files also needs to be considered, because through memory-mapped files, files can be directly read and written, so they may be used by hackers to attack.
In short, memory-mapped files are an efficient way to access files, which can improve the efficiency of accessing files, reduce the overhead of IO operations, and allow multiple processes to share the contents of files. But it is also necessary to pay attention to its disadvantages in order to make full use of its advantages and avoid the influence of its disadvantages.