Cons and pros
Maybe you are wondering why do we use mmap for file access instead of
the standard read and write system calls. We will list some
advantages and disadvantages of mmap below.
Advantages of mmap
Reading from and writing to a memory-mapped file avoids the extraneous copy that occurs when using the
readorwritesystem calls, where the data must be copied to and from a user-space buffer.Aside from any potential page faults, reading from and writing to a memory-mapped file does not incur any system call or context switch overhead. It is as simple as accessing memory.
When multiple processes map the same object into memory, the data is shared among all the processes. This can save a lot of memory, which is common in the kind of server systems. Read-only and shared writable mappings are shared in their entirety; private writable mappings have their not-yet-COW (copy-on-write) pages shared.
Disadvantages of mmap
Memory mappings are always an integer number of pages in size. Thus, the difference between the size of the backing file and an integer number of pages is "wasted" as slack space. For small files, a significant percentage of the mapping may be wasted. For example, with 4 KB pages, a 7 byte mapping wastes 4,089 bytes.
The memory mappings must fit into the process' address space. With a 32-bit address space, a very large number of various-sized mappings can result in fragmentation of the address space, making it hard to find large free contiguous regions. This problem, of course, is much less apparent with a 64-bit address space.
There is overhead in creating and maintaining the memory mappings and associated data structures inside the kernel. This overhead is generally obviated by the elimination of the double copy mentioned in the previous section, particularly for larger and frequently accessed files.
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