Question

I Understand the usage of the mmap. Considering simple read/write operation on the file, involves, opening the file, and allocating the buffer, read [ which requires context switch, ], and then the data available to the user in the buffer, and changes in the buffer will not reflect into the file unless it is written explictly.

Instead , if we use mmap, writting directly to the buffer is nothing but writting into the file.

The Question:

1) File is in the hard disk, mmaped into the process, Each time i write into mmaped memory, is it written directly to the file?. In this case, does not it require any context switch, because, the changes are done directly into the file itself. If mmap is faster than legacy file accessing, where we see the time saving?

Kindly explain. correct me if i m wrong also.

Answer 1

Updates to the file are not immediately visible in the disk, but are visible after an unmap or following an msync call. Hence, there is no system call during the updates, and the kernel is not involved. However, since the file is lazily read page by page, as needed, OS may need to read-in portions of the file as you cross page boundaries. Most obvious advantage of memory mapping is that it eliminates kernel-space to user-space data copies. There is also no need for system calls to seek to a specific position in a file.