When should I use mmap for file access?

Question!

POSIX environments provide at least two ways of accessing files. There's the standard system calls open(), read(), write(), and friends, but there's also the option of using mmap() to map the file into virtual memory.

When is it preferable to use one over the other? What're their individual advantages that merit including two interfaces?



Answers

mmap has the advantage when you have random access on big files. Another advantage is that you access it with memory operations (memcpy, pointer arithmetic), without bothering with the buffering. Normal I/O can sometimes be quite difficult when using buffers when you have structures bigger than your buffer. The code to handle that is often difficult to get right, mmap is often easier. This said, there are certain traps when working with mmap. As people have already mentioned, mmap is quite costly to set up, so it is worth using only for a given size (varying from machine to machine).

For pure sequential accesses to the file, it is also not always the better solution, though an appropriate call to madvise can mitigate the problem.

You have to be careful with alignment restrictions of your architecture(SPARC, itanium), with read/write IO the buffers are often properly aligned and do not trap when dereferencing a casted pointer.

You also have to be careful that you do not access outside of the map. It can easily happen if you use string functions on your map, and your file does not contain a \0 at the end. It will work most of the time when your file size is not a multiple of the page size as the last page is filled with 0 (the mapped area is always in the size of a multiple of your page size).



One area where I found mmap() to not be an advantage was when reading small files (under 16K). The overhead of page faulting to read the whole file was very high compared with just doing a single read() system call. This is because the kernel can sometimes satisify a read entirely in your time slice, meaning your code doesn't switch away. With a page fault, it seemed more likely that another program would be scheduled, making the file operation have a higher latency.



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