Question

In a 32-bit machine each process gets a 4GB virtual space. In this case one can worry that we might face trouble due to fragmentation. But in the case of a 64-bit machine we theoretically have a huge addressable virtual memory, so why is memory fragmentation still an issue (if it is) in a 64-bit machine?

Answer 1

Each virtual address that you try to access is mapped by the operating system to physical memory. Physical memory is allocated in pages (e.g. 4K in size). If you manage to allocate a byte at offset 1000000*n and do it for n from 1 to 1000000 (I think you could do that with mmap), then the OS will have to back that with a million pages of physical memory, which is something like 4G. That physical memory will not be available for anything else. If you had allocated the bytes contiguously, you’d only need about 1M of physical memory (256 pages) for your million bytes.

You can get in a similar bad situation if you allocate 4G for legitimate reasons, and then deallocate parts of it, keeping a bit of every page allocated. The OS will not be able to actually reuse the freed memory for anything else because there are no physical pages that are fully free. So that’s a fragmentation problem.

In theory, you could imagine that virtual addresses 1000000 and 2000000 would map to the same page of physical memory, avoiding the fragmentation. But in practice, and for good reasons, the virtual memory mapping is done on a page by page basis. You can read more about it here: http://en.wikipedia.org/wiki/Page_table.