MMapDirectory need twice more virtual memory than actually need?

[luocanrao]

Environment: 64 bit linux,memory 8G

When I used pmap instruction to see virtual memory, I found two big anon
memory which grows with the index file size.

I had the two following pictures to show the problem, can you explain?

         This is why I got out of memory exception in 32 bit machine.
Because my index file size is only 800m.

                                                                 Anon memory
picture



                                                                 Indexfile
memory picture