If you need to stream a file, mmap'ed variant will eat memory up to file size. Given large enough file, it will eat all you memory and then will begin to page out unused portions of the file. Of course, details on when and where will vary depending on VM-system implementation.
The thing is, you really need only a small portion of a file. Memory mapping is useful if you need all data to be (virtually) present in RAM. Regards, Dmitry.
