The os.mmap function does not take file offset as
requirement. As a result, if python want to mmap
a piece of the file data at the very end of the
file, it needs to mmap the every thing before that.

Without offset argument, it is hard to work with
large files.

I make a patch to add the offset arguments to mmap
function call a while back. I wish it get included
in the future version of python.

http://mail.python.org/pipermail/python-list/2005-May/324213.html


Thanks