mmap -- memory mapped file

[Alex Martelli]

Carl Mackey <cjmackey at seas.upenn.edu> wrote:

> hi, i'm new to this list and new to python as well.
> 
> i have a question on the memory mapped file ability python has. when i
> use a mmap on a file, will it copy the whole thing to ram or just
> whatever part of it i'm working on? basically, i'm wondering if it would
> be ok for me to have multiple mmap's open on very large files as i read
> or write from them.

Python relies on the operating system to do the "memory mapping"
sensibly; generally, that means the whole file is mapped in the
*virtual* memory of the process -- the process's address space -- but
only the necessary pages actually get into RAM.  This should work fine
with any modern Linux, *BSD, MacOSX, or just about any other Unix that
is still sold, or, also, on Windows (at least in the NT/2000/XP line, I
wouldn't be so sure on some ancient Windows/98 box:-).

Still, assuming that my "very large files" you mean several gigabytes,
mmap will not work well on them with a 32-bit machine, or a 64-bit
machine hobbled by a 32-bit operating system -- the process's address
space being limited to no more 4GB, you may be unable to mmap even just
one "very large file" (this is totally unrelated to how much RAM you may
have: the limitation is with the *address space* of each process).


Alex