Why deleting large variables doesn't free up any memory?

[Ben Hutchings]

In article <b6khbt$ftf$1 at panix2.panix.com>, Aahz wrote:
> In article <169a7283.0304040936.69e2b9a1 at posting.google.com>,
> sdieselil <sdieselil at yahoo.com> wrote:
>>
>>I'm using Python 2.2.2 under FreeBSD. When I create a large variable
>>(using for example "a = range(1000000)") 'top' command indicates that
>>Python ate 12MB of swap space. But when I execute "del a" nothing
>>happens, it doesn't release these 12MB! Using "gc.collect()" also
>>doesn't help. What's the problem? What happened to Python's garbage
>>collector?
> 
> That's got nothing to do with Python; generally speaking, once the OS
> allocates memory to a process, the process keeps it until it exits.
> However, if you allocate and de-allocate big chunks of memory, you'll
> notice that Python's size stays static -- the memory gets re-used.

Actually it's an issue for the heap manager, not the OS.  Under Windows
and Unix at least, it is possible for a process to return memory to
the OS.  In practice allocation is done in chunks of at least a page
and a page can't be freed until every object in it has gone.  Further-
more the memory will probably be needed again, and if it isn't it will
be paged out to disk.  So most heap managers don't bother to do this,
except for very large allocations.