memory usage, temporary and otherwise

[mk]

Obviously, don't try this on low-memory machine:

 >>> a={}
 >>> for i in range(10000000):
...     a[i]='spam'*10
...
 >>> import sys
 >>> sys.getsizeof(a)
201326728
 >>> id(a[1])
3085643936L
 >>> id(a[100])
3085713568L

 >>> ids={}
 >>> for i in range(len(a)):
...     ids[id(a[i])]=True
...
 >>> len(ids.keys())
10000000

Hm, apparently Python didn't spot that 'spam'*10 in a's values is really 
the same string, right?

So sys.getsizeof returns some 200MB for this dictionary. But according 
to top RSS of the python process is 300MB. ps auxw says the same thing 
(more or less).

Why the 50% overhead? (and I would swear that a couple of times RSS 
according to top grew to 800MB).

Regards,
mk