> BTW, after utilize lxml instead of ElementTree, such phenomenon of increasing memory usage disappeared.

If you looked at the link I posted, you'll see that lxml had some similar issues and solved it by calling malloc_trim systematically when freeing memory.
It could also be heap fragmentation, though.

To go further, it'd be nice if you could provide the output of
valgrind --tool=memcheck --leak-check=full --suppressions=Misc/valgrind-python.supp python <test script>
after uncommenting relevant lines in Misc/valgrind-python.supp (see http://svn.python.org/projects/python/trunk/Misc/README.valgrind ).
It will either confirm a memory leak or malloc issue (I still favour the later).

By the way, does

while True:
    XML(gen_xml())

lead to a constant memory usage increase ?