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The MALLOC_MMAP_THRESHOLD improvement is less visible here:
$ MALLOC_MMAP_THRESHOLD_=1024 ../opt/python issue11849_test.py 
*** Python 3.3.0 alpha
--- USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
 0 antoine 7703 0.0 0.1 57756 8560 pts/2 S+ 01:16 0:00 ../opt/python issue11849_test.py
 1 antoine 7703 62.0 1.0 138892 86100 pts/2 S+ 01:16 0:01 ../opt/python issue11849_test.py
 2 antoine 7703 84.6 2.0 213580 160552 pts/2 S+ 01:16 0:02 ../opt/python issue11849_test.py
 3 antoine 7703 97.0 2.9 288080 234972 pts/2 S+ 01:16 0:03 ../opt/python issue11849_test.py
 4 antoine 7703 85.6 3.9 362852 309408 pts/2 S+ 01:16 0:05 ../opt/python issue11849_test.py
 5 antoine 7703 93.4 4.8 437616 383844 pts/2 S+ 01:16 0:06 ../opt/python issue11849_test.py
 6 antoine 7703 99.0 5.7 512380 458276 pts/2 S+ 01:16 0:07 ../opt/python issue11849_test.py
 7 antoine 7703 89.6 6.7 591360 535672 pts/2 S+ 01:16 0:08 ../opt/python issue11849_test.py
 8 antoine 7703 94.9 7.6 661676 607156 pts/2 S+ 01:16 0:10 ../opt/python issue11849_test.py
 9 antoine 7703 95.5 8.6 740652 684556 pts/2 S+ 01:16 0:11 ../opt/python issue11849_test.py
END antoine 7703 96.1 7.5 650432 597736 pts/2 S+ 01:16 0:13 ../opt/python issue11849_test.py
 GC antoine 7703 97.2 6.5 570316 519228 pts/2 S+ 01:16 0:13 ../opt/python issue11849_test.py
*** antoine 7703 90.8 6.5 569876 518792 pts/2 S+ 01:16 0:13 ../opt/python issue11849_test.py
By the way, an easy fix is to use cElementTree instead of ElementTree. It still won't release all memory but it will eat a lot less of it, and be much faster as well.