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Some posix module functions unnecessarily release the GIL.
For example, posix_dup, posix_dup2 and posix_pipe all release the GIL, but those are non-blocking syscalls (the don't imply any I/O, only modifying the process file descriptors table).
This leads to the famous convoy effect (see http://bugs.python.org/issue7946).
For example:
$ cat /tmp/test_dup2.py 
import os
import threading
import sys
import time
def do_loop():
 while True:
 pass
t = threading.Thread(target=do_loop)
t.setDaemon(True)
t.start()
f = os.open(sys.argv[1], os.O_RDONLY)
for i in range(4, 1000):
 os.dup2(f, i)
Whith GIL release/acquire:
$ time ./python /tmp/test_dup2.py /etc/fstab 
real 0m5.238s
user 0m5.223s
sys 0m0.009s
$ time ./python /tmp/test_pipe.py 
real 0m3.083s
user 0m3.074s
sys 0m0.007s
Without GIL release/acquire:
$ time ./python /tmp/test_dup2.py /etc/fstab 
real 0m0.094s
user 0m0.077s
sys 0m0.010s
$ time ./python /tmp/test_pipe.py 
real 0m0.088s
user 0m0.074s
sys 0m0.008s