Paul <[email protected]> added the comment:
I've dug into stracing this python program in 2.7 vs. 3.7.
directread.py
import mmap
import os
fd = os.open('/dev/dm-2', os.O_DIRECT | os.O_RDWR) # mapped block device
fo = os.fdopen(fd, 'rb+')
m = mmap.mmap(-1, 4096)
fo.readinto(m)
Python 2.7 result:
...
open("/dev/dm-2", O_RDWR|O_DIRECT) = 3
...
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0) =
0x7f743db31000
...
read(0x3, 0x7f743db31000, 0x1000) = 0x1000
...
Python 3.7 result:
...
open("/dev/dm-2", O_RDWR|O_DIRECT|O_CLOEXEC) = 3
...
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0) =
0x7f5e087ee000
...
read(0x3, 0x256c8a0, 0x1000) = -1 (errno 22)
Notice that Python 3 isn't using the mmap buffer for the read. Why is it using
a stack buffer?
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<https://bugs.python.org/issue5396>
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