Bugs item #1663329, was opened at 2007-02-19 11:17
Message generated for change (Comment added) made by loewis
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Category: Performance
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: H. von Bargen (hvbargen)
Assigned to: Nobody/Anonymous (nobody)
Summary: subprocess/popen close_fds perform poor if SC_OPEN_MAX is hi
Initial Comment:
If the value of sysconf("SC_OPEN_MAX") is high
and you try to start a subprocess with subprocess.py or os.popen2 with
close_fds=True, then starting the other process is very slow.
This boils down to the following code in subprocess.py:
def _close_fds(self, but):
for i in xrange(3, MAXFD):
if i == but:
continue
try:
os.close(i)
except:
pass
resp. the similar code in popen2.py:
def _run_child(self, cmd):
if isinstance(cmd, basestring):
cmd = ['/bin/sh', '-c', cmd]
for i in xrange(3, MAXFD):
try:
os.close(i)
except OSError:
pass
There has been an optimization already (range has been replaced by xrange to
reduce memory impact), but I think the problem is that for high values of
MAXFD, usually a high percentage of the os.close statements will fail, raising
an exception (which is an "expensive" operation).
It has been suggested already to add a C implementation called "rclose" or
"close_range" that tries to close all FDs in a given range (min, max) without
the overhead of Python exception handling.
I'd like emphasize that this is not a theoretical, but a real world problem:
We have a Python application in a production environment on Sun Solaris. Some
other software running on the same server needed a high value of 260000 for
SC_OPEN_MAX (set with ulimit -n XXX or in some /etc/-file (don't know which
one).
Suddenly calling any other process with subprocess.Popen (..., close_fds=True)
now took 14 seconds (!) instead of some microseconds.
This caused a huge performance degradation, since the subprocess itself only
needs only a few seconds.
See also:
Patches item #1607087 "popen() slow on AIX due to large FOPEN_MAX value".
This contains a fix, but only for AIX - and I think the patch does not support
the "but" argument used in subprocess.py.
The correct solution should be coded in C, and should
do the same as the _close_fds routine in subprocess.py.
It could be optimized to make use of (operating-specific) system calls to close
all handles from (but+1) to MAX_FD with "closefrom" or "fcntl" as proposed in
the patch.
----------------------------------------------------------------------
>Comment By: Martin v. Löwis (loewis)
Date: 2007-02-21 00:45
Message:
Logged In: YES
user_id=21627
Originator: NO
Wouldn't it be simpler for you to just don't pass close_fds=True to popen?
----------------------------------------------------------------------
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