[ python-Bugs-1598181 ] subprocess.py: O(N**2) bottleneck

[SourceForge.net]

Bugs item #1598181, was opened at 2006-11-16 22:40
Message generated for change (Comment added) made by rwgk
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1598181&group_id=5470

Please note that this message will contain a full copy of the comment thread,
including the initial issue submission, for this request,
not just the latest update.
Category: Python Library
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Ralf W. Grosse-Kunstleve (rwgk)
Assigned to: Nobody/Anonymous (nobody)
Summary: subprocess.py: O(N**2) bottleneck

Initial Comment:
subprocess.py (Python 2.5, current SVN, probably all versions) contains this O(N**2) code:

  bytes_written = os.write(self.stdin.fileno(), input[:512])
  input = input[bytes_written:]

For large but reasonable "input" the second line is rate limiting. Luckily, it is very easy to remove this bottleneck. I'll upload a simple patch. Below is a small script that demonstrates the huge speed difference. The output on my machine is:

creating input
0.888417959213
slow slicing input
61.1553330421
creating input
0.863168954849
fast slicing input
0.0163860321045
done

The numbers are times in seconds.

This is the source:

import time
import sys
size = 1000000
t0 = time.time()
print "creating input"
input = "\n".join([str(i) for i in xrange(size)])
print time.time()-t0
t0 = time.time()
print "slow slicing input"
n_out_slow = 0
while True:
  out = input[:512]
  n_out_slow += 1
  input = input[512:]
  if not input:
    break
print time.time()-t0
t0 = time.time()
print "creating input"
input = "\n".join([str(i) for i in xrange(size)])
print time.time()-t0
t0 = time.time()
print "fast slicing input"
n_out_fast = 0
input_done = 0
while True:
  out = input[input_done:input_done+512]
  n_out_fast += 1
  input_done += 512
  if input_done >= len(input):
    break
print time.time()-t0
assert n_out_fast == n_out_slow
print "done"


----------------------------------------------------------------------

>Comment By: Ralf W. Grosse-Kunstleve (rwgk)
Date: 2006-11-16 22:43

Message:
Logged In: YES 
user_id=71407
Originator: YES

Sorry, I didn't know the tracker would destroy the indentation.
I'm uploading the demo source as a separate file.


----------------------------------------------------------------------

You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1598181&group_id=5470