某些网站的urllib2超时不正确

Question

timeout中的urllib2.urlopen参数似乎不一致。这是为什么？

我在下面的一些示例网址上测试了参数，测量了超时之前通话的总时间。有些页面是准确的，有些则不是。这是urllib2的错误吗？任何人都可以解释为什么会这样吗？

下面是我用来测试超时功能的一些示例代码：

import datetime, urllib2, logging, pprint
timeout_sites = [
  "http://10.255.255.1", # Unreachable
  "http://example.com:81", # No DNS name
  "http://google.com:81", # Closed port
  "http://httpbin.org/delay/10" # Waits 10 secs, then responds
]
# Calculate the timeout times and put them in here.
timeouts = {}
for url in timeout_sites:
  timeouts[url] = []
  for t in range(0, 5, 1):
    # Save the start time.
    start = datetime.datetime.utcnow()
    try:
      urllib2.urlopen(url, timeout = t)
    except Exception as e:
      # We're expecting a timeout exception. Measure the total time.
      ttime = (datetime.datetime.utcnow() - start).total_seconds()
      timeouts[url].append((t, ttime))
      logging.info("%s: Timeout=%s, Actual=%s (e=%s %s)"%(url, t, ttime, type(e), e))
    else:
      raise Exception("Expecting itmeout")
pprint.pprint(timeouts)

我希望测量的超时时间相对接近指定的时间，但在某些情况下它似乎会偏离两倍：

{'http://10.255.255.1': [(0, 0.001),
                         (1, 1.0),
                         (2, 2.002),
                         (3, 3.001),
                         (4, 4.001)],
 'http://example.com:81': [(0, 0.003),
                           (1, 2.002),
                           (2, 4.003),
                           (3, 6.003),
                           (4, 8.003)],
 'http://google.com:81': [(0, 0.033),
                          (1, 2.002),
                          (2, 4.003),
                          (3, 6.002),
                          (4, 8.002)],
 'http://httpbin.org/delay/10': [(0, 0.132),
                                 (1, 1.029),
                                 (2, 2.033),
                                 (3, 3.031),
                                 (4, 4.037)]}