CherryPy 60x as slow in the test with 8 requesting threads compared to 7

Question

I'm curious why when testing Python web server CherryPy using abwith -c 7(7 parallel streams), he can use the server 1500 requests / sec (about what I expect), but when I turn on -c 8, it drops to 25 requests / sec. I run CherryPy with numthreads = 10 (but this is no different if I use numthreads = 8 or 20) on a 64-bit four-core Windows machine with Python 2.6.

I half suspect that the Python GIL is part of the problem, but I don't know why this happens when I receive up to 8 requesting threads at once. On a quad-core machine, I expect it to change to -c 4, but it is not.

I am using the CherryPy single page web server that comes with web.py , and here is the WSGI application that I am testing against:

from web.wsgiserver import CherryPyWSGIServer

def application(environ, start_response):
    start_response("200 OK", [("Content-type", "text/plain")])
    return ["Hello World!",]

server = CherryPyWSGIServer(('0.0.0.0', 80), application, numthreads=10)
try:
    server.start()
except KeyboardInterrupt:
    server.stop()

Output abfor 7 and 8 simultaneous streams:

C:\\> ab -n 1000 -c 7 http://localhost/
...
Concurrency Level:      7
Time taken for tests:   0.670 seconds
Complete requests:      1000
Failed requests:        0
Write errors:           0
Total transferred:      130000 bytes
HTML transferred:       12000 bytes
Requests per second:    1492.39 [#/sec] (mean)
Time per request:       4.690 [ms] (mean)
Time per request:       0.670 [ms] (mean, across all concurrent requests)
Transfer rate:          189.46 [Kbytes/sec] received

C:\\> ab -n 1000 -c 8 http://localhost/
...
Concurrency Level:      8
Time taken for tests:   7.169 seconds
Complete requests:      158
Failed requests:        0
Write errors:           0
Total transferred:      20540 bytes
HTML transferred:       1896 bytes
Requests per second:    22.04 [#/sec] (mean)
Time per request:       362.973 [ms] (mean)
Time per request:       45.372 [ms] (mean, across all concurrent requests)
Transfer rate:          2.80 [Kbytes/sec] received

Answer 1

In my Linux window, this is due to the retransmission of the TCP packet from ab, although I'm not quite sure why:

No.     Time        Source                Destination           Protocol Info                                                            Delta
  10682 21.218156   127.0.0.1             127.0.0.1             TCP      http-alt > 57246 [SYN, ACK] Seq=0 Ack=0 Win=32768 Len=0 MSS=16396 TSV=17307504 TSER=17306704 WS=6 21.218156
  10683 21.218205   127.0.0.1             127.0.0.1             TCP      57246 > http-alt [ACK] Seq=82 Ack=1 Win=513 Len=0 TSV=17307504 TSER=17307504 SLE=0 SRE=1 0.000049
  10701 29.306438   127.0.0.1             127.0.0.1             HTTP     [TCP Retransmission] GET / HTTP/1.0                             8.088233
  10703 29.306536   127.0.0.1             127.0.0.1             TCP      http-alt > 57246 [ACK] Seq=1 Ack=82 Win=512 Len=0 TSV=17309526 TSER=17309526 0.000098
  10704 29.308555   127.0.0.1             127.0.0.1             TCP      [TCP segment of a reassembled PDU]                              0.002019
  10705 29.308628   127.0.0.1             127.0.0.1             TCP      57246 > http-alt [ACK] Seq=82 Ack=107 Win=513 Len=0 TSV=17309526 TSER=17309526 0.000073
  10707 29.309718   127.0.0.1             127.0.0.1             TCP      [TCP segment of a reassembled PDU]                              0.001090
  10708 29.309754   127.0.0.1             127.0.0.1             TCP      57246 > http-alt [ACK] Seq=82 Ack=119 Win=513 Len=0 TSV=17309526 TSER=17309526 0.000036
  10710 29.309992   127.0.0.1             127.0.0.1             HTTP     HTTP/1.1 200 OK  (text/plain)                                   0.000238
  10711 29.310572   127.0.0.1             127.0.0.1             TCP      57246 > http-alt [FIN, ACK] Seq=82 Ack=120 Win=513 Len=0 TSV=17309527 TSER=17309526 0.000580
  10712 29.310661   127.0.0.1             127.0.0.1             TCP      http-alt > 57246 [ACK] Seq=120 Ack=83 Win=512 Len=0 TSV=17309527 TSER=17309527 0.000089

The original "GET" package was also not found by Wireshark. For some reason, it abtries to send a request and fails, even if the TCP connection was double ACK'd just fine. The TCP client stack then waits a few seconds for a packet that has never been sent as ACK'd, and when it does not see the ACK, retries and succeeds.

. , CherryPy. ab, HTTP/1.0 1.1, keepalive, localhost ( ), Windows (), , CPU... .