Nginx proxy overhead with / out keep alive requests

[paphillon]

I have a test setup to measure the nginx overhead when plugged in front
of a Jboss tomcat server.
In tomcat I have deployed a test.jsp and use ab to measure the
performance with the following scenarios 

With out keep alive ab option
ab --> <jboss tomcat URL> (ab -n 5000 -c 5
http://jboss.tomcat.url/test.jsp)
ab --> <nginx URL> --> <jboss tomcat URL>  (ab -n 5000 -c 5
http://nginx.proxy.url/test.jsp)

With keep alive ab option (-k)
ab  --> <jboss tomcat URL>  (ab -n 5000 -c 5 -k
http://jboss.tomcat.url/test.jsp)
ab  --> <nginx URL> --> <jboss tomcat URL>  (ab -n 5000 -c 5 -k
http://nginx.proxy.url/test.jsp)

The performance numbers WITHOUT keep alive is almost same BUT WITH keep
alive option the performance numbers are very different and nginx takes
about 5 secs more than the page accessed directly via tomcat. Tomcat
takes only 0.503430 on an average 

Why should there be so much of deviation with Keep alive? Is there
anything I am missing?

-------------Tomcat per result using ab keep alive (ab -n 5000 -c 5 -k
http://jboss.tomcat.url/test.jsp)-----------------------

Server Software:        Apache-Coyote/1.1
Server Hostname:      jboss.tomcat.host
Server Port:            9080

Document Path:          /test.jsp
Document Length:        301 bytes

Concurrency Level:      5
Time taken for tests:   0.503430 seconds
Complete requests:      5000
Failed requests:        0
Write errors:           0
Keep-Alive requests:    4952
Total transferred:      2944760 bytes
HTML transferred:       1505000 bytes
Requests per second:    9931.87 [#/sec] (mean)
Time per request:       0.503 [ms] (mean)
Time per request:       0.101 [ms] (mean, across all concurrent
requests)
Transfer rate:          5710.82 [Kbytes/sec] received

-------------Nginx perf result using ab keep alive (ab -n 5000 -c 5 -k
http://nginx.proxy.url/test.jsp) --------------------------

Server Software:        nginx
Server Hostname:      nginx.proxy.url
Server Port:            80

Document Path:          /test.jsp
Document Length:        301 bytes

Concurrency Level:      5
Time taken for tests:   5.440499 seconds
Complete requests:      5000
Failed requests:        0
Write errors:           0
Keep-Alive requests:    4952
Total transferred:      2654760 bytes
HTML transferred:       1505000 bytes
Requests per second:    919.03 [#/sec] (mean)
Time per request:       5.440 [ms] (mean)
Time per request:       1.088 [ms] (mean, across all concurrent
requests)
Transfer rate:          476.43 [Kbytes/sec] received

-------------Tomcat per result WITHOUT ab keep alive (ab -n 5000 -c 5
http://jboss.tomcat.url/test.jsp)-----------------------

Server Software:        Apache-Coyote/1.1
Server Hostname:      jboss.tomcat.host
Server Port:            9080

Document Path:          /test.jsp
Document Length:        301 bytes

Concurrency Level:      5
Time taken for tests:   4.658429 seconds
Complete requests:      5000
Failed requests:        0
Write errors:           0
Total transferred:      2920000 bytes
HTML transferred:       1505000 bytes
Requests per second:    1073.32 [#/sec] (mean)
Time per request:       4.658 [ms] (mean)
Time per request:       0.932 [ms] (mean, across all concurrent
requests)
Transfer rate:          612.01 [Kbytes/sec] received

-------------Nginx perf result WITHOUT ab keep alive (ab -n 5000 -c 5
http://nginx.proxy.url/test.jsp) --------------------------

Server Software:        nginx
Server Hostname:      nginx.proxy.url
Server Port:            80

Document Path:          /test.jsp
Document Length:        301 bytes

Concurrency Level:      5
Time taken for tests:   4.916966 seconds
Complete requests:      5000
Failed requests:        0
Write errors:           0
Total transferred:      2630000 bytes
HTML transferred:       1505000 bytes
Requests per second:    1016.89 [#/sec] (mean)
Time per request:       4.917 [ms] (mean)
Time per request:       0.983 [ms] (mean, across all concurrent
requests)
Transfer rate:          522.27 [Kbytes/sec] received

=========Configuration=================

Tomcat maxKeepAlive connector is set to 100

Nginx keepalive_requests 100;
worker_processes  4;
worker_connections  4098;
use epoll;
multi_accept on;

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,227472,227472#msg-227472