upstream keepalive - call for testing

[magicbear]

addition information:
the server may dead when processed about 20 million requests. Not very
high frequencies to dead.

2011/08/27 13:31:21 [alert] 30433#0: worker process 1944 exited on
signal 11 (core dumped)

2011/08/28 14:40:35 [alert] 30433#0: worker process 14510 exited on
signal 11 (core dumped)

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,213207,214395#msg-214395