I'm using sysctl.conf settings from [1] and [2], but still I only able to open ~520K connections on largest cluster compute EC2 instance with CentOS. The same number was reported in this older thread [3].

[1] http://www.metabrew.com/article/a-million-user-comet-application-with-mochiweb-part-1/

[2] http://www.metabrew.com/article/a-million-user-comet-application-with-mochiweb-part-3/

[3] Linux Kernel Tuning for C500k

http://news.ycombinator.com/item?id=1740823