[SR-Users] Kamailio Registration load Issue

Hi Guys, I am running a simple REGISTER load test on: 1) Kamailio sever with 2 cores - mem 5G 2) Kamailio server with 4 cores - mem 16G Both are EC2 instances. At -r = 500 i.e. 500 reg/sec sipP test works fine with very few re-trans. But when i increase it to 800 reg/sec it starts retransmissions in bulk. I don't see server's CPU, load-avg or memory shooting. Running everything by default, even using kamctl to start the instance. So I have not tuned any params yet. My main concern is how can I make server choke and get its actual capacity, and avoid these retransmissions as apparently kamailio is not even utilizing 2 cores and CPU usage is under 10 always. Results of both server is same i.e. 500 reg/sec max so I am sure there is some problem that needs a fix, but need some hints here. Also used multiple SIPp's to rule out if it is SIPp issue, but after 500 I see same problem. Regards, JSZ