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