[SR-Users] Question about Kamailio Performance

My goal: make sure when using kamailio as engine I can achieve: on systems with 4GB memory, Kamailio can serve a population over 300 000 online subscribers <from: https://www.kamailio.org/w/features/ > Test bed: CPU: 4 cores , 4G RAM Kamailio 5 with Mysql.  Test source：another server running SIPp 10k subscriber in Mysql .  6000 Register Per Second ,  reuse 10k user to register , it works fine.  If register 50 every second for per user it seem 300 k subscribers should work.  (about 300% cpu used of totally 400% cpu. All requests finished without retransmit and fail. ) add 90k subscriber into Mysql . 100k subscirbers in Mysql . 1000 Register Per Second , a lot of failure and can not work .  (cpu are not used much at that time. IO is not high.) My question :  What is the root cause of bad performance when there is 100k subscribers' data in the Mysql database.  How can I deploy a kamailio with 300k  subscriber in database, and support 6000 register per second at the same time ? Maybe use ram database instead of Mysql?    Thanks with regards, David