Nginx upstream keepalive value for 100K+ Concurrent connection

I have 100K+ Concurrent connection during high traffic load. I am using Nginx and then nodejs to process request.

In Nginx I am using upstream to redirect request to nodejs. What should be the value of keepalive value in upstream to process 100K+ conenction at same time.

upstream prodpass {
    server 127.0.0.1:3001;
    keepalive 2500; --------> This value?
  }

Other nginx config -

events {
  # max_clients = worker_processes * worker_connections / 4;
  worker_connections 100000;
  use epoll;
  multi_accept on;
}

worker_rlimit_nofile 300000;

ulimit -Hn ------> 1000000

via Swapnil Saurav