Nginx upstream connection prematurely closed by up

问题:

I have node server running behind Nginx.

Time to time I get this error -

upstream prematurely closed connection while reading response header from upstream,

My upstream settings -

upstream prodpass {
    server 127.0.0.1:3001;
    keepalive 64;
  }

We have more than 5000 connections at the same time open. What should be the value of keepalive?