node.js - Node JS server hangs up on high load. DB pool connections are used up completely stop responding -

we using node.js(v 3.7.2) pg pool module (v 5.1.0) , set maxconnections 20. until load manageable 3 connections @ pool being used. typically see handful of database connections given server( have 6 servers each having 2 worker node processes). when approach loads of 18k users or more, causes node.js requests backed up. when server gets far behind, seems enter unhealthy state , never recovers , number of connections used connection pool jumps 20 , connections never enter pool again. connections seem stay in idle state on db side. have take server out of load balanced pool , restart before can placed in pool again. during time other servers may working fine. see database loads spike during times not able correlate issues. @ least 1 time saw active connections in postgresql server had been rebooted. had manually kill in postgresql before system started working again.