Earlier quoted context omitted.
pgbouncer
How do you pgbounce a bunch of poller connections? pgbouncer has an event-based acceptor on the front, and a connection pool on the back. If all the connections in the pool are taken, then it will queue up the incoming connections until a backend connection is available. If all backend connections are long-polling, then incoming connections will just hang.
Having 100-1000x more active connections than you have cores on the box is worse than useless, it's actively detrimental.