Hi Borislav – Thank You for the update and all the information. It does look like we are on the same boat. And I feel the same too - maxing out on max_connections is just a symptom. pgbouncer may help alleviate the problem (though in your case it didn’t) and is definitely good to have either way.
One thing I wanted to mention is that we upgraded from CentOS 5.6 to 6.3 in order to fix a kernel bug we hit on 5.6. It was only after we moved to 6.3 that we started seeing issues with max_connections. And it is worth mentioning that there was no application change when we moved from 5.6 to 6.3. We then upgraded to 6.5 and still have the same problem. And from your case, 6.4 also has the same problem. I think that is why we wanted to downgrade to 5.8 to see if it resolves the issue.
Thank You
Ramya