I have Odoo 9 with postgreSQL 9.6, with nginx and the longpolling well configured.

Everything is working fine, and suddenly, the server crashes and this message appears in the log:

OperationalError: FATAL:  sorry, too many clients already
2018-05-16 13:00:02,496 141 INFO ? openerp.sql_db: Connection to the database failed
2018-05-16 13:00:02,496 141 ERROR ? openerp.service.server: Worker (141) Exception occured, exiting...
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/openerp/service/server.py", line 721, in run
    self.process_work()
  File "/usr/lib/python2.7/dist-packages/openerp/service/server.py", line 790, in process_work
    db_names = self._db_list()
  File "/usr/lib/python2.7/dist-packages/openerp/service/server.py", line 783, in _db_list
    db_names = openerp.service.db.list_dbs(True)
  File "/usr/lib/python2.7/dist-packages/openerp/service/db.py", line 323, in list_dbs
    with closing(db.cursor()) as cr:
  File "/usr/lib/python2.7/dist-packages/openerp/sql_db.py", line 643, in cursor
    return Cursor(self.__pool, self.dbname, self.dsn, serialized=serialized)
  File "/usr/lib/python2.7/dist-packages/openerp/sql_db.py", line 177, in __init__
    self._cnx = pool.borrow(dsn)
  File "/usr/lib/python2.7/dist-packages/openerp/sql_db.py", line 526, in _locked
    return fun(self, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/openerp/sql_db.py", line 594, in borrow
    **connection_info)
  File "/usr/lib/python2.7/dist-packages/psycopg2/__init__.py", line 164, in connect
    conn = _connect(dsn, connection_factory=connection_factory, async=async)

I have to restart the server to temporal fix, but I don't know what is the cause.

I have a 4 cores CPU with 8 GB RAM.

I set these parameters in Odoo.conf file:

...
workers = 7
max_cron_threads = 2
db_maxconn = 64

What is the reason of the crash? Is it an odoo config error or a postgreSQL config error?