MySQL 5.7 Release Notes
If clients encounter Too many connections
errors when attempting to connect to the
mysqld server, all available connections
are in use by other clients.
The permitted number of connections is controlled by the
max_connections system
variable. To support more connections, set
max_connections to a larger
value.
mysqld actually permits
max_connections
+ 1 client connections. The extra connection is reserved for
use by accounts that have the
SUPER privilege. By granting
the privilege to administrators and not to normal users (who
should not need it), an administrator who also has the
PROCESS privilege can connect
to the server and use SHOW
PROCESSLIST to diagnose problems even if the maximum
number of unprivileged clients are connected. See
Section 13.7.5.29, “SHOW PROCESSLIST Statement”.
For more information about how the server handles client connections, see Section 5.1.11.1, “Connection Interfaces”.