mysql客戶端連接服務端時，會請求到服務器端的連接池中。
在my.cnf中配置了連接數的最大值，MySQL服務器允許的最大連接數16384。
max_connections = 1000 #這里為1000
查看當前的連接數可以采用如下方式：
$ sudo netstat -antxp | grep mysql
tcp 0 0 0.0.0.0:3306 0.0.0.0:* LISTEN 1880/mysqld
unix 2 [ ACC ] STREAM LISTENING 18993 1880/mysqld /tmp/mysql.sock
unix 3 [ ] STREAM CONNECTED 34308 1880/mysqld /tmp/mysql.sock
unix 3 [ ] STREAM CONNECTED 33985 1880/mysqld /tmp/mysql.sock
unix 3 [ ] STREAM CONNECTED 33984 4762/mysql
unix 3 [ ] STREAM CONNECTED 34307 4779/mysql
mysql>show full processlist;
+----+------+-----------+------+---------+------+-------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+------+---------+------+-------+------------------+
| 2 | root | localhost | NULL | Query | 0 | NULL | show processlist |
| 3 | root | localhost | NULL | Sleep | 271 | | NULL |
+----+------+-----------+------+---------+------+-------+------------------+
