mysql客户端连接服务端时，会请求到服务器端的连接池中。
在my.cnf中配置了连接数的最大值，MySQL服务器允许的最大连接数16384。
max_connections = 1000 #这里为1000
查看当前的连接数可以采用如下方式：
$ 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 |
+----+------+-----------+------+---------+------+-------+------------------+
