测试环境的连接满了，连root用户都无法登录了。但是开发又不愿意现在就重启系统。MySQL通常会为SUPER用户保留一个连接，但是如果应用程序以SUPER用户的身份进行连接，这无济于事了。而这个测试环境用的就是root做测试的。只好借助gdb来临时调大连接数：

# gdb -p $(cat /usr/local/mysql/data/mysqldb.pid) -ex "set max_connections=2500" -batch  
[New LWP 4458]
[New LWP 4456]
[New LWP 4455]
[New LWP 4454]
[New LWP 4453]
[New LWP 4452]
[New LWP 4451]
[New LWP 4450]
[New LWP 4449]
[New LWP 4448]
[New LWP 4447]
[New LWP 4446]
[New LWP 4445]
[New LWP 4444]
[New LWP 4443]
[New LWP 4440]
[New LWP 4439]
[New LWP 4438]
[New LWP 4437]
[New LWP 4436]
[New LWP 4435]
[New LWP 4434]
[New LWP 4433]
[New LWP 4432]
[New LWP 4431]
[New LWP 4430]
[New LWP 4429]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
0x00007f41a6adbddd in poll () from /lib64/libc.so.6
[Inferior 1 (process 4428) detached]

或者这样:

# gdb -p $(pidof mysqld) -ex "set max_connections=2500" -batch