今天测试人员反映，一台测试环境mysql服务器查询很慢，有时登录不上， 在top中发现cpu被大量占用，系统内存还有，jave程序运行的很慢，登录服务器，
show full processlist 刷屏了很多进程，还有大量 unauthenticated user 状态的
连接如下：
| 6465371 | unauthenticated user | connecting host | NULL | Connect | NULL | login | NULL | 0 | 0 |
| 6465372 | unauthenticated user | connecting host | NULL | Connect | NULL | login | NULL | 0 | 0 |
| 6465373 | unauthenticated user | connecting host | NULL | Connect | NULL | login | NULL | 0 | 0 |
| 6465374 | unauthenticated user | connecting host | NULL | Connect | NULL | login | NULL | 0 | 0 |
| 6465375 | unauthenticated user | connecting host | NULL | Connect | NULL | login | NULL | 0 | 0 |
| 6465376 | unauthenticated user | connecting host | NULL | Connect | NULL | login | NULL | 0 | 0 |
| 6465377 | unauthenticated user | connecting host | NULL | Connect | NULL | login | NULL | 0 | 0 |
| 6465378 | unauthenticated user | connecting host | NULL | Connect | NULL | login | NULL | 0 | 0 |
看下手册中的解释是：
unauthenticated user refers to a thread that has become associated with a client connection but for which authentication of the client user has not yet been done。
即：有一个线程在处理客户端的连接，但是该客户端还没通过用户验证。
原因可能有：
1、服务器在做DNS反响解析，解决办法有2个：
a、在 hosts 中添加客户端ip，如
192.168.100.18 web-javeb、MySQL启动参数增加一个skip-name-resolve，即不启用DNS反响解析
在配置文件里面添加skip-name-resolve，然后重启服务生效；
2、服务器的线程还处于排队状态，因此可以加大 back_log
back_log为只读参数 需要重启mysql服务
back_log定义mysql在暂时停止回答请求之前的短时间内多少请求可以被存在堆栈中。
如果期望在端时间内有很多连接，则需要增加它
在配置文件里面修改back_log的值；
换句话说，就是该值定义了TCP/IP连接侦听队列的大小
操作系统在这个队列大小上有它自己的限制，视图设定back_log高于操作系统的限制将是无效的。
在mysql中的back_log的设置取决于操作系统，
在Linux下这个参数的值不能大于系统参数tcp_max_syn_backlog的值
通过以下命令可以查看当前值：cat /proc/sys/net/ipv4/tcp_max_syn_backlog通过以下命令进行修改sysctl -w net.ipv4.tcp_max_syn_backlog=n
3、程序上有bug或者异常连接为关闭，或者黑客攻击，导致连接数爆增。
分析原因：根据上面几点，排除了原因1,2.锁定是jave程序连接问题，看到进程里面有这个
6473469 root 192.168.10.155:47401 xxx_doc Query 0 executing /* mysql-connector-java-5.1.35 ( Revision: 5fb9c5849535c13917c2cf9baaece6ef9693ef27 ) */SHOW VARIABLES WHERE Variable_name =‘language‘ OR Variable_na me = ‘net_write_timeout‘ OR Variable_name = ‘interactive_timeout‘ OR Variable_name = ‘wait_timeout‘ OR Variable_name = ‘character_set_client‘ OR Variable_name = ‘character_set_connection‘ OR Variable_name = ‘character_set‘ OR Variable_na me = ‘character_set_server‘ OR Variable_name = ‘tx_isolation‘ OR Variable_name = ‘transaction_isolation‘ OR Variable_name = ‘character_set_results‘ OR Variable_name = ‘timezone‘ OR Variable_name = ‘time_zone‘ OR Variable_name = ‘system_t
ime_zone‘ OR Variable_name = ‘lower_case_table_names‘ OR Variable_name = ‘max_allowed_packet‘ OR Variable_name = ‘net_buffer_length‘ OR Variable_name = ‘sql_mode‘ OR Variable_name = ‘query_cache_type‘ OR Variable_name = ‘query_cache_size ‘ OR Variable_name = ‘license‘ OR Variable_name = ‘init_connect‘
查看进程连接数：
[email protected](none) 18:05> show status like ‘Thread%‘;
+-------------------------+-------+
| Variable_name | Value |
+-------------------------+-------+
| Threadpool_idle_threads | 10 |
| Threadpool_threads | 15 |
| Threads_cached | 0 |
| Threads_connected | 998 | ====已经用完，（剩下2个是预留的）
| Threads_created | 16944 |
| Threads_running | 5 |
+-------------------------+-------+
[email protected](none) 18:05> show variables like ‘%connect%‘;
+-----------------------------------------------+-----------------+
| Variable_name | Value |
+-----------------------------------------------+-----------------+
| character_set_connection | utf8 |
| collation_connection | utf8_general_ci |
| connect_timeout | 20 |
| disconnect_on_expired_password | ON |
| extra_max_connections | 1 |
| init_connect | |
| max_connect_errors | 10000 |
| max_connections | 1000 | =====最大连接数1000
| max_user_connections | 1000 |
| performance_schema_session_connect_attrs_size | 512 |
+-----------------------------------------------+-----------------+
10 rows in set (0.00 sec
查看error日志：
[Warning] Too many connections
叫开发检查程序是否 有异常，发现是有人在测试jdbc连接没配置好，造成的