MySQL锁阻塞分析

2022-10-01 22:20:54

日常维护中，经常会碰到线程被阻塞，导致数据库响应非常慢，下面就看看如何获取是哪个线程导致了阻塞的。

blog地址：http://blog.csdn.net/hw_libo/article/details/39080809

1. 环境说明

RHEL 6.4 x86_64 + MySQL 5.6.19
事务隔离级别：RR

2. 测试过程

3. 查看锁阻塞线程信息

这里用几中方法进行分析：

3.1 使用show processlist查看

MySQL [(none)]> show processlist;
+----+------+-----------+------+---------+------+--------------+------------------------------------------+
| Id | User | Host      | db   | Command | Time | State        | Info                                     |
+----+------+-----------+------+---------+------+--------------+------------------------------------------+
|  2 | root | localhost | NULL | Query   |    0 | init         | show processlist                         |
|  3 | root | localhost | test | Query   |   70 | Sending data | select count(*) from t3 a,t3 b           |
|  4 | root | localhost | test | Query   |   65 | updating     | delete from emp where empno=7788         |
|  7 | root | localhost | test | Query   |   68 | updating     | update emp set sal=3500 where empno=7788 |
+----+------+-----------+------+---------+------+--------------+------------------------------------------+
4 rows in set (0.00 sec)

如果数据库存在较多线程的话，这种方法确实不太好确认的。

3.2 直接使用show engine innodb status查看

------------
TRANSACTIONS
------------
Trx id counter 4131
Purge done for trx's n:o < 4119 undo n:o < 0 state: running but idle
History list length 126
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0, not started
MySQL thread id 2, OS thread handle 0x7f953ffff700, query id 115 localhost root init
show engine innodb status
---TRANSACTION 4130, ACTIVE 41 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 360, 1 row lock(s)
MySQL thread id 4, OS thread handle 0x7f953ff9d700, query id 112 localhost root updating
delete from emp where empno=7788
------- TRX HAS BEEN WAITING 41 SEC FOR THIS LOCK TO BE GRANTED:   ## 等待了41s
RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4130 lock_mode X locks rec but not gap waiting
Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0  ## 线程4在等待往test.emp中的主键上加X锁，page num=3
 0: len 4; hex 80001e6c; asc    l;;
 1: len 6; hex 000000001018; asc       ;;
 2: len 7; hex 91000001420084; asc     B  ;;
 3: len 5; hex 53434f5454; asc SCOTT;;
 4: len 7; hex 414e414c595354; asc ANALYST;;
 5: len 4; hex 80001d8e; asc     ;;
 6: len 4; hex 208794f0; asc     ;;
 7: len 4; hex 80000bb8; asc     ;;
 8: SQL NULL;
 9: len 4; hex 80000014; asc     ;;

------------------
---TRANSACTION 4129, ACTIVE 45 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 360, 1 row lock(s)
MySQL thread id 7, OS thread handle 0x7f953ff6c700, query id 111 localhost root updating
update emp set sal=3500 where empno=7788
------- TRX HAS BEEN WAITING 45 SEC FOR THIS LOCK TO BE GRANTED:   ## 等待了45s
RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4129 lock_mode X locks rec but not gap waiting
Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0  ## 线程7在等待往test.emp中的主键上加X锁，page num=3
 0: len 4; hex 80001e6c; asc    l;;
 1: len 6; hex 000000001018; asc       ;;
 2: len 7; hex 91000001420084; asc     B  ;;
 3: len 5; hex 53434f5454; asc SCOTT;;
 4: len 7; hex 414e414c595354; asc ANALYST;;
 5: len 4; hex 80001d8e; asc     ;;
 6: len 4; hex 208794f0; asc     ;;
 7: len 4; hex 80000bb8; asc     ;;
 8: SQL NULL;
 9: len 4; hex 80000014; asc     ;;

------------------
---TRANSACTION 4128, ACTIVE 51 sec
2 lock struct(s), heap size 360, 1 row lock(s)
MySQL thread id 3, OS thread handle 0x7f953ffce700, query id 110 localhost root cleaning up

我们知道，主要根因还是thread=3引起的，但从innodb status中却无法分析得到这个结果。

从上面来看，线程4和线程7都在等待往test.emp中的主键上加X锁，page num=3，但是线程7等待的时间为45s，而线程4等待的时间为41s，是较线程7之后申请的锁，所以可以判断是线程7阻塞了线程4。至于线程7为什么出现等待，这里分析不到根因。

3.3 使用mysqladmin debug查看

# mysqladmin -S /tmp/mysql3306.sock debug

然后在error日志中，会看到：

Thread database.table_name          Locked/Waiting        Lock_type


3       test.t3                     Locked - read         Low priority read lock
7       test.emp                    Locked - write        High priority write lock

这种方法中，能找到线程ID=3和7是阻塞者，但还是不太准确，判断不出来线程7也是被线程ID=3阻塞的。

3.4 使用innodb_lock_monitor来获取阻塞锁线程

MySQL [test]> CREATE TABLE innodb_lock_monitor (a INT) ENGINE=INNODB;  ## 随便在一个数据库中创建这个表，就会打开lock monitor
Query OK, 0 rows affected, 1 warning (0.07 sec)

MySQL [test]> show warnings\G
*************************** 1. row ***************************
  Level: Warning
   Code: 131
Message: Using the table name innodb_lock_monitor to enable diagnostic output is deprecated and may be removed in future releases. Use INFORMATION_SCHEMA or PERFORMANCE_SCHEMA tables or SET GLOBAL innodb_status_output=ON.
1 row in set (0.00 sec)

说明：这个在5.6中有一个warning，但不影响使用。

然后再使用show engine innodb status查看：

------------
TRANSACTIONS
------------
Trx id counter 4667
Purge done for trx's n:o < 4659 undo n:o < 0 state: running but idle
History list length 138
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0, not started
MySQL thread id 9, OS thread handle 0x7f813c5f7700, query id 152 localhost root init
show engine innodb status
---TRANSACTION 4663, ACTIVE 78 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 360, 1 row lock(s)
MySQL thread id 4, OS thread handle 0x7f813c628700, query id 149 localhost root updating
delete from emp where empno=7788
------- TRX HAS BEEN WAITING 78 SEC FOR THIS LOCK TO BE GRANTED:   ## 等待了78s
RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4663 lock_mode X locks rec but not gap waiting
Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0  ## 线程4在等待往test.emp中的主键上加X锁，page num=3
 0: len 4; hex 80001e6c; asc    l;;
 1: len 6; hex 000000001018; asc       ;;
 2: len 7; hex 91000001420084; asc     B  ;;
 3: len 5; hex 53434f5454; asc SCOTT;;
 4: len 7; hex 414e414c595354; asc ANALYST;;
 5: len 4; hex 80001d8e; asc     ;;
 6: len 4; hex 208794f0; asc     ;;
 7: len 4; hex 80000bb8; asc     ;;
 8: SQL NULL;
 9: len 4; hex 80000014; asc     ;;

------------------
TABLE LOCK table `test`.`emp` trx id 4663 lock mode IX   ## 在给主键行上加X锁之前，先要在表上加意向锁IX
RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4663 lock_mode X locks rec but not gap waiting
Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0
 0: len 4; hex 80001e6c; asc    l;;
 1: len 6; hex 000000001018; asc       ;;
 2: len 7; hex 91000001420084; asc     B  ;;
 3: len 5; hex 53434f5454; asc SCOTT;;
 4: len 7; hex 414e414c595354; asc ANALYST;;
 5: len 4; hex 80001d8e; asc     ;;
 6: len 4; hex 208794f0; asc     ;;
 7: len 4; hex 80000bb8; asc     ;;
 8: SQL NULL;
 9: len 4; hex 80000014; asc     ;;

---TRANSACTION 4662, ACTIVE 81 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 360, 1 row lock(s)
MySQL thread id 7, OS thread handle 0x7f813c5c6700, query id 148 localhost root updating
update emp set sal=3500 where empno=7788
------- TRX HAS BEEN WAITING 81 SEC FOR THIS LOCK TO BE GRANTED:  ## 等待了81s
RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4662 lock_mode X locks rec but not gap waiting
Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0  ## 线程7在等待往test.emp中的主键上加X锁，page num=3
 0: len 4; hex 80001e6c; asc    l;;
 1: len 6; hex 000000001018; asc       ;;
 2: len 7; hex 91000001420084; asc     B  ;;
 3: len 5; hex 53434f5454; asc SCOTT;;
 4: len 7; hex 414e414c595354; asc ANALYST;;
 5: len 4; hex 80001d8e; asc     ;;
 6: len 4; hex 208794f0; asc     ;;
 7: len 4; hex 80000bb8; asc     ;;
 8: SQL NULL;
 9: len 4; hex 80000014; asc     ;;

------------------
TABLE LOCK table `test`.`emp` trx id 4662 lock mode IX   ## 在给主键行上加X锁之前，先要在表上加意向锁IX
RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4662 lock_mode X locks rec but not gap waiting
Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0
 0: len 4; hex 80001e6c; asc    l;;
 1: len 6; hex 000000001018; asc       ;;
 2: len 7; hex 91000001420084; asc     B  ;;
 3: len 5; hex 53434f5454; asc SCOTT;;
 4: len 7; hex 414e414c595354; asc ANALYST;;
 5: len 4; hex 80001d8e; asc     ;;
 6: len 4; hex 208794f0; asc     ;;
 7: len 4; hex 80000bb8; asc     ;;
 8: SQL NULL;
 9: len 4; hex 80000014; asc     ;;

---TRANSACTION 4615, ACTIVE 1579 sec, thread declared inside InnoDB 1222
mysql tables in use 2, locked 0
2 lock struct(s), heap size 360, 1 row lock(s)
MySQL thread id 3, OS thread handle 0x7f813c659700, query id 147 localhost root Sending data
select count(*) from t3 a,t3 b   ## 这是线程3当前正在执行的SQL
Trx read view will not see trx with id >= 4662, sees < 4659
TABLE LOCK table `test`.`emp` trx id 4615 lock mode IX  ## 线程3中正在拥有表上的意向IX锁，并且有test.emp表上主键的行级X锁，page num=3
RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4615 lock_mode X locks rec but not gap
Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0
 0: len 4; hex 80001e6c; asc    l;;
 1: len 6; hex 000000001018; asc       ;;
 2: len 7; hex 91000001420084; asc     B  ;;
 3: len 5; hex 53434f5454; asc SCOTT;;
 4: len 7; hex 414e414c595354; asc ANALYST;;
 5: len 4; hex 80001d8e; asc     ;;
 6: len 4; hex 208794f0; asc     ;;
 7: len 4; hex 80000bb8; asc     ;;
 8: SQL NULL;
 9: len 4; hex 80000014; asc     ;;

为什么线程3当前执行的是一个select t3表操作，但却锁住了test.emp表上page num=3？
有可能是线程3之前对test.emp表的操作事务没有及时提交导致。
所以得出：线程3阻塞了线程7，而线程7又阻塞了线程4，所以根因就是线程3，让线程3尽快提交或是kill掉即可。

4. 结论

在分析innodb中锁阻塞时，几种方法的对比情况：

（1）使用show processlist查看不靠谱；
（2）直接使用show engine innodb status查看，无法判断到问题的根因；
（3）使用mysqladmin debug查看，能看到所有产生锁的线程，但无法判断哪个才是根因；
（4）开启innodb_lock_monitor后，再使用show engine innodb status查看，能够找到锁阻塞的根因。