MySQL锁阻塞的深入分析

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。

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。

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