MySQL锁阻塞的深入分析
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2022-04-07 18:21:01
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：rr2. 测试...
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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