[Todo] 乐观悲观锁 自旋互斥锁等等

乐观锁、悲观锁、要实践

/blog/1860954 《mysql悲观锁总结和实践》

/blog/1863407 《mysql乐观锁总结和实践》

/sql/optimistic-lock-and-pessimistic-lock

注意，以下的表里面的列名，一定要用 `` 反引号来包括。

mysql> create table `t_goods` (-> `id` bigint(11) NOT NULL AUTO_INCREMENT,-> `status` bigint(11) DEFAULT 0,-> `name` varchar(32) DEFAULT NULL,-> `version` bigint(11) DEFAULT 1,-> PRIMARY KEY (`id`)-> ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;Query OK, 0 rows affected (0.13 sec)

mysql> insert into t_goods (`name`) values ('weapon');Query OK, 1 row affected (0.07 sec)mysql> insert into t_goods (`name`) values ('equipment');Query OK, 1 row affected (0.10 sec)

mysql> select * from t_goods;

+----+--------+-----------+---------+

| id | status | name | version |

+----+--------+-----------+---------+

| 1 | 0 | weapon | 1 |

| 2 | 0 | equipment | 1 |

+----+--------+-----------+---------+

2 rows in set (0.00 sec)

实验1，select for update 指定主键，只锁行：

首先要关闭autocommit：

mysql> set autocommit=0;Query OK, 0 rows affected (0.00 sec)mysql> show variables like 'autocommit';+---------------+-------+| Variable_name | Value |+---------------+-------+| autocommit | OFF |+---------------+-------+1 row in set (0.00 sec)

如果不关闭，经过实验，的确不会相互影响。

关闭autocommit之后，普通的sql不放在事务里面也可以。

console A:mysql> select * from t_goods where id = 1 for update;+----+--------+--------+---------+| id | status | name | version |+----+--------+--------+---------+| 1 |0 | weapon | 1 |+----+--------+--------+---------+1 row in set (0.00 sec)console B:mysql> select * from t_goods where id = 1;+----+--------+--------+---------+| id | status | name | version |+----+--------+--------+---------+| 1 |0 | weapon | 1 |+----+--------+--------+---------+1 row in set (0.00 sec)mysql> select * from t_goods where id = 2 for update;+----+--------+-----------+---------+| id | status | name| version |+----+--------+-----------+---------+| 2 |0 | equipment | 1 |+----+--------+-----------+---------+1 row in set (0.00 sec)mysql> select * from t_goods where id = 1 for update;ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

可以看出，不加for update不影响，加了for update不是同一行，不影响（仅对于主键查询有关）。

实验2，select for update 指定非主键，锁全部：

Console A:mysql> select * from t_goods where name = 'weapon' for update;+----+--------+--------+---------+| id | status | name | version |+----+--------+--------+---------+| 1 |0 | weapon | 1 |+----+--------+--------+---------+1 row in set (0.00 sec)Console B:mysql> select * from t_goods;+----+--------+-----------+---------+| id | status | name| version |+----+--------+-----------+---------+| 1 |0 | weapon | 1 || 2 |0 | equipment | 1 |+----+--------+-----------+---------+2 rows in set (0.00 sec)mysql> select * from t_goods where name = 'equipment' for update;ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transactionmysql> select * from t_goods where id = 2 for update;ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

实验3，查询主键没查到，不锁：

Console A:mysql> select * from t_goods where id=3 for update;Empty set (0.01 sec)Console B:mysql> select * from t_goods for update;+----+--------+-----------+---------+| id | status | name| version |+----+--------+-----------+---------+| 1 |0 | weapon | 1 || 2 |0 | equipment | 1 |+----+--------+-----------+---------+2 rows in set (0.00 sec)

说明，主键没查到数据，不加锁。

实验4，查询非主键，没查到，锁全部，table lock.

Console A:mysql> select * from t_goods where name = 'abc' for update;Empty set (0.00 sec)Console B:mysql> select * from t_goods for update;ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transactionmysql> select * from t_goods where id = 1 for update;ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

实验5：查询主键不明确，为范围，大于小于，只锁相关的行；

Console A:mysql> select * from t_goods where id > 1 for update;+----+--------+-----------+---------+| id | status | name| version |+----+--------+-----------+---------+| 2 |0 | equipment | 1 |+----+--------+-----------+---------+1 row in set (0.00 sec)Console B:mysql> select * from t_goods where id = 1 for update;+----+--------+--------+---------+| id | status | name | version |+----+--------+--------+---------+| 1 |0 | weapon | 1 |+----+--------+--------+---------+1 row in set (0.00 sec)mysql> select * from t_goods where id = 2 for update;ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

实验5：查询主键不明确，!=或者<>，锁全部；

Console A:mysql> select * from t_goods where id != 1 for update;+----+--------+-----------+---------+| id | status | name| version |+----+--------+-----------+---------+| 2 |0 | equipment | 1 |+----+--------+-----------+---------+1 row in set (0.00 sec)Console B:mysql> select * from t_goods where id = 1 for update;ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transactionConsole A:mysql> select * from t_goods where id <> 1 for update;+----+--------+-----------+---------+| id | status | name| version |+----+--------+-----------+---------+| 2 |0 | equipment | 1 |+----+--------+-----------+---------+1 row in set (0.00 sec)Console B:mysql> select * from t_goods where id = 1 for update;ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

实验6，对于普通索引，也类似于主键的效果：

未加索引之前，锁全表：console A:mysql> select * from t_goods where status = 1 for update;+----+--------+--------+---------+| id | status | name | version |+----+--------+--------+---------+| 1 |1 | weapon | 1 |+----+--------+--------+---------+1 row in set (0.00 sec)console B:mysql> select * from t_goods where id = 2 for update;ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction加了索引之后，只锁行：console A:mysql> alter table t_goods add index index_name(`status`);Query OK, 0 rows affected (0.15 sec)Records: 0 Duplicates: 0 Warnings: 0mysql> select * from t_goods where status = 1 for update;+----+--------+--------+---------+| id | status | name | version |+----+--------+--------+---------+| 1 |1 | weapon | 1 |+----+--------+--------+---------+1 row in set (0.00 sec)console B:mysql> select * from t_goods where id = 2 for update;+----+--------+-----------+---------+| id | status | name| version |+----+--------+-----------+---------+| 2 |0 | equipment | 1 |+----+--------+-----------+---------+1 row in set (0.00 sec)mysql> select * from t_goods where id = 1 for update;ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

乐观锁：

从业务层面加锁，一般是加上version字段，然后sql以如下形式处理：

update t_goods set status=2,version=version+1where id=#{id} and version=#{version};

自旋锁与互斥锁

/a675311/article/details/49096435

自旋锁就是不听的忙检测，拿不到锁就返回。

pthread中提供的锁有：pthread_mutex_t, pthread_spinlock_t, pthread_rwlock_t。pthread_mutex_t是互斥锁，同一瞬间只能有一个线程能够获取锁，其他线程在等待获取锁的时候会进入休眠状态。因此pthread_mutex_t消耗的CPU资源很小，但是性能不高，因为会引起线程切换。pthread_spinlock_t是自旋锁，同一瞬间也只能有一个线程能够获取锁，不同的是，其他线程在等待获取锁的过程中并不进入睡眠状态，而是在CPU上进入“自旋”等待。自旋锁的性能很高，但是只适合对很小的代码段加锁（或短期持有的锁），自旋锁对CPU的占用相对较高。pthread_rwlock_t是读写锁，同时可以有多个线程获得读锁，同时只允许有一个线程获得写锁。其他线程在等待锁的时候同样会进入睡眠。读写锁在互斥锁的基础上，允许多个线程“读”，在某些场景下能提高性能。诸如pthread中的pthread_cond_t, pthread_barrier_t, semaphone等，更像是一种同步原语，不属于单纯的锁。

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