[Todo] 乐观悲观锁,自旋互斥锁等等
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乐观锁、悲观锁、要实践
http://chenzhou123520.iteye.com/blog/1860954 《mysql悲观锁总结和实践》
http://chenzhou123520.iteye.com/blog/1863407 《mysql乐观锁总结和实践》
http://outofmemory.cn/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 transaction mysql> 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 transaction mysql> 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 transaction 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 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: 0 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; +----+--------+-----------+---------+ | 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+1 where id=#{id} and version=#{version};
自旋锁与互斥锁
http://blog.csdn.net/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。
http://www.cnblogs.com/hdflzh/p/3716156.html
http://blog.csdn.net/pi9nc/article/details/39177343
Java锁相关
http://blog.csdn.net/Evankaka/article/details/44153709 (这一篇要重点看,讲了Thread Runnable等)
http://blog.csdn.net/Evankaka/article/details/51866242(Java锁技术内幕上)
http://blog.csdn.net/evankaka/article/details/51932044(Java锁技术内幕中)
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