[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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