What happens when redis runs out of memory

1. Memory recovery

Use redis We need to set the validity period for some key value pairs ,redis Through 4 A separate command to set the expiration time for a key value ：

（1）expire key ttl ： take key The expiration time for is set to ttl second

（2）pexpire key ttl ： take key The expiration time of the value is set to ttl millisecond

（3）expireat key timestamp ： take key The expiration time of is set to the specified timestamp Number of seconds

（4）pexpireat key timestamp ： take key The expiration time of the value is set to the specified timestamp Number of milliseconds

Whatever command you use ,redis The bottom floor is all through pexpireat To set the expiration time

View remaining expiration time ： Can pass ttl and pttl Two commands to view

ttl key return key Seconds left to expire

pttl key return key The number of milliseconds remaining to expire

If the expiration time is not set, this 2 All commands return -1 , If an illegal expiration time is set , All return to -2.

2. Expiration strategy

If you delete an expired key , We have three strategies

（1） Delete regularly ： Set a timer for each key , Once the expiration time is up , Delete the key . This strategy is memory friendly , But yes. CPU unfriendly , Because each timer will occupy a certain amount of CPU resources .

（2） Lazy deletion ： No matter whether the key has expired or not, it will not be deleted automatically , Wait until each time you get the key to determine whether it is expired , Delete the key if it is out of date , Otherwise, it returns the value of the key . This strategy is not memory friendly , It can waste a lot of memory .

（3） Scan regularly ： The system scans regularly at regular intervals , Delete the expired key when it is found . This strategy is relatively a compromise between the above two strategies , It should be noted that the regular frequency should be well controlled according to the actual situation , One drawback of using this scheme is that keys that have expired may also be returned .

stay Redis in , The choice is strategy （2） And strategy （3） Comprehensive use of , however redis The regular scan of will only scan the key with expiration time set , Because the key to set the expiration time Redis Will be stored separately , So you don't have to scan all the keys ：

typedef struct redisDb{

dict *dict ; // All key value pairs

dict * expires; // Set the key value pair of expiration time

dict * blocking_keys ;// blocked key

dict * watched_keys; //WATCHED keys

}

Redis Of 8 Kind of elimination strategy

First redis A parameter is provided maxmemory To configure the redis Maximum memory used ：maxmemory <bytes> Or by order config set maxmemory 1GB To dynamically modify . If the parameter is not set , So in 32 Bit in the operating system Redis Use... At most 3GB Memory , And in the 64 There are no restrictions in the bit operating system .

Redis Provided in 8 Kind of elimination strategy , You can use the parameter maxmemory-policy To configure ：

 

ttl： Set the expiration time of key in , Those with less time left will be eliminated first .

lru： Least recently used key, Priority to be eliminated .

lfu： The least visited recently key, Priority to be eliminated .

random： Randomly eliminate the contents of memory .

noeviction： Can no longer write Redis, Does not process the contents of memory , Is the default elimination strategy .