redis中的maxmemory

前言

• 当redis中的存储数据的容量达到最大时，若果我们再往里面存储数据，redis在默认的配置下会抛出异常，那么当mem_used内存已经超过maxmemory，我们采取的措施是什么哪？

清理策略

• 1.volatile-lru:从设置了过期时间的数据集中，选择最近最久未使用的数据释放。
• 2.allkeys-lru:从数据集中(包括设置过期时间以及未设置过期时间的数据集中)，选择最近最久未使用的数据释放。
• 3.volatile-random:从设置了过期时间的数据集中，随机选择一个数据进行释放。
• 4.allkeys-random:从数据集中(包括了设置过期时间以及未设置过期时间)随机选择一个数据进行入释放。
• 5.volatile-ttl：从设置了过期时间的数据集中，选择马上就要过期的数据进行释放操作。
• 6.noeviction：不删除任意数据(但redis还会根据引用计数器进行释放呦~),这时如果内存不够时，会直接返回错误。

默认配置是noeviction

过程解析

• 当mem_used内存已经超过maxmemory的设定，对于所有的读写请求，都会触发redis.c/freeMemoryIfNeeded(void)函数以清理超出的内存。注意这个清理过程是阻塞的，直到清理出足够的内存空间。所以如果在达到maxmemory并且调用方还在不断写入的情况下，可能会反复触发主动清理策略，导致请求会有一定的延迟。

redis.conf的默认配置

# Redis calls an internal function to perform many background tasks, like 
# closing connections of clients in timeout, purging expired keys that are 
# never requested, and so forth. 
# 
# Not all tasks are performed with the same frequency, but Redis checks for 
# tasks to perform according to the specified "hz" value. 
# 
# By default "hz" is set to 10. Raising the value will use more CPU when 
# Redis is idle, but at the same time will make Redis more responsive when 
# there are many keys expiring at the same time, and timeouts may be 
# handled with more precision. 
# 
# The range is between 1 and 500, however a value over 100 is usually not 
# a good idea. Most users should use the default of 10 and raise this up to 
# 100 only in environments where very low latency is required. 
hz 10 

# MAXMEMORY POLICY: how Redis will select what to remove when maxmemory 
# is reached. You can select among five behaviors: 
# 
# volatile-lru -> remove the key with an expire set using an LRU algorithm 
# allkeys-lru -> remove any key according to the LRU algorithm 
# volatile-random -> remove a random key with an expire set 
# allkeys-random -> remove a random key, any key 
# volatile-ttl -> remove the key with the nearest expire time (minor TTL) 
# noeviction -> don't expire at all, just return an error on write operations 
# 
# Note: with any of the above policies, Redis will return an error on write 
#       operations, when there are no suitable keys for eviction. 
# 
#       At the date of writing these commands are: set setnx setex append 
#       incr decr rpush lpush rpushx lpushx linsert lset rpoplpush sadd 
#       sinter sinterstore sunion sunionstore sdiff sdiffstore zadd zincrby 
#       zunionstore zinterstore hset hsetnx hmset hincrby incrby decrby 
#       getset mset msetnx exec sort 
# 
# The default is: 
# 
maxmemory-policy noeviction 

# LRU and minimal TTL algorithms are not precise algorithms but approximated 
# algorithms (in order to save memory), so you can tune it for speed or 
# accuracy. For default Redis will check five keys and pick the one that was 
# used less recently, you can change the sample size using the following 
# configuration directive. 
# 
# The default of 5 produces good enough results. 10 Approximates very closely 
# true LRU but costs a bit more CPU. 3 is very fast but not very accurate. 
# 
maxmemory-samples 5

小结

• maxmemory-samples在redis-3.0.0中的默认配置为5，如果增加，会提高LRU或TTL的精准度，redis作者测试的结果是当这个配置为10时已经非常接近全量LRU的精准度了，并且增加maxmemory-samples会导致在主动清理时消耗更多的CPU时间。