关于Redis的内存淘汰策略详解

一、什么是内存淘汰？

如果在做项目时，不计任何后果地把任何数据都往 Redis 写入，使用不合理很容易导致数据超过Redis 的最大内存，这种情况就会导致如下问题。

• Redis 中有很多无效的缓存，这些缓存数据会降低 IO 性能。
• 随着系统的运行，Redis 的数据越来越多，会导致物理内存不足。虽然可以通过使用虚拟内存，将很少访问的数据交换到磁盘上，腾出内存空间的方法来解决物理内存不足的问题，但是由于这部分数据存储在磁盘上，如果在高并发场景中，频繁访问虚拟内存空间会严重降低系统性能。

所以遇到 Redis 内存不足的问题时，我们一般有几种方法：

• 对每个存储到 Redis 中的 key 设置过期时间，这个根据实际业务场景来决定。否则，再大的内存都会随着系统运行被消耗完；
• 增加内存；
• 使用内存淘汰策略。

当内存空间使用达到限制时，Redis 会根据配置策略来选择不同处理方式，要么返回 errors，要么按照不同的策略算法来清除一些旧数据，达到回收内存的目的，这就是 Redis 的内存淘汰，有些文章中，内存淘汰也叫缓存回收。

本文以 Linux 系统安装的 4.0.8 版本的 Redis 为例，对内存淘汰策略进行总结。Redis 的最大内存上限、淘汰算法的配置都在 redis.conf 文件中有说明，现在把 redis.conf 中对内存管理的部分内容摘录如下。

############################## MEMORY MANAGEMENT ################################
# Set a memory usage limit to the specified amount of bytes.
# When the memory limit is reached Redis will try to remove keys
# according to the eviction policy selected (see maxmemory-policy).
#
# If Redis can't remove keys according to the policy, or if the policy is
# set to 'noeviction', Redis will start to reply with errors to commands
# that would use more memory, like SET, LPUSH, and so on, and will continue
# to reply to read-only commands like GET.
#
# This option is usually useful when using Redis as an LRU or LFU cache, or to
# set a hard memory limit for an instance (using the 'noeviction' policy).
#
# WARNING: If you have slaves attached to an instance with maxmemory on,
# the size of the output buffers needed to feed the slaves are subtracted
# from the used memory count, so that network problems / resyncs will
# not trigger a loop where keys are evicted, and in turn the output
# buffer of slaves is full with DELs of keys evicted triggering the deletion
# of more keys, and so forth until the database is completely emptied.
#
# In short... if you have slaves attached it is suggested that you set a lower
# limit for maxmemory so that there is some free RAM on the system for slave
# output buffers (but this is not needed if the policy is 'noeviction').
#
# maxmemory <bytes>
# MAXMEMORY POLICY: how Redis will select what to remove when maxmemory
# is reached. You can select among five behaviors:
#
# volatile-lru -> Evict using approximated LRU among the keys with an expire set.
# allkeys-lru -> Evict any key using approximated LRU.
# volatile-lfu -> Evict using approximated LFU among the keys with an expire set.
# allkeys-lfu -> Evict any key using approximated LFU.
# volatile-random -> Remove a random key among the ones 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 evict anything, just return an error on write operations.
#
# LRU means Least Recently Used
# LFU means Least Frequently Used
#
# Both LRU, LFU and volatile-ttl are implemented using approximated
# randomized algorithms.
#
# 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, LFU 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 more CPU. 3 is faster but not very accurate.
#
# maxmemory-samples 5

maxmemory-samples 5