1.定时过期expilre     expire key TTL 10定时器  主动淘汰
2.惰性过期  被动淘汰
3getCommand 
expireIfNeed()


设置内存上线   set   memory   上线
activeExpireCycle()


redis使用了   惰性过期 + 定期过期


达到了最大内存上线后根据内存淘汰规则


# 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  不在接收新的指令存进内存