这里要讨论的为过期的数据是如何被清除的，也就是网上常常讨论的过期清除策略。
需要注意的是，redis除了会对过期的数据进行淘汰，也可以通过对内存大小进行限制，并对超出内存限制后进行数据淘汰。此时淘汰的数据未必是过期的，只是因为内存达到限制而被淘汰。需要注意一下两者的区别，数据淘汰算法包括LRU、LFU等。
好，回归过期数据清除策略这里。redis的数据清除策略包括2种。惰性清除和定期清除。
惰性清除很简单，redis会在用户访问数据时，如果发现数据是过期的，那么就删除这个数据，并且给用户返回相应的结果。那么redis是如何发现这个数据是过期的呢？我们知道redis默认有16个库，0~15号库，每一个库对应源码中的一个db结构体，db中有一个dict类型成员变量expire，dict，即字典，也就是哈希表，是redis底层数据结构中的一种，expire哈希表中存储的键值对，key为键，value为其对应的过期时间，expire中存储了该库中所有键的过期时间。
那么定期清除呢？我们先贴上一段redis配置文件中的文字。
# Redis reclaims expired keys in two ways: upon access when those keys are
# found to be expired, and also in background, in what is called the
# "active expire key". The key space is slowly and interactively scanned
# looking for expired keys to reclaim, so that it is possible to free memory
# of keys that are expired and will never be accessed again in a short time.
#
(redis通过2种方法清除过期键：在操作时发现这些key是过期的，还有也是
通过后台清除的，被称作'定期清除key'。key空间被缓慢的、交互式的清除
过期键，所以它可能会清除过期的key来释放内存，并且短时间不会被再次
访问)
# The default effort of the expire cycle will try to avoid having more than
# ten percent of expired keys still in memory, and will try to avoid consuming
# more than 25% of total memory and to add latency to the system. However
# it is possible to increase the expire "effort" that is normally set to
# "1", to a greater value, up to the value "10". At its maximum value the
# system will use more CPU, longer cycles (and technically may introduce
# more latency), and will tolerate less already expired keys still present
# in the system. It's a tradeoff between memory, CPU and latency.
#
（清除循环作为一种默认的尝试，会尝试避免在内存中存在超过10%的过期
Key，并且避免(过期key占用的内存)超过总内存的25%，最终将内存归还系
统。它可以增加这种清除的尝试，通常它会被设置为1，到一个更大的值，
最大可以达到10。它的最大值意味着系统会使用更的的cpu资源，更久的循
环时间(理论上可能会导致延迟)，并且容忍更少的现在存在于系统中的过期
的key。它是一种在内存、CPU、延迟之间的一种平衡和折衷）
# active-expire-effort 1
也就是说redis定期清除会主动的进行清除。active-expire-effort控制了每次循环的时间有多长。清除达标的条件为避免在内存中存在超过10%的过期Key，并且避免(过期key占用的内存)超过总内存的25%。不达标的话会继续循环清除，直到达标为止。
另外一个参数hz控制了诸如清除循环、关闭超时客户端等后台任务的频率。综合控制了定期清除的行为。
hz相关注释如下：
# 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
参考文章：
[1]，Redis（6）删除策略（定时删除、惰性删除、定期删除）和数据逐出策略
