Redis过期键删除策略

Redis key过期的方式有三种：

被动删除：当读/写一个已经过期的key时，会触发惰性删除策略，直接删除掉这个过期key
主动删除：由于惰性删除策略无法保证冷数据被及时删掉，所以Redis会定期主动淘汰一批已过期的key
当前已用内存超过maxmemory限定时，触发主动清理策略

被动删除

只有key被操作时(如GET)，REDIS才会被动检查该key是否过期，如果过期则删除之并且返回NIL。

1、这种删除策略对CPU是友好的，删除操作只有在不得不的情况下才会进行，不会其他的expire key上浪费无谓的CPU时间。

2、但是这种策略对内存不友好，一个key已经过期，但是在它被操作之前不会被删除，仍然占据内存空间。如果有大量的过期键存在但是又很少被访问到，那会造成大量的内存空间浪费。expireIfNeeded(redisDb *db, robj *key)函数位于src/db.c。

/*-----------------------------------------------------------------------------* Expires API*----------------------------------------------------------------------------*/int removeExpire(redisDb *db, robj *key) {/* An expire may only be removed if there is a corresponding entry in the* main dict. Otherwise, the key will never be freed. */redisAssertWithInfo(NULL,key,dictFind(db->dict,key->ptr) != NULL);return dictDelete(db->expires,key->ptr) == DICT_OK;
}void setExpire(redisDb *db, robj *key, long long when) {dictEntry *kde, *de;/* Reuse the sds from the main dict in the expire dict */kde = dictFind(db->dict,key->ptr);redisAssertWithInfo(NULL,key,kde != NULL);de = dictReplaceRaw(db->expires,dictGetKey(kde));dictSetSignedIntegerVal(de,when);
}/* Return the expire time of the specified key, or -1 if no expire* is associated with this key (i.e. the key is non volatile) */
long long getExpire(redisDb *db, robj *key) {dictEntry *de;/* No expire? return ASAP */if (dictSize(db->expires) == 0 ||(de = dictFind(db->expires,key->ptr)) == NULL) return -1;/* The entry was found in the expire dict, this means it should also* be present in the main dict (safety check). */redisAssertWithInfo(NULL,key,dictFind(db->dict,key->ptr) != NULL);return dictGetSignedIntegerVal(de);
}/* Propagate expires into slaves and the AOF file.* When a key expires in the master, a DEL operation for this key is sent* to all the slaves and the AOF file if enabled.** This way the key expiry is centralized in one place, and since both* AOF and the master->slave link guarantee operation ordering, everything* will be consistent even if we allow write operations against expiring* keys. */
void propagateExpire(redisDb *db, robj *key) {robj *argv[2];argv[0] = shared.del;argv[1] = key;incrRefCount(argv[0]);incrRefCount(argv[1]);if (server.aof_state != REDIS_AOF_OFF)feedAppendOnlyFile(server.delCommand,db->id,argv,2);replicationFeedSlaves(server.slaves,db->id,argv,2);decrRefCount(argv[0]);decrRefCount(argv[1]);
}int expireIfNeeded(redisDb *db, robj *key) {mstime_t when = getExpire(db,key);mstime_t now;if (when < 0) return 0; /* No expire for this key *//* Don't expire anything while loading. It will be done later. */if (server.loading) return 0;/* If we are in the context of a Lua script, we claim that time is* blocked to when the Lua script started. This way a key can expire* only the first time it is accessed and not in the middle of the* script execution, making propagation to slaves / AOF consistent.* See issue #1525 on Github for more information. */now = server.lua_caller ? server.lua_time_start : mstime();/* If we are running in the context of a slave, return ASAP:* the slave key expiration is controlled by the master that will* send us synthesized DEL operations for expired keys.** Still we try to return the right information to the caller,* that is, 0 if we think the key should be still valid, 1 if* we think the key is expired at this time. */if (server.masterhost != NULL) return now > when;/* Return when this key has not expired */if (now <= when) return 0;/* Delete the key */server.stat_expiredkeys++;propagateExpire(db,key);notifyKeyspaceEvent(REDIS_NOTIFY_EXPIRED,"expired",key,db->id);return dbDelete(db,key);
}/*-----------------------------------------------------------------------------* Expires Commands*----------------------------------------------------------------------------*//* This is the generic command implementation for EXPIRE, PEXPIRE, EXPIREAT* and PEXPIREAT. Because the commad second argument may be relative or absolute* the "basetime" argument is used to signal what the base time is (either 0* for *AT variants of the command, or the current time for relative expires).** unit is either UNIT_SECONDS or UNIT_MILLISECONDS, and is only used for* the argv[2] parameter. The basetime is always specified in milliseconds. */
void expireGenericCommand(redisClient *c, long long basetime, int unit) {robj *key = c->argv[1], *param = c->argv[2];long long when; /* unix time in milliseconds when the key will expire. */if (getLongLongFromObjectOrReply(c, param, &when, NULL) != REDIS_OK)return;if (unit == UNIT_SECONDS) when *= 1000;when += basetime;/* No key, return zero. */if (lookupKeyRead(c->db,key) == NULL) {addReply(c,shared.czero);return;}/* EXPIRE with negative TTL, or EXPIREAT with a timestamp into the past* should never be executed as a DEL when load the AOF or in the context* of a slave instance.** Instead we take the other branch of the IF statement setting an expire* (possibly in the past) and wait for an explicit DEL from the master. */if (when <= mstime() && !server.loading && !server.masterhost) {robj *aux;redisAssertWithInfo(c,key,dbDelete(c->db,key));server.dirty++;/* Replicate/AOF this as an explicit DEL. */aux = createStringObject("DEL",3);rewriteClientCommandVector(c,2,aux,key);decrRefCount(aux);signalModifiedKey(c->db,key);notifyKeyspaceEvent(REDIS_NOTIFY_GENERIC,"del",key,c->db->id);addReply(c, shared.cone);return;} else {setExpire(c->db,key,when);addReply(c,shared.cone);signalModifiedKey(c->db,key);notifyKeyspaceEvent(REDIS_NOTIFY_GENERIC,"expire",key,c->db->id);server.dirty++;return;}
}void expireCommand(redisClient *c) {expireGenericCommand(c,mstime(),UNIT_SECONDS);
}void expireatCommand(redisClient *c) {expireGenericCommand(c,0,UNIT_SECONDS);
}void pexpireCommand(redisClient *c) {expireGenericCommand(c,mstime(),UNIT_MILLISECONDS);
}void pexpireatCommand(redisClient *c) {expireGenericCommand(c,0,UNIT_MILLISECONDS);
}void ttlGenericCommand(redisClient *c, int output_ms) {long long expire, ttl = -1;/* If the key does not exist at all, return -2 */if (lookupKeyRead(c->db,c->argv[1]) == NULL) {addReplyLongLong(c,-2);return;}/* The key exists. Return -1 if it has no expire, or the actual* TTL value otherwise. */expire = getExpire(c->db,c->argv[1]);if (expire != -1) {ttl = expire-mstime();if (ttl < 0) ttl = 0;}if (ttl == -1) {addReplyLongLong(c,-1);} else {addReplyLongLong(c,output_ms ? ttl : ((ttl+500)/1000));}
}void ttlCommand(redisClient *c) {ttlGenericCommand(c, 0);
}void pttlCommand(redisClient *c) {ttlGenericCommand(c, 1);
}void persistCommand(redisClient *c) {dictEntry *de;de = dictFind(c->db->dict,c->argv[1]->ptr);if (de == NULL) {addReply(c,shared.czero);} else {if (removeExpire(c->db,c->argv[1])) {addReply(c,shared.cone);server.dirty++;} else {addReply(c,shared.czero);}}
}

但仅是这样是不够的，因为可能存在一些key永远不会被再次访问到，这些设置了过期时间的key也是需要在过期后被删除的，我们甚至可以将这种情况看作是一种内存泄露----无用的垃圾数据占用了大量的内存，而服务器却不会自己去释放它们，这对于运行状态非常依赖于内存的Redis服务器来说，肯定不是一个好消息

主动删除

先说一下时间事件，对于持续运行的服务器来说，服务器需要定期对自身的资源和状态进行必要的检查和整理，从而让服务器维持在一个健康稳定的状态，这类操作被统称为常规操作（cron job）

在 Redis 中，常规操作由 redis.c/serverCron 实现，它主要执行以下操作

更新服务器的各类统计信息，比如时间、内存占用、数据库占用情况等。
清理数据库中的过期键值对。
对不合理的数据库进行大小调整。
关闭和清理连接失效的客户端。
尝试进行 AOF 或 RDB 持久化操作。
如果服务器是主节点的话，对附属节点进行定期同步。
如果处于集群模式的话，对集群进行定期同步和连接测试。

Redis 将 serverCron 作为时间事件来运行，从而确保它每隔一段时间就会自动运行一次，又因为 serverCron 需要在 Redis 服务器运行期间一直定期运行，所以它是一个循环时间事件： serverCron 会一直定期执行，直到服务器关闭为止。

在 Redis 2.6 版本中，程序规定 serverCron 每秒运行 10 次，平均每 100 毫秒运行一次。从 Redis 2.8 开始，用户可以通过修改 hz 选项来调整 serverCron的每秒执行次数，具体信息请参考 redis.conf 文件中关于 hz 选项的说明

也叫定时删除，这里的“定期”指的是Redis定期触发的清理策略，由位于src/redis.c的activeExpireCycle(void)函数来完成。

serverCron是由redis的事件框架驱动的定位任务，这个定时任务中会调用activeExpireCycle函数，针对每个db在限制的时间REDIS_EXPIRELOOKUPS_TIME_LIMIT内迟可能多的删除过期key，之所以要限制时间是为了防止过长时间的阻塞影响redis的正常运行。这种主动删除策略弥补了被动删除策略在内存上的不友好。

因此，Redis会周期性的随机测试一批设置了过期时间的key并进行处理。测试到的已过期的key将被删除。典型的方式为,Redis每秒做10次如下的步骤：

随机测试100个设置了过期时间的key
删除所有发现的已过期的key
若删除的key超过25个则重复步骤1

这是一个基于概率的简单算法，基本的假设是抽出的样本能够代表整个key空间，redis持续清理过期的数据直至将要过期的key的百分比降到了25%以下。这也意味着在任何给定的时刻已经过期但仍占据着内存空间的key的量最多为每秒的写操作量除以4.

Redis-3.0.0中的默认值是10，代表每秒钟调用10次后台任务。

除了主动淘汰的频率外，Redis对每次淘汰任务执行的最大时长也有一个限定，这样保证了每次主动淘汰不会过多阻塞应用请求，以下是这个限定计算公式：

#define ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC 25 /* CPU max % for keys collection */
...
timelimit = 1000000*ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC/server.hz/100;

hz调大将会提高Redis主动淘汰的频率，如果你的Redis存储中包含很多冷数据占用内存过大的话，可以考虑将这个值调大，但Redis作者建议这个值不要超过100。我们实际线上将这个值调大到100，观察到CPU会增加2%左右，但对冷数据的内存释放速度确实有明显的提高（通过观察keyspace个数和used_memory大小）。

可以看出timelimit和server.hz是一个倒数的关系，也就是说hz配置越大，timelimit就越小。换句话说是每秒钟期望的主动淘汰频率越高，则每次淘汰最长占用时间就越短。这里每秒钟的最长淘汰占用时间是固定的250ms（1000000*ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC/100），而淘汰频率和每次淘汰的最长时间是通过hz参数控制的。

从以上的分析看，当redis中的过期key比率没有超过25%之前，提高hz可以明显提高扫描key的最小个数。假设hz为10，则一秒内最少扫描200个key（一秒调用10次*每次最少随机取出20个key），如果hz改为100，则一秒内最少扫描2000个key；另一方面，如果过期key比率超过25%，则扫描key的个数无上限，但是cpu时间每秒钟最多占用250ms。

当REDIS运行在主从模式时，只有主结点才会执行上述这两种过期删除策略，然后把删除操作”del key”同步到从结点。

maxmemory

当前已用内存超过maxmemory限定时，触发主动清理策略

volatile-lru：只对设置了过期时间的key进行LRU（默认值）
allkeys-lru ：删除lru算法的key
volatile-random：随机删除即将过期key
allkeys-random：随机删除
volatile-ttl ：删除即将过期的
noeviction ：永不过期，返回错误当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。

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。

清理时会根据用户配置的maxmemory-policy来做适当的清理（一般是LRU或TTL），这里的LRU或TTL策略并不是针对redis的所有key，而是以配置文件中的maxmemory-samples个key作为样本池进行抽样清理。

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，建议：

尽量不要触发maxmemory，最好在mem_used内存占用达到maxmemory的一定比例后，需要考虑调大hz以加快淘汰，或者进行集群扩容。
如果能够控制住内存，则可以不用修改maxmemory-samples配置；如果Redis本身就作为LRU cache服务（这种服务一般长时间处于maxmemory状态，由Redis自动做LRU淘汰），可以适当调大maxmemory-samples。

以下是上文中提到的配置参数的说明

# 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

Replication link和AOF文件中的过期处理

为了获得正确的行为而不至于导致一致性问题，当一个key过期时DEL操作将被记录在AOF文件并传递到所有相关的slave。也即过期删除操作统一在master实例中进行并向下传递，而不是各salve各自掌控。这样一来便不会出现数据不一致的情形。当slave连接到master后并不能立即清理已过期的key（需要等待由master传递过来的DEL操作），slave仍需对数据集中的过期状态进行管理维护以便于在slave被提升为master会能像master一样独立的进行过期处理。