redis源码笔记 - redis对过期值的处理（in redis.c）

redis允许对key设置超时时间，实现过期key的自动淘汰。这篇blog分析下，其自适应（adaptive）的淘汰机制。

redis每隔100ms定时执行的循环（serverCron function）里有如下语句：

 655     /* Expire a few keys per cycle, only if this is a master.
 656      * On slaves we wait for DEL operations synthesized by the master
 657      * in order to guarantee a strict consistency. */
 658     if (server.masterhost == NULL) activeExpireCycle();

正如文中注释所示，只有master执行expire cycle，slave会等候由master传递的DEL消息，保证master-slave在过期值处理上的一致性。（后边代码会看到，redis对过期值的选择是随机抽取的，master-slave完全可能抽取不同的值，因此要求master通过DEL消息实现同步，同时这种expire机制也是不可靠的expire，即key超时后有可能不会被删除）。

activeExpireCycle函数如下：

477 /* Try to expire a few timed out keys. The algorithm used is adaptive and
 478  * will use few CPU cycles if there are few expiring keys, otherwise
 479  * it will get more aggressive to avoid that too much memory is used by
 480  * keys that can be removed from the keyspace. */
 481 void activeExpireCycle(void) {
 482     int j;
 483 
 484     for (j = 0; j < server.dbnum; j++) {
 485         int expired;
 486         redisDb *db = server.db+j;
 487 
 488         /* Continue to expire if at the end of the cycle more than 25%
 489          * of the keys were expired. */
 490         do {
 491             long num = dictSize(db->expires);
 492             time_t now = time(NULL);
 493 
 494             expired = 0;
 495             if (num > REDIS_EXPIRELOOKUPS_PER_CRON)
 496                 num = REDIS_EXPIRELOOKUPS_PER_CRON;
 497             while (num--) {
 498                 dictEntry *de;
 499                 time_t t;
 500 
 501                 if ((de = dictGetRandomKey(db->expires)) == NULL) break;
 502                 t = (time_t) dictGetEntryVal(de);
 503                 if (now > t) {
 504                     sds key = dictGetEntryKey(de);
 505                     robj *keyobj = createStringObject(key,sdslen(key));
 506 
 507                     propagateExpire(db,keyobj);     //将删除操作传播给各个slaves,在此之前，还将del操作记录aof
 508                     dbDelete(db,keyobj);   //这个函数先从db->expires中删除，然后删除db->dict
 509                     decrRefCount(keyobj);
 510                     expired++;
 511                     server.stat_expiredkeys++;
 512                 }
 513             }
 514         } while (expired > REDIS_EXPIRELOOKUPS_PER_CRON/4);
 515     }
 516 }

ExpireCycle每次尝试处理10个key，如果10个key中有>2.5个超时，则继续处理10个key。其用意在于，如果超时的key比例很高，则一次迭代处理很多个，否则等待下次serverCron循环再随机抽取。