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。