redis允許對key設置超時時間，實現過期key的自動淘汰。這篇blog分析下，其自適應（adaptive）的淘汰機制。
redis每隔100ms定時執行的循環（serverCron function）里有如下語句：
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();
正如文中注釋所示，只有master執行expire cycle，slave會等候由master傳遞的DEL消息，保證master-slave在過期值處理上的一致性。（后邊代碼會看到，redis對過期值的選擇是隨機抽取的，master-slave完全可能抽取不同的值，因此要求master通過DEL消息實現同步，同時這種expire機制也是不可靠的expire，即key超時后有可能不會被刪除）。
activeExpireCycle函數如下：
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); //將刪除操作傳播給各個slaves,在此之前，還將del操作記錄aof 508 dbDelete(db,keyobj); //這個函數先從db->expires中刪除，然后刪除db->dict 509 decrRefCount(keyobj); 510 expired++; 511 server.stat_expiredkeys++; 512 } 513 } 514 } while (expired > REDIS_EXPIRELOOKUPS_PER_CRON/4); 515 } 516 }
ExpireCycle每次嘗試處理10個key，如果10個key中有>2.5個超時，則繼續處理10個key。其用意在於，如果超時的key比例很高，則一次迭代處理很多個，否則等待下次serverCron循環再隨機抽取。