typedef struct redisDb {dict *dict;                 /* The keyspace for this DB */dict *expires;              /* Timeout of keys with a timeout set */dict *blocking_keys;        /* Keys with clients waiting for data (BLPOP)*/dict *ready_keys;           /* Blocked keys that received a PUSH */dict *watched_keys;         /* WATCHED keys for MULTI/EXEC CAS */int id;                     /* Database ID */long long avg_ttl;          /* Average TTL, just for stats */unsigned long expires_cursor; /* Cursor of the active expire cycle. */list *defrag_later;         /* List of key names to attempt to defrag one by one, gradually. */
} redisDb;

#define ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP 20 /* Keys for each DB loop. */
#define ACTIVE_EXPIRE_CYCLE_FAST_DURATION 1000 /* Microseconds. */
#define ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC 25 /* Max % of CPU to use. */
#define ACTIVE_EXPIRE_CYCLE_ACCEPTABLE_STALE 10 /* % of stale keys after whichwe do extra efforts. */void activeExpireCycle(int type) {/* Adjust the running parameters according to the configured expire* effort. The default effort is 1, and the maximum configurable effort* is 10. */unsigned longeffort = server.active_expire_effort-1, /* Rescale from 0 to 9. */config_keys_per_loop = ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP +ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP/4*effort,config_cycle_fast_duration = ACTIVE_EXPIRE_CYCLE_FAST_DURATION +ACTIVE_EXPIRE_CYCLE_FAST_DURATION/4*effort,config_cycle_slow_time_perc = ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC +2*effort,config_cycle_acceptable_stale = ACTIVE_EXPIRE_CYCLE_ACCEPTABLE_STALE-effort;/* This function has some global state in order to continue the work* incrementally across calls. */static unsigned int current_db = 0; /* Last DB tested. */static int timelimit_exit = 0;      /* Time limit hit in previous call? */static long long last_fast_cycle = 0; /* When last fast cycle ran. */int j, iteration = 0;int dbs_per_call = CRON_DBS_PER_CALL;long long start = ustime(), timelimit, elapsed;/* When clients are paused the dataset should be static not just from the* POV of clients not being able to write, but also from the POV of* expires and evictions of keys not being performed. */if (clientsArePaused()) return;if (type == ACTIVE_EXPIRE_CYCLE_FAST) {/* Don't start a fast cycle if the previous cycle did not exit* for time limit, unless the percentage of estimated stale keys is* too high. Also never repeat a fast cycle for the same period* as the fast cycle total duration itself. */if (!timelimit_exit &&server.stat_expired_stale_perc < config_cycle_acceptable_stale)return;if (start < last_fast_cycle + (long long)config_cycle_fast_duration*2)return;last_fast_cycle = start;}/* We usually should test CRON_DBS_PER_CALL per iteration, with* two exceptions:** 1) Don't test more DBs than we have.* 2) If last time we hit the time limit, we want to scan all DBs* in this iteration, as there is work to do in some DB and we don't want* expired keys to use memory for too much time. */if (dbs_per_call > server.dbnum || timelimit_exit)dbs_per_call = server.dbnum;/* We can use at max 'config_cycle_slow_time_perc' percentage of CPU* time per iteration. Since this function gets called with a frequency of* server.hz times per second, the following is the max amount of* microseconds we can spend in this function. */timelimit = config_cycle_slow_time_perc*1000000/server.hz/100;timelimit_exit = 0;if (timelimit <= 0) timelimit = 1;if (type == ACTIVE_EXPIRE_CYCLE_FAST)timelimit = config_cycle_fast_duration; /* in microseconds. *//* Accumulate some global stats as we expire keys, to have some idea* about the number of keys that are already logically expired, but still* existing inside the database. */long total_sampled = 0;long total_expired = 0;for (j = 0; j < dbs_per_call && timelimit_exit == 0; j++) {/* Expired and checked in a single loop. */unsigned long expired, sampled;redisDb *db = server.db+(current_db % server.dbnum);/* Increment the DB now so we are sure if we run out of time* in the current DB we'll restart from the next. This allows to* distribute the time evenly across DBs. */current_db++;/* Continue to expire if at the end of the cycle more than 25%* of the keys were expired. */do {unsigned long num, slots;long long now, ttl_sum;int ttl_samples;iteration++;/* If there is nothing to expire try next DB ASAP. */if ((num = dictSize(db->expires)) == 0) {db->avg_ttl = 0;break;}slots = dictSlots(db->expires);now = mstime();/* When there are less than 1% filled slots, sampling the key* space is expensive, so stop here waiting for better times...* The dictionary will be resized asap. */if (num && slots > DICT_HT_INITIAL_SIZE &&(num*100/slots < 1)) break;/* The main collection cycle. Sample random keys among keys* with an expire set, checking for expired ones. */expired = 0;sampled = 0;ttl_sum = 0;ttl_samples = 0;if (num > config_keys_per_loop)num = config_keys_per_loop;/* Here we access the low level representation of the hash table* for speed concerns: this makes this code coupled with dict.c,* but it hardly changed in ten years.** Note that certain places of the hash table may be empty,* so we want also a stop condition about the number of* buckets that we scanned. However scanning for free buckets* is very fast: we are in the cache line scanning a sequential* array of NULL pointers, so we can scan a lot more buckets* than keys in the same time. */long max_buckets = num*20;long checked_buckets = 0;while (sampled < num && checked_buckets < max_buckets) {for (int table = 0; table < 2; table++) {if (table == 1 && !dictIsRehashing(db->expires)) break;unsigned long idx = db->expires_cursor;idx &= db->expires->ht[table].sizemask;dictEntry *de = db->expires->ht[table].table[idx];long long ttl;/* Scan the current bucket of the current table. */checked_buckets++;while(de) {/* Get the next entry now since this entry may get* deleted. */dictEntry *e = de;de = de->next;ttl = dictGetSignedIntegerVal(e)-now;if (activeExpireCycleTryExpire(db,e,now)) expired++;if (ttl > 0) {/* We want the average TTL of keys yet* not expired. */ttl_sum += ttl;ttl_samples++;}sampled++;}}db->expires_cursor++;}total_expired += expired;total_sampled += sampled;/* Update the average TTL stats for this database. */if (ttl_samples) {long long avg_ttl = ttl_sum/ttl_samples;/* Do a simple running average with a few samples.* We just use the current estimate with a weight of 2%* and the previous estimate with a weight of 98%. */if (db->avg_ttl == 0) db->avg_ttl = avg_ttl;db->avg_ttl = (db->avg_ttl/50)*49 + (avg_ttl/50);}/* We can't block forever here even if there are many keys to* expire. So after a given amount of milliseconds return to the* caller waiting for the other active expire cycle. */if ((iteration & 0xf) == 0) { /* check once every 16 iterations. */elapsed = ustime()-start;if (elapsed > timelimit) {timelimit_exit = 1;server.stat_expired_time_cap_reached_count++;break;}}/* We don't repeat the cycle for the current database if there are* an acceptable amount of stale keys (logically expired but yet* not reclained). */} while ((expired*100/sampled) > config_cycle_acceptable_stale);}elapsed = ustime()-start;server.stat_expire_cycle_time_used += elapsed;latencyAddSampleIfNeeded("expire-cycle",elapsed/1000);/* Update our estimate of keys existing but yet to be expired.* Running average with this sample accounting for 5%. */double current_perc;if (total_sampled) {current_perc = (double)total_expired/total_sampled;} elsecurrent_perc = 0;server.stat_expired_stale_perc = (current_perc*0.05)+(server.stat_expired_stale_perc*0.95);
}