Add per-db average TTL information in INFO output.

Example:

db0:keys=221913,expires=221913,avg_ttl=655

The algorithm uses a running average with only two samples (current and
previous). Keys found to be expired are considered at TTL zero even if
the actual TTL can be negative.

The TTL is reported in milliseconds.

src/redis.c

@@ -733,10 +733,14 @@ void activeExpireCycle(int type) {
          * of the keys were expired. */
         do {
             unsigned long num, slots;
-            long long now;
+            long long now, ttl_sum;
+            int ttl_samples;
 
             /* If there is nothing to expire try next DB ASAP. */
-            if ((num = dictSize(db->expires)) == 0) break;
+            if ((num = dictSize(db->expires)) == 0) {
+                db->avg_ttl = 0;
+                break;
+            }
             slots = dictSlots(db->expires);
             now = mstime();
 
@@ -749,14 +753,33 @@ void activeExpireCycle(int type) {
             /* The main collection cycle. Sample random keys among keys
              * with an expire set, checking for expired ones. */
             expired = 0;
+            ttl_sum = 0;
+            ttl_samples = 0;
+
             if (num > ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP)
                 num = ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP;
+
             while (num--) {
                 dictEntry *de;
+                long long ttl;
 
                 if ((de = dictGetRandomKey(db->expires)) == NULL) break;
+                ttl = dictGetSignedIntegerVal(de)-now;
                 if (activeExpireCycleTryExpire(db,de,now)) expired++;
+                if (ttl < 0) ttl = 0;
+                ttl_sum += ttl;
+                ttl_samples++;
             }
+
+            /* Update the average TTL stats for this database. */
+            if (ttl_samples) {
+                long long avg_ttl = ttl_sum/ttl_samples;
+
+                if (db->avg_ttl == 0) db->avg_ttl = avg_ttl;
+                /* Smooth the value averaging with the previous one. */
+                db->avg_ttl = (db->avg_ttl+avg_ttl)/2;
+            }
+
             /* 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. */
@@ -1490,6 +1513,7 @@ void initServer() {
         server.db[j].ready_keys = dictCreate(&setDictType,NULL);
         server.db[j].watched_keys = dictCreate(&keylistDictType,NULL);
         server.db[j].id = j;
+        server.db[j].avg_ttl = 0;
     }
     server.pubsub_channels = dictCreate(&keylistDictType,NULL);
     server.pubsub_patterns = listCreate();
@@ -2496,8 +2520,9 @@ sds genRedisInfoString(char *section) {
             keys = dictSize(server.db[j].dict);
             vkeys = dictSize(server.db[j].expires);
             if (keys || vkeys) {
-                info = sdscatprintf(info, "db%d:keys=%lld,expires=%lld\r\n",
-                    j, keys, vkeys);
+                info = sdscatprintf(info,
+                    "db%d:keys=%lld,expires=%lld,avg_ttl=%lld\r\n",
+                    j, keys, vkeys, server.db[j].avg_ttl);
             }
         }
     }

src/redis.h

@@ -398,6 +398,7 @@ typedef struct redisDb {
     dict *ready_keys;           /* Blocked keys that received a PUSH */
     dict *watched_keys;         /* WATCHED keys for MULTI/EXEC CAS */
     int id;
+    long long avg_ttl;          /* Average TTL, just for stats */
 } redisDb;
 
 /* Client MULTI/EXEC state */