activeExpireCycle(): process only a small number of DBs per iteration.

This small number of DBs is set to 16 so actually in the default
configuraiton Redis should behave exactly like in the past.
However the difference is that when the user configures a very large
number of DBs we don't do an O(N) operation, consuming a non trivial
amount of CPU per serverCron() iteration.

src/redis.c

@@ -617,9 +617,13 @@ void updateDictResizePolicy(void) {
 /* Try to expire a few timed out keys. The algorithm used is adaptive and
  * will use few CPU cycles if there are few expiring keys, otherwise
  * it will get more aggressive to avoid that too much memory is used by
- * keys that can be removed from the keyspace. */
+ * keys that can be removed from the keyspace.
+ *
+ * No more than REDIS_DBCRON_DBS_PER_SEC databases are tested at every
+ * iteration. */
 void activeExpireCycle(void) {
-    int j, iteration = 0;
+    static unsigned int current_db = 0;
+    unsigned int j, iteration = 0;
     long long start = ustime(), timelimit;
 
     /* We can use at max REDIS_EXPIRELOOKUPS_TIME_PERC percentage of CPU time
@@ -629,9 +633,14 @@ void activeExpireCycle(void) {
     timelimit = 1000000*REDIS_EXPIRELOOKUPS_TIME_PERC/server.hz/100;
     if (timelimit <= 0) timelimit = 1;
 
-    for (j = 0; j < server.dbnum; j++) {
+    for (j = 0; j < REDIS_DBCRON_DBS_PER_SEC; j++) {
         int expired;
-        redisDb *db = server.db+j;
+        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. */