Support TTL stats with the new expire datastructure

Former-commit-id: 271df3dad4f55f20177a8a9a065778f4943835f1

src/db.cpp

@@ -1122,11 +1122,13 @@ int dbSwapDatabases(int id1, int id2) {
     db1->setexpire = db2->setexpire;
     db1->expireitr = db2->expireitr;
     db1->avg_ttl = db2->avg_ttl;
+    db1->last_expire_set = db2->last_expire_set;
 
     db2->pdict = aux.pdict;
     db2->setexpire = aux.setexpire;
     db2->expireitr = aux.expireitr;
     db2->avg_ttl = aux.avg_ttl;
+    db2->last_expire_set = aux.last_expire_set;
 
     /* Now we need to handle clients blocked on lists: as an effect
      * of swapping the two DBs, a client that was waiting for list
@@ -1220,6 +1222,19 @@ void setExpire(client *c, redisDb *db, robj *key, long long when) {
     expireEntry e((sds)dictGetKey(kde), when);
     ((robj*)dictGetVal(kde))->SetFExpires(true);
 
+    /* Update TTL stats (exponential moving average) */
+    /*  Note: We never have to update this on expiry since we reduce it by the current elapsed time here */
+    long long now = g_pserver->mstime;
+    db->avg_ttl -= (now - db->last_expire_set); // reduce the TTL by the time that has elapsed
+    if (db->setexpire->empty())
+        db->avg_ttl = 0;
+    else
+        db->avg_ttl -= db->avg_ttl / db->setexpire->size(); // slide one entry out the window
+    if (db->avg_ttl < 0)
+        db->avg_ttl = 0;    // TTLs are never negative
+    db->avg_ttl += (double)(when-now) / (db->setexpire->size()+1);    // add the new entry
+    db->last_expire_set = now;
+
     db->setexpire->insert(e);
 
     int writable_slave = listLength(g_pserver->masters) && g_pserver->repl_slave_ro == 0;

src/expire.cpp

@@ -155,8 +155,8 @@ void activeExpireCycle(int type) {
         /* If there is nothing to expire try next DB ASAP. */
         if (db->setexpire->empty())
         {
-            // TODO: Compute db->avg_ttl somewhere... but probably not here
             db->avg_ttl = 0;
+            db->last_expire_set = now;
             continue;
         }
         

src/server.cpp

@@ -2926,6 +2926,7 @@ void initServer(void) {
         g_pserver->db[j].watched_keys = dictCreate(&keylistDictType,NULL);
         g_pserver->db[j].id = j;
         g_pserver->db[j].avg_ttl = 0;
+        g_pserver->db[j].last_expire_set = 0;
         g_pserver->db[j].defrag_later = listCreate();
     }
 
@@ -4566,10 +4567,17 @@ sds genRedisInfoString(const char *section) {
 
             keys = dictSize(g_pserver->db[j].pdict);
             vkeys = g_pserver->db[j].setexpire->size();
+
+            // Adjust TTL by the current time
+            g_pserver->db[j].avg_ttl -= (g_pserver->mstime - g_pserver->db[j].last_expire_set);
+            if (g_pserver->db[j].avg_ttl < 0)
+                g_pserver->db[j].avg_ttl = 0;
+            g_pserver->db[j].last_expire_set = g_pserver->mstime;
+            
             if (keys || vkeys) {
                 info = sdscatprintf(info,
                     "db%d:keys=%lld,expires=%lld,avg_ttl=%lld\r\n",
-                    j, keys, vkeys, g_pserver->db[j].avg_ttl);
+                    j, keys, vkeys, static_cast<long long>(g_pserver->db[j].avg_ttl));
             }
         }
     }

src/server.h

@@ -844,7 +844,8 @@ typedef struct redisDb {
     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 */
+    long long last_expire_set;  /* when the last expire was set */
+    double avg_ttl;             /* Average TTL, just for stats */
     list *defrag_later;         /* List of key names to attempt to defrag one by one, gradually. */
 } redisDb;