LFU: do some changes about LFU to find hotkeys

Firstly, use access time to replace the decreas time of LFU.
For function LFUDecrAndReturn,
it should only try to get decremented counter,
not update LFU fields, we will update it in an explicit way.
And we will times halve the counter according to the times of
elapsed time than server.lfu_decay_time.
Everytime a key is accessed, we should update the LFU
including update access time, and increment the counter after
call function LFUDecrAndReturn.
If a key is overwritten, the LFU should be also updated.
Then we can use `OBJECT freq` command to get a key's frequence,
and LFUDecrAndReturn should be called in `OBJECT freq` command
in case of the key has not been accessed for a long time,
because we update the access time only when the key is read or
overwritten.

src/db.c

@@ -38,6 +38,15 @@
  * C-level DB API
  *----------------------------------------------------------------------------*/
 
+/* Update LFU when an object is accessed.
+ * Firstly, decrement the counter if the decrement time is reached.
+ * Then logarithmically increment the counter, and update the access time. */
+void updateLFU(robj *val) {
+    unsigned long counter = LFUDecrAndReturn(val);
+    counter = LFULogIncr(counter);
+    val->lru = (LFUGetTimeInMinutes()<<8) | counter;
+}
+
 /* Low level key lookup API, not actually called directly from commands
  * implementations that should instead rely on lookupKeyRead(),
  * lookupKeyWrite() and lookupKeyReadWithFlags(). */
@@ -54,9 +63,7 @@ robj *lookupKey(redisDb *db, robj *key, int flags) {
             !(flags & LOOKUP_NOTOUCH))
         {
             if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
-                unsigned long ldt = val->lru >> 8;
-                unsigned long counter = LFULogIncr(val->lru & 255);
-                val->lru = (ldt << 8) | counter;
+                updateLFU(val);
             } else {
                 val->lru = LRU_CLOCK();
             }
@@ -180,6 +187,9 @@ void dbOverwrite(redisDb *db, robj *key, robj *val) {
         int saved_lru = old->lru;
         dictReplace(db->dict, key->ptr, val);
         val->lru = saved_lru;
+        /* LFU should be not only copied but also updated
+         * when a key is overwritten. */
+        updateLFU(val);
     } else {
         dictReplace(db->dict, key->ptr, val);
     }