Obtain LRU clock in a resolution dependent way.

For testing purposes it is handy to have a very high resolution of the
LRU clock, so that it is possible to experiment with scripts running in
just a few seconds how the eviction algorithms works.

This commit allows Redis to use the cached LRU clock, or a value
computed on demand, depending on the resolution. So normally we have the
good performance of a precomputed value, and a clock that wraps in many
days using the normal resolution, but if needed, changing a define will
switch behavior to an high resolution LRU clock.

src/db.c

@@ -50,7 +50,7 @@ robj *lookupKey(redisDb *db, robj *key) {
          * Don't do it if we have a saving child, as this will trigger
          * a copy on write madness. */
         if (server.rdb_child_pid == -1 && server.aof_child_pid == -1)
-            val->lru = server.lruclock;
+            val->lru = LRU_CLOCK();
         return val;
     } else {
         return NULL;

src/object.c

@@ -651,10 +651,11 @@ char *strEncoding(int encoding) {
 /* Given an object returns the min number of seconds the object was never
  * requested, using an approximated LRU algorithm. */
 unsigned long long estimateObjectIdleTime(robj *o) {
-    if (server.lruclock >= o->lru) {
-        return (server.lruclock - o->lru) * REDIS_LRU_CLOCK_RESOLUTION;
+    unsigned long long lruclock = LRU_CLOCK();
+    if (lruclock >= o->lru) {
+        return (lruclock - o->lru) * REDIS_LRU_CLOCK_RESOLUTION;
     } else {
-        return ((REDIS_LRU_CLOCK_MAX - o->lru) + server.lruclock) *
+        return (lruclock + (REDIS_LRU_CLOCK_MAX - o->lru)) *
                     REDIS_LRU_CLOCK_RESOLUTION;
     }
 }

src/redis.c

@@ -843,9 +843,8 @@ void activeExpireCycle(int type) {
     }
 }
 
-void updateLRUClock(void) {
-    server.lruclock = (mstime()/REDIS_LRU_CLOCK_RESOLUTION) &
-                                                REDIS_LRU_CLOCK_MAX;
+unsigned int getLRUClock(void) {
+    return (mstime()/REDIS_LRU_CLOCK_RESOLUTION) & REDIS_LRU_CLOCK_MAX;
 }
 
 /* Add a sample to the operations per second array of samples. */
@@ -1044,19 +1043,18 @@ int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
 
     run_with_period(100) trackOperationsPerSecond();
 
-    /* We have just 22 bits per object for LRU information.
-     * So we use an (eventually wrapping) LRU clock with 10 seconds resolution.
-     * 2^22 bits with 10 seconds resolution is more or less 1.5 years.
+    /* We have just REDIS_LRU_BITS bits per object for LRU information.
+     * So we use an (eventually wrapping) LRU clock.
      *
-     * Note that even if this will wrap after 1.5 years it's not a problem,
-     * everything will still work but just some object will appear younger
-     * to Redis. But for this to happen a given object should never be touched
-     * for 1.5 years.
+     * Note that even if the counter wraps it's not a big problem,
+     * everything will still work but some object will appear younger
+     * to Redis. However for this to happen a given object should never be
+     * touched for all the time needed to the counter to wrap, which is
+     * not likely.
      *
      * Note that you can change the resolution altering the
-     * REDIS_LRU_CLOCK_RESOLUTION define.
-     */
-    updateLRUClock();
+     * REDIS_LRU_CLOCK_RESOLUTION define. */
+    server.lruclock = getLRUClock();
 
     /* Record the max memory used since the server was started. */
     if (zmalloc_used_memory() > server.stat_peak_memory)
@@ -1421,7 +1419,7 @@ void initServerConfig() {
     server.migrate_cached_sockets = dictCreate(&migrateCacheDictType,NULL);
     server.loading_process_events_interval_bytes = (1024*1024*2);
 
-    updateLRUClock();
+    server.lruclock = getLRUClock();
     resetServerSaveParams();
 
     appendServerSaveParams(60*60,1);  /* save after 1 hour and 1 change */

src/redis.h

@@ -394,6 +394,12 @@ typedef struct redisObject {
     void *ptr;
 } robj;
 
+/* Macro used to obtain the current LRU clock.
+ * If the current resolution is lower than the frequency we refresh the
+ * LRU clock (as it should be in production servers) we return the
+ * precomputed value, otherwise we need to resort to a function call. */
+#define LRU_CLOCK() ((1000/server.hz <= REDIS_LRU_CLOCK_RESOLUTION) ? server.lruclock : getLRUClock())
+
 /* Macro used to initialize a Redis object allocated on the stack.
  * Note that this macro is taken near the structure definition to make sure
  * we'll update it when the structure is changed, to avoid bugs like
@@ -1055,7 +1061,7 @@ char *strEncoding(int encoding);
 int compareStringObjects(robj *a, robj *b);
 int collateStringObjects(robj *a, robj *b);
 int equalStringObjects(robj *a, robj *b);
-unsigned long estimateObjectIdleTime(robj *o);
+unsigned long long estimateObjectIdleTime(robj *o);
 #define sdsEncodedObject(objptr) (objptr->encoding == REDIS_ENCODING_RAW || objptr->encoding == REDIS_ENCODING_EMBSTR)
 
 /* Synchronous I/O with timeout */
@@ -1156,6 +1162,7 @@ void adjustOpenFilesLimit(void);
 void closeListeningSockets(int unlink_unix_socket);
 void updateCachedTime(void);
 void resetServerStats(void);
+unsigned int getLRUClock(void);
 
 /* Set data type */
 robj *setTypeCreate(robj *value);