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.

1 files changed, 12 insertions, 14 deletions

diff --git a/src/redis.c b/src/redis.c
index b8ed87325..7719745c8 100644
--- a/src/redis.c
+++ b/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 */