Scripting: don't call lua_gc() after Lua script run.

Calling lua_gc() after every script execution is too expensive, and
apparently does not make the execution smoother: the same peak latency
was measured before and after the commit.

This change accounts for scripts execution speedup in the order of 10%.

Author: antirez

src/scripting.c

@@ -943,7 +943,23 @@ void evalGenericCommand(redisClient *c, int evalsha) {
     }
     server.lua_caller = NULL;
     selectDb(c,server.lua_client->db->id); /* set DB ID from Lua client */
-    lua_gc(lua,LUA_GCSTEP,1);
+
+    /* Call the Lua garbage collector from time to time to avoid a
+     * full cycle performed by Lua, which adds too latency.
+     *
+     * The call is performed every LUA_GC_CYCLE_PERIOD executed commands
+     * (and for LUA_GC_CYCLE_PERIOD collection steps) because calling it
+     * for every command uses too much CPU. */
+    #define LUA_GC_CYCLE_PERIOD 50
+    {
+        static long gc_count = 0;
+
+        gc_count++;
+        if (gc_count == LUA_GC_CYCLE_PERIOD) {
+            lua_gc(lua,LUA_GCSTEP,LUA_GC_CYCLE_PERIOD);
+            gc_count = 0;
+        }
+    }
 
     if (err) {
         addReplyErrorFormat(c,"Error running script (call to %s): %s\n",