--- old/src/share/vm/services/memTrackWorker.hpp	Fri Jul 13 08:04:47 2012
+++ new/src/share/vm/services/memTrackWorker.hpp	Fri Jul 13 08:04:46 2012
@@ -67,7 +67,7 @@
   NOT_PRODUCT(int _last_gen_in_use;)
 
   inline int generations_in_use() const {
-    return (_tail <= _head ? (_head - _tail + 1) : (MAX_GENERATIONS - (_tail - _head) + 1));
+    return (_tail >= _head ? (_tail - _head + 1) : (MAX_GENERATIONS - (_head - _tail) + 1));
   }
 };