--- a/src/share/vm/runtime/arguments.cpp	Thu Mar 28 19:13:22 2013 -0700
+++ b/src/share/vm/runtime/arguments.cpp	Fri Mar 29 08:38:00 2013 -0700
@@ -3320,6 +3320,13 @@
   }
   check_deprecated_gcs();
   check_deprecated_gc_flags();
+  if (AssumeMP && !UseSerialGC) {
+    if (FLAG_IS_DEFAULT(ParallelGCThreads) && ParallelGCThreads == 1) {
+      warning("If the number of processors is expected to increase from one, then"
+              " you should configure the number of parallel GC threads appropriately"
+              " using -XX:ParallelGCThreads=N");
+    }
+  }
 #else // INCLUDE_ALL_GCS
   assert(verify_serial_gc_flags(), "SerialGC unset");
 #endif // INCLUDE_ALL_GCS