--- old/src/share/vm/gc/serial/defNewGeneration.cpp	2015-05-20 11:02:00.843278190 +0200
+++ new/src/share/vm/gc/serial/defNewGeneration.cpp	2015-05-20 11:02:00.691273054 +0200
@@ -627,7 +627,9 @@
          "save marks have not been newly set.");
 
   {
-    // SerialGC runs with n_workers == 0.
+    // DefNew needs to run with n_threads == 0, to make sure the serial
+    // version of the card table scanning code is used.
+    // See: CardTableModRefBS::non_clean_card_iterate_possibly_parallel.
     StrongRootsScope srs(0);
 
     gch->gen_process_roots(&srs,