--- a/hotspot/src/share/vm/gc/g1/g1RootProcessor.cpp	Wed May 20 10:45:40 2015 +0200
+++ b/hotspot/src/share/vm/gc/g1/g1RootProcessor.cpp	Thu May 21 09:10:47 2015 +0200
@@ -253,7 +253,7 @@
 
   {
     G1GCParPhaseTimesTracker x(phase_times, G1GCPhaseTimes::ThreadRoots, worker_i);
-    bool is_par = _g1h->n_par_threads() > 0;
+    bool is_par = _g1h->n_par_threads() > 1;
     Threads::possibly_parallel_oops_do(is_par, strong_roots, thread_stack_clds, strong_code);
   }
 }

--- a/hotspot/src/share/vm/gc/shared/genCollectedHeap.cpp	Wed May 20 10:45:40 2015 +0200
+++ b/hotspot/src/share/vm/gc/shared/genCollectedHeap.cpp	Thu May 21 09:10:47 2015 +0200
@@ -609,7 +609,7 @@
   // Only process code roots from thread stacks if we aren't visiting the entire CodeCache anyway
   CodeBlobClosure* roots_from_code_p = (so & SO_AllCodeCache) ? NULL : code_roots;
 
-  bool is_par = n_par_threads() > 0;
+  bool is_par = n_par_threads() > 1;
   Threads::possibly_parallel_oops_do(is_par, strong_roots, roots_from_clds_p, roots_from_code_p);
 
   if (!_process_strong_tasks->is_task_claimed(GCH_PS_Universe_oops_do)) {