--- a/src/hotspot/share/gc/g1/g1CollectedHeap.cpp	Tue May 29 09:26:00 2018 +0200
+++ b/src/hotspot/share/gc/g1/g1CollectedHeap.cpp	Mon Jun 18 12:11:59 2018 +0200
@@ -1815,7 +1815,8 @@
                            (ParallelGCThreads > 1) || (ConcGCThreads > 1), // mt discovery
                            MAX2(ParallelGCThreads, ConcGCThreads),         // degree of mt discovery
                            false,                                          // Reference discovery is not atomic
-                           &_is_alive_closure_cm);                         // is alive closure
+                           &_is_alive_closure_cm,                          // is alive closure
+                           true);                                          // allow changes to number of processing threads
 
   // STW ref processor
   _ref_processor_stw =
@@ -1825,7 +1826,8 @@
                            (ParallelGCThreads > 1),              // mt discovery
                            ParallelGCThreads,                    // degree of mt discovery
                            true,                                 // Reference discovery is atomic
-                           &_is_alive_closure_stw);              // is alive closure
+                           &_is_alive_closure_stw,               // is alive closure
+                           true);                                // allow changes to number of processing threads
 }
 
 CollectorPolicy* G1CollectedHeap::collector_policy() const {
@@ -3791,25 +3793,22 @@
   G1ParScanThreadStateSet*  _pss;
   RefToScanQueueSet*        _queues;
   WorkGang*                 _workers;
-  uint                      _active_workers;
 
 public:
   G1STWRefProcTaskExecutor(G1CollectedHeap* g1h,
                            G1ParScanThreadStateSet* per_thread_states,
                            WorkGang* workers,
-                           RefToScanQueueSet *task_queues,
-                           uint n_workers) :
+                           RefToScanQueueSet *task_queues) :
     _g1h(g1h),
     _pss(per_thread_states),
     _queues(task_queues),
-    _workers(workers),
-    _active_workers(n_workers)
+    _workers(workers)
   {
-    g1h->ref_processor_stw()->set_active_mt_degree(n_workers);
+    g1h->ref_processor_stw()->set_active_mt_degree(workers->active_workers());
   }
 
   // Executes the given task using concurrent marking worker threads.
-  virtual void execute(ProcessTask& task);
+  virtual void execute(ProcessTask& task, uint ergo_workers);
 };
 
 // Gang task for possibly parallel reference processing
@@ -3865,13 +3864,16 @@
 // Driver routine for parallel reference processing.
 // Creates an instance of the ref processing gang
 // task and has the worker threads execute it.
-void G1STWRefProcTaskExecutor::execute(ProcessTask& proc_task) {
+void G1STWRefProcTaskExecutor::execute(ProcessTask& proc_task, uint ergo_workers) {
   assert(_workers != NULL, "Need parallel worker threads.");
 
-  ParallelTaskTerminator terminator(_active_workers, _queues);
+  assert(_workers->active_workers() >= ergo_workers,
+         "Ergonomically chosen workers (%u) should be less than or equal to active workers (%u)",
+         ergo_workers, _workers->active_workers());
+  ParallelTaskTerminator terminator(ergo_workers, _queues);
   G1STWRefProcTaskProxy proc_task_proxy(proc_task, _g1h, _pss, _queues, &terminator);
 
-  _workers->run_task(&proc_task_proxy);
+  _workers->run_task(&proc_task_proxy, ergo_workers);
 }
 
 // End of weak reference support closures
@@ -3922,7 +3924,7 @@
            "Mismatch between the number of GC workers %u and the maximum number of Reference process queues %u",
            no_of_gc_workers,  rp->max_num_queues());
 
-    G1STWRefProcTaskExecutor par_task_executor(this, per_thread_states, workers(), _task_queues, no_of_gc_workers);
+    G1STWRefProcTaskExecutor par_task_executor(this, per_thread_states, workers(), _task_queues);
     stats = rp->process_discovered_references(&is_alive,
                                               &keep_alive,
                                               &drain_queue,