--- old/src/hotspot/share/gc/g1/g1ConcurrentMark.cpp	2018-10-15 15:13:13.657398191 +0200
+++ new/src/hotspot/share/gc/g1/g1ConcurrentMark.cpp	2018-10-15 15:13:13.505399220 +0200
@@ -1545,7 +1545,11 @@
   // and overflow handling in G1CMTask::do_marking_step() knows
   // how many workers to wait for.
   _cm->set_concurrency(ergo_workers);
-  _workers->run_task(&proc_task_proxy, ergo_workers);
+  if (ergo_workers == 1) {
+    proc_task_proxy.work(0);
+  } else {
+    _workers->run_task(&proc_task_proxy, ergo_workers);
+  }
 }
 
 void G1ConcurrentMark::weak_refs_work(bool clear_all_soft_refs) {