--- old/src/hotspot/share/gc/shenandoah/shenandoahRootProcessor.hpp	2020-06-01 12:25:23.301028046 +0200
+++ new/src/hotspot/share/gc/shenandoah/shenandoahRootProcessor.hpp	2020-06-01 12:25:22.985028456 +0200
@@ -226,9 +226,18 @@
 template <bool CONCURRENT, bool SINGLE_THREADED>
 class ShenandoahClassLoaderDataRoots {
 private:
+  ShenandoahSharedSemaphore     _semaphore;
   ShenandoahPhaseTimings::Phase _phase;
+
+  static uint worker_count(uint n_workers) {
+    // Limit concurrency a bit, otherwise it wastes resources when workers are tripping
+    // over each other. This also leaves free workers to process other parts of the root
+    // set, while admitted workers are busy with doing the CLDG walk.
+    return MAX2(1u, MIN2(ShenandoahSharedSemaphore::max_tokens(), n_workers / 2));
+  }
+
 public:
-  ShenandoahClassLoaderDataRoots(ShenandoahPhaseTimings::Phase phase);
+  ShenandoahClassLoaderDataRoots(ShenandoahPhaseTimings::Phase phase, uint n_workers);
   ~ShenandoahClassLoaderDataRoots();
 
   void always_strong_cld_do(CLDClosure* clds, uint worker_id);