--- a/src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp	Wed Jun 05 08:59:17 2019 +0200
+++ b/src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp	Wed Jun 05 09:23:09 2019 +0200
@@ -67,12 +67,34 @@
     FLAG_SET_DEFAULT(UseNUMAInterleaving, true);
   }
 
-  FLAG_SET_DEFAULT(ParallelGCThreads,
-                   WorkerPolicy::parallel_worker_threads());
+  // Set up default number of concurrent threads. We want to have cycles complete fast
+  // enough, but we also do not want to steal too much CPU from the concurrently running
+  // application. Using 1/4 of available threads for concurrent GC seems a good
+  // compromise here.
+  if (FLAG_IS_DEFAULT(ConcGCThreads)) {
+    FLAG_SET_DEFAULT(ConcGCThreads, MAX2(1, os::processor_count() / 4));
+  }
+
+  if (ConcGCThreads == 0) {
+    vm_exit_during_initialization("Shenandoah expects ConcGCThreads > 0, check -XX:ConcGCThreads=#");
+  }
 
-  if (FLAG_IS_DEFAULT(ConcGCThreads)) {
-    uint conc_threads = MAX2((uint) 1, ParallelGCThreads);
-    FLAG_SET_DEFAULT(ConcGCThreads, conc_threads);
+  // Set up default number of parallel threads. We want to have decent pauses performance
+  // which would use parallel threads, but we also do not want to do too many threads
+  // that will overwhelm the OS scheduler. Using 1/2 of available threads seems to be a fair
+  // compromise here. Due to implementation constraints, it should not be lower than
+  // the number of concurrent threads.
+  if (FLAG_IS_DEFAULT(ParallelGCThreads)) {
+    FLAG_SET_DEFAULT(ParallelGCThreads, MAX2(1, os::processor_count() / 2));
+  }
+
+  if (ParallelGCThreads == 0) {
+    vm_exit_during_initialization("Shenandoah expects ParallelGCThreads > 0, check -XX:ParallelGCThreads=#");
+  }
+
+  if (ParallelGCThreads < ConcGCThreads) {
+    warning("Shenandoah expects ConcGCThreads <= ParallelGCThreads, adjusting ParallelGCThreads automatically");
+    FLAG_SET_DEFAULT(ParallelGCThreads, ConcGCThreads);
   }
 
   if (FLAG_IS_DEFAULT(ParallelRefProcEnabled)) {