--- a/hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/cmsCollectorPolicy.cpp	Thu Jan 03 16:28:22 2013 -0800
+++ b/hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/cmsCollectorPolicy.cpp	Sun Dec 30 08:47:52 2012 +0100
@@ -56,7 +56,7 @@
   if (_generations == NULL)
     vm_exit_during_initialization("Unable to allocate gen spec");
 
-  if (ParNewGeneration::in_use()) {
+  if (UseParNewGC) {
     if (UseAdaptiveSizePolicy) {
       _generations[0] = new GenerationSpec(Generation::ASParNew,
                                            _initial_gen0_size, _max_gen0_size);
@@ -96,7 +96,7 @@
 
 void ConcurrentMarkSweepPolicy::initialize_gc_policy_counters() {
   // initialize the policy counters - 2 collectors, 3 generations
-  if (ParNewGeneration::in_use()) {
+  if (UseParNewGC) {
     _gc_policy_counters = new GCPolicyCounters("ParNew:CMS", 2, 3);
   }
   else {
@@ -119,7 +119,7 @@
 
   assert(size_policy() != NULL, "A size policy is required");
   // initialize the policy counters - 2 collectors, 3 generations
-  if (ParNewGeneration::in_use()) {
+  if (UseParNewGC) {
     _gc_policy_counters = new CMSGCAdaptivePolicyCounters("ParNew:CMS", 2, 3,
       size_policy());
   }

--- a/hotspot/src/share/vm/gc_implementation/parNew/parNewGeneration.cpp	Thu Jan 03 16:28:22 2013 -0800
+++ b/hotspot/src/share/vm/gc_implementation/parNew/parNewGeneration.cpp	Sun Dec 30 08:47:52 2012 +0100
@@ -1623,7 +1623,3 @@
 const char* ParNewGeneration::name() const {
   return "par new generation";
 }
-
-bool ParNewGeneration::in_use() {
-  return UseParNewGC && ParallelGCThreads > 0;
-}

--- a/hotspot/src/share/vm/gc_implementation/parNew/parNewGeneration.hpp	Thu Jan 03 16:28:22 2013 -0800
+++ b/hotspot/src/share/vm/gc_implementation/parNew/parNewGeneration.hpp	Sun Dec 30 08:47:52 2012 +0100
@@ -361,8 +361,6 @@
     delete _task_queues;
   }
 
-  static bool in_use();
-
   virtual void ref_processor_init();
   virtual Generation::Name kind()        { return Generation::ParNew; }
   virtual const char* name() const;

--- a/hotspot/src/share/vm/memory/collectorPolicy.cpp	Thu Jan 03 16:28:22 2013 -0800
+++ b/hotspot/src/share/vm/memory/collectorPolicy.cpp	Sun Dec 30 08:47:52 2012 +0100
@@ -827,7 +827,7 @@
   if (_generations == NULL)
     vm_exit_during_initialization("Unable to allocate gen spec");
 
-  if (UseParNewGC && ParallelGCThreads > 0) {
+  if (UseParNewGC) {
     _generations[0] = new GenerationSpec(Generation::ParNew, _initial_gen0_size, _max_gen0_size);
   } else {
     _generations[0] = new GenerationSpec(Generation::DefNew, _initial_gen0_size, _max_gen0_size);
@@ -840,10 +840,9 @@
 
 void MarkSweepPolicy::initialize_gc_policy_counters() {
   // initialize the policy counters - 2 collectors, 3 generations
-  if (UseParNewGC && ParallelGCThreads > 0) {
+  if (UseParNewGC) {
     _gc_policy_counters = new GCPolicyCounters("ParNew:MSC", 2, 3);
-  }
-  else {
+  } else {
     _gc_policy_counters = new GCPolicyCounters("Copy:MSC", 2, 3);
   }
 }

--- a/hotspot/src/share/vm/memory/tenuredGeneration.cpp	Thu Jan 03 16:28:22 2013 -0800
+++ b/hotspot/src/share/vm/memory/tenuredGeneration.cpp	Sun Dec 30 08:47:52 2012 +0100
@@ -62,7 +62,7 @@
                                        _virtual_space.reserved_size(),
                                        _the_space, _gen_counters);
 #ifndef SERIALGC
-  if (UseParNewGC && ParallelGCThreads > 0) {
+  if (UseParNewGC) {
     typedef ParGCAllocBufferWithBOT* ParGCAllocBufferWithBOTPtr;
     _alloc_buffers = NEW_C_HEAP_ARRAY(ParGCAllocBufferWithBOTPtr,
                                       ParallelGCThreads, mtGC);

--- a/hotspot/src/share/vm/runtime/arguments.cpp	Thu Jan 03 16:28:22 2013 -0800
+++ b/hotspot/src/share/vm/runtime/arguments.cpp	Sun Dec 30 08:47:52 2012 +0100
@@ -1083,10 +1083,6 @@
   }
 }
 
-// If the user has chosen ParallelGCThreads > 0, we set UseParNewGC
-// if it's not explictly set or unset. If the user has chosen
-// UseParNewGC and not explicitly set ParallelGCThreads we
-// set it, unless this is a single cpu machine.
 void Arguments::set_parnew_gc_flags() {
   assert(!UseSerialGC && !UseParallelOldGC && !UseParallelGC && !UseG1GC,
          "control point invariant");
@@ -1095,42 +1091,41 @@
   // Turn off AdaptiveSizePolicy for parnew until it is complete.
   disable_adaptive_size_policy("UseParNewGC");
 
-  if (ParallelGCThreads == 0) {
-    FLAG_SET_DEFAULT(ParallelGCThreads,
-                     Abstract_VM_Version::parallel_worker_threads());
-    if (ParallelGCThreads == 1) {
-      FLAG_SET_DEFAULT(UseParNewGC, false);
-      FLAG_SET_DEFAULT(ParallelGCThreads, 0);
-    }
+  if (FLAG_IS_DEFAULT(ParallelGCThreads)) {
+    FLAG_SET_DEFAULT(ParallelGCThreads, Abstract_VM_Version::parallel_worker_threads());
+    assert(ParallelGCThreads > 0, "We should always have at least one thread by default");
+  } else if (ParallelGCThreads == 0) {
+    jio_fprintf(defaultStream::error_stream(),
+        "The ParNew GC can not be combined with -XX:ParallelGCThreads=0\n");
+    vm_exit(1);
+  }
+
+  // By default YoungPLABSize and OldPLABSize are set to 4096 and 1024 respectively,
+  // these settings are default for Parallel Scavenger. For ParNew+Tenured configuration
+  // we set them to 1024 and 1024.
+  // See CR 6362902.
+  if (FLAG_IS_DEFAULT(YoungPLABSize)) {
+    FLAG_SET_DEFAULT(YoungPLABSize, (intx)1024);
   }
-  if (UseParNewGC) {
-    // By default YoungPLABSize and OldPLABSize are set to 4096 and 1024 respectively,
-    // these settings are default for Parallel Scavenger. For ParNew+Tenured configuration
-    // we set them to 1024 and 1024.
-    // See CR 6362902.
-    if (FLAG_IS_DEFAULT(YoungPLABSize)) {
-      FLAG_SET_DEFAULT(YoungPLABSize, (intx)1024);
+  if (FLAG_IS_DEFAULT(OldPLABSize)) {
+    FLAG_SET_DEFAULT(OldPLABSize, (intx)1024);
+  }
+
+  // AlwaysTenure flag should make ParNew promote all at first collection.
+  // See CR 6362902.
+  if (AlwaysTenure) {
+    FLAG_SET_CMDLINE(uintx, MaxTenuringThreshold, 0);
+  }
+  // When using compressed oops, we use local overflow stacks,
+  // rather than using a global overflow list chained through
+  // the klass word of the object's pre-image.
+  if (UseCompressedOops && !ParGCUseLocalOverflow) {
+    if (!FLAG_IS_DEFAULT(ParGCUseLocalOverflow)) {
+      warning("Forcing +ParGCUseLocalOverflow: needed if using compressed references");
     }
-    if (FLAG_IS_DEFAULT(OldPLABSize)) {
-      FLAG_SET_DEFAULT(OldPLABSize, (intx)1024);
-    }
-
-    // AlwaysTenure flag should make ParNew promote all at first collection.
-    // See CR 6362902.
-    if (AlwaysTenure) {
-      FLAG_SET_CMDLINE(uintx, MaxTenuringThreshold, 0);
-    }
-    // When using compressed oops, we use local overflow stacks,
-    // rather than using a global overflow list chained through
-    // the klass word of the object's pre-image.
-    if (UseCompressedOops && !ParGCUseLocalOverflow) {
-      if (!FLAG_IS_DEFAULT(ParGCUseLocalOverflow)) {
-        warning("Forcing +ParGCUseLocalOverflow: needed if using compressed references");
-      }
-      FLAG_SET_DEFAULT(ParGCUseLocalOverflow, true);
-    }
-    assert(ParGCUseLocalOverflow || !UseCompressedOops, "Error");
+    FLAG_SET_DEFAULT(ParGCUseLocalOverflow, true);
   }
+  assert(ParGCUseLocalOverflow || !UseCompressedOops, "Error");
 }
 
 // Adjust some sizes to suit CMS and/or ParNew needs; these work well on