--- a/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Wed Apr 01 10:49:08 2015 +0200
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Tue Mar 17 11:19:05 2015 -0700
@@ -3712,7 +3712,14 @@
 
     TraceCPUTime tcpu(G1Log::finer(), true, gclog_or_tty);
 
-    uint active_workers = workers()->active_workers();
+    uint active_workers = AdaptiveSizePolicy::calc_active_workers(workers()->total_workers(),
+                                                                  workers()->active_workers(),
+                                                                  Threads::number_of_non_daemon_threads());
+    assert(UseDynamicNumberOfGCThreads ||
+           active_workers == workers()->total_workers(),
+           "If not dynamic should be using all the  workers");
+    workers()->set_active_workers(active_workers);
+
     double pause_start_sec = os::elapsedTime();
     g1_policy()->phase_times()->note_gc_start(active_workers, mark_in_progress());
     log_gc_header();
@@ -5410,15 +5417,10 @@
   hot_card_cache->reset_hot_cache_claimed_index();
   hot_card_cache->set_use_cache(false);
 
-  uint n_workers;
-  n_workers =
-    AdaptiveSizePolicy::calc_active_workers(workers()->total_workers(),
-                                   workers()->active_workers(),
-                                   Threads::number_of_non_daemon_threads());
+  const uint n_workers = workers()->active_workers();
   assert(UseDynamicNumberOfGCThreads ||
          n_workers == workers()->total_workers(),
          "If not dynamic should be using all the  workers");
-  workers()->set_active_workers(n_workers);
   set_par_threads(n_workers);
 
 

--- a/hotspot/src/share/vm/gc_implementation/g1/g1GCPhaseTimes.cpp	Wed Apr 01 10:49:08 2015 +0200
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1GCPhaseTimes.cpp	Tue Mar 17 11:19:05 2015 -0700
@@ -154,28 +154,28 @@
     _has_new_data = true;
   }
 
-  double average(){
-    calculate_totals();
+  double average(uint active_threads){
+    calculate_totals(active_threads);
     return _average;
   }
 
-  T sum() {
-    calculate_totals();
+  T sum(uint active_threads) {
+    calculate_totals(active_threads);
     return _sum;
   }
 
-  T minimum() {
-    calculate_totals();
+  T minimum(uint active_threads) {
+    calculate_totals(active_threads);
     return _min;
   }
 
-  T maximum() {
-    calculate_totals();
+  T maximum(uint active_threads) {
+    calculate_totals(active_threads);
     return _max;
   }
 
   void reset() PRODUCT_RETURN;
-  void verify() PRODUCT_RETURN;
+  void verify(uint active_threads) PRODUCT_RETURN;
 
   void set_enabled(bool enabled) { _enabled = enabled; }
 
@@ -183,7 +183,7 @@
 
  private:
 
-  void calculate_totals(){
+  void calculate_totals(uint active_threads){
     if (!_has_new_data) {
       return;
     }
@@ -191,13 +191,14 @@
     _sum = (T)0;
     _min = _data[0];
     _max = _min;
-    for (uint i = 0; i < _length; ++i) {
+    assert(active_threads <= _length, "Wrong number of active threads");
+    for (uint i = 0; i < active_threads; ++i) {
       T val = _data[i];
       _sum += val;
       _min = MIN2(_min, val);
       _max = MAX2(_max, val);
     }
-    _average = (double)_sum / (double)_length;
+    _average = (double)_sum / (double)active_threads;
     _has_new_data = false;
   }
 };
@@ -226,17 +227,18 @@
 }
 
 template <class T>
-void WorkerDataArray<T>::verify() {
+void WorkerDataArray<T>::verify(uint active_threads) {
   if (!_enabled) {
     return;
   }
 
-  for (uint i = 0; i < _length; i++) {
+  assert(active_threads <= _length, "Wrong number of active threads");
+  for (uint i = 0; i < active_threads; i++) {
     assert(_data[i] != WorkerDataArray<T>::uninitialized(),
         err_msg("Invalid data for worker %u in '%s'", i, _title));
   }
   if (_thread_work_items != NULL) {
-    _thread_work_items->verify();
+    _thread_work_items->verify(active_threads);
   }
 }
 
@@ -321,7 +323,7 @@
   }
 
   for (int i = 0; i < GCParPhasesSentinel; i++) {
-    _gc_par_phases[i]->verify();
+    _gc_par_phases[i]->verify(_active_gc_threads);
   }
 }
 
@@ -378,7 +380,7 @@
 
 // return the average time for a phase in milliseconds
 double G1GCPhaseTimes::average_time_ms(GCParPhases phase) {
-  return _gc_par_phases[phase]->average() * 1000.0;
+  return _gc_par_phases[phase]->average(_active_gc_threads) * 1000.0;
 }
 
 double G1GCPhaseTimes::get_time_ms(GCParPhases phase, uint worker_i) {
@@ -386,15 +388,15 @@
 }
 
 double G1GCPhaseTimes::sum_time_ms(GCParPhases phase) {
-  return _gc_par_phases[phase]->sum() * 1000.0;
+  return _gc_par_phases[phase]->sum(_active_gc_threads) * 1000.0;
 }
 
 double G1GCPhaseTimes::min_time_ms(GCParPhases phase) {
-  return _gc_par_phases[phase]->minimum() * 1000.0;
+  return _gc_par_phases[phase]->minimum(_active_gc_threads) * 1000.0;
 }
 
 double G1GCPhaseTimes::max_time_ms(GCParPhases phase) {
-  return _gc_par_phases[phase]->maximum() * 1000.0;
+  return _gc_par_phases[phase]->maximum(_active_gc_threads) * 1000.0;
 }
 
 size_t G1GCPhaseTimes::get_thread_work_item(GCParPhases phase, uint worker_i) {
@@ -404,22 +406,22 @@
 
 size_t G1GCPhaseTimes::sum_thread_work_items(GCParPhases phase) {
   assert(_gc_par_phases[phase]->thread_work_items() != NULL, "No sub count");
-  return _gc_par_phases[phase]->thread_work_items()->sum();
+  return _gc_par_phases[phase]->thread_work_items()->sum(_active_gc_threads);
 }
 
 double G1GCPhaseTimes::average_thread_work_items(GCParPhases phase) {
   assert(_gc_par_phases[phase]->thread_work_items() != NULL, "No sub count");
-  return _gc_par_phases[phase]->thread_work_items()->average();
+  return _gc_par_phases[phase]->thread_work_items()->average(_active_gc_threads);
 }
 
 size_t G1GCPhaseTimes::min_thread_work_items(GCParPhases phase) {
   assert(_gc_par_phases[phase]->thread_work_items() != NULL, "No sub count");
-  return _gc_par_phases[phase]->thread_work_items()->minimum();
+  return _gc_par_phases[phase]->thread_work_items()->minimum(_active_gc_threads);
 }
 
 size_t G1GCPhaseTimes::max_thread_work_items(GCParPhases phase) {
   assert(_gc_par_phases[phase]->thread_work_items() != NULL, "No sub count");
-  return _gc_par_phases[phase]->thread_work_items()->maximum();
+  return _gc_par_phases[phase]->thread_work_items()->maximum(_active_gc_threads);
 }
 
 class G1GCParPhasePrinter : public StackObj {
@@ -455,14 +457,16 @@
   }
 
   void print_time_values(LineBuffer& buf, G1GCPhaseTimes::GCParPhases phase_id, WorkerDataArray<double>* phase) {
-    for (uint i = 0; i < phase->_length; ++i) {
+    uint active_length = _phase_times->_active_gc_threads;
+    for (uint i = 0; i < active_length; ++i) {
       buf.append("  %.1lf", _phase_times->get_time_ms(phase_id, i));
     }
     buf.print_cr();
   }
 
   void print_count_values(LineBuffer& buf, G1GCPhaseTimes::GCParPhases phase_id, WorkerDataArray<size_t>* thread_work_items) {
-    for (uint i = 0; i < thread_work_items->_length; ++i) {
+    uint active_length = _phase_times->_active_gc_threads;
+    for (uint i = 0; i < active_length; ++i) {
       buf.append("  " SIZE_FORMAT, _phase_times->get_thread_work_item(phase_id, i));
     }
     buf.print_cr();

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hotspot/test/gc/ergonomics/TestDynamicNumberOfGCThreads.java	Tue Mar 17 11:19:05 2015 -0700
@@ -0,0 +1,69 @@
+/*
+ * Copyright (c) 2015, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test TestDynamicNumberOfGCThreads
+ * @bug 8017462
+ * @summary Ensure that UseDynamicNumberOfGCThreads runs
+ * @requires vm.gc=="null"
+ * @key gc
+ * @library /testlibrary
+ */
+
+import com.oracle.java.testlibrary.ProcessTools;
+import com.oracle.java.testlibrary.OutputAnalyzer;
+
+public class TestDynamicNumberOfGCThreads {
+  public static void main(String[] args) throws Exception {
+
+    testDynamicNumberOfGCThreads("UseConcMarkSweepGC");
+
+    testDynamicNumberOfGCThreads("UseG1GC");
+
+    testDynamicNumberOfGCThreads("UseParallelGC");
+  }
+
+  private static void verifyDynamicNumberOfGCThreads(OutputAnalyzer output) {
+    output.shouldContain("new_active_workers");
+    output.shouldHaveExitValue(0);
+  }
+
+  private static void testDynamicNumberOfGCThreads(String gcFlag) throws Exception {
+    // UseDynamicNumberOfGCThreads and TraceDynamicGCThreads enabled
+    ProcessBuilder pb_enabled =
+      ProcessTools.createJavaProcessBuilder("-XX:+" + gcFlag, "-Xmx10M", "-XX:+PrintGCDetails",  "-XX:+UseDynamicNumberOfGCThreads", "-XX:+TraceDynamicGCThreads", GCTest.class.getName());
+    verifyDynamicNumberOfGCThreads(new OutputAnalyzer(pb_enabled.start()));
+  }
+
+  static class GCTest {
+    private static byte[] garbage;
+    public static void main(String [] args) {
+      System.out.println("Creating garbage");
+      // create 128MB of garbage. This should result in at least one GC
+      for (int i = 0; i < 1024; i++) {
+        garbage = new byte[128 * 1024];
+      }
+      System.out.println("Done");
+    }
+  }
+}