--- a/hotspot/src/share/vm/gc/g1/g1CollectedHeap.cpp	Thu Mar 31 11:21:20 2016 +0000
+++ b/hotspot/src/share/vm/gc/g1/g1CollectedHeap.cpp	Thu Mar 31 17:05:29 2016 +0200
@@ -3228,7 +3228,7 @@
                                                                   Threads::number_of_non_daemon_threads());
     workers()->set_active_workers(active_workers);
 
-    g1_policy()->note_gc_start(active_workers);
+    g1_policy()->note_gc_start();
 
     TraceCollectorStats tcs(g1mm()->incremental_collection_counters());
     TraceMemoryManagerStats tms(false /* fullGC */, gc_cause());

--- a/hotspot/src/share/vm/gc/g1/g1CollectorPolicy.cpp	Thu Mar 31 11:21:20 2016 +0000
+++ b/hotspot/src/share/vm/gc/g1/g1CollectorPolicy.cpp	Thu Mar 31 17:05:29 2016 +0200
@@ -186,8 +186,8 @@
   _collection_set->start_incremental_building();
 }
 
-void G1CollectorPolicy::note_gc_start(uint num_active_workers) {
-  phase_times()->note_gc_start(num_active_workers);
+void G1CollectorPolicy::note_gc_start() {
+  phase_times()->note_gc_start();
 }
 
 // Create the jstat counters for the policy.

--- a/hotspot/src/share/vm/gc/g1/g1CollectorPolicy.hpp	Thu Mar 31 11:21:20 2016 +0000
+++ b/hotspot/src/share/vm/gc/g1/g1CollectorPolicy.hpp	Thu Mar 31 17:05:29 2016 +0200
@@ -317,7 +317,7 @@
 
   void init();
 
-  virtual void note_gc_start(uint num_active_workers);
+  virtual void note_gc_start();
 
   // Create jstat counters for the policy.
   virtual void initialize_gc_policy_counters();

--- a/hotspot/src/share/vm/gc/g1/g1GCPhaseTimes.cpp	Thu Mar 31 11:21:20 2016 +0000
+++ b/hotspot/src/share/vm/gc/g1/g1GCPhaseTimes.cpp	Thu Mar 31 17:05:29 2016 +0200
@@ -33,7 +33,7 @@
 #include "runtime/timer.hpp"
 #include "runtime/os.hpp"
 
-static const char* Indents[5] = {"", "  ", "    ", "     ", "       "};
+static const char* Indents[5] = {"", "  ", "    ", "      ", "        "};
 
 G1GCPhaseTimes::G1GCPhaseTimes(uint max_gc_threads) :
   _max_gc_threads(max_gc_threads)
@@ -94,11 +94,8 @@
   _gc_par_phases[PreserveCMReferents] = new WorkerDataArray<double>(max_gc_threads, "Parallel Preserve CM Refs (ms):");
 }
 
-void G1GCPhaseTimes::note_gc_start(uint active_gc_threads) {
-  assert(active_gc_threads > 0, "The number of threads must be > 0");
-  assert(active_gc_threads <= _max_gc_threads, "The number of active threads must be <= the max number of threads");
+void G1GCPhaseTimes::note_gc_start() {
   _gc_start_counter = os::elapsed_counter();
-  _active_gc_threads = active_gc_threads;
   _cur_expand_heap_time_ms = 0.0;
   _external_accounted_time_ms = 0.0;
 
@@ -109,31 +106,55 @@
   }
 }
 
+#define ASSERT_PHASE_UNINITIALIZED(phase) \
+    assert(_gc_par_phases[phase]->get(i) == uninitialized, "Phase " #phase " reported for thread that was not started");
+
+double G1GCPhaseTimes::worker_time(GCParPhases phase, uint worker) {
+  double value = _gc_par_phases[phase]->get(worker);
+  if (value != WorkerDataArray<double>::uninitialized()) {
+    return value;
+  }
+  return 0.0;
+}
+
 void G1GCPhaseTimes::note_gc_end() {
   _gc_pause_time_ms = TimeHelper::counter_to_millis(os::elapsed_counter() - _gc_start_counter);
-  for (uint i = 0; i < _active_gc_threads; i++) {
-    double worker_time = _gc_par_phases[GCWorkerEnd]->get(i) - _gc_par_phases[GCWorkerStart]->get(i);
-    record_time_secs(GCWorkerTotal, i , worker_time);
+
+  double uninitialized = WorkerDataArray<double>::uninitialized();
+
+  for (uint i = 0; i < _max_gc_threads; i++) {
+    double worker_start = _gc_par_phases[GCWorkerStart]->get(i);
+    if (worker_start != uninitialized) {
+      assert(_gc_par_phases[GCWorkerEnd]->get(i) != uninitialized, "Worker started but not ended.");
+      double total_worker_time = _gc_par_phases[GCWorkerEnd]->get(i) - _gc_par_phases[GCWorkerStart]->get(i);
+      record_time_secs(GCWorkerTotal, i , total_worker_time);
 
-    double worker_known_time =
-        _gc_par_phases[ExtRootScan]->get(i) +
-        _gc_par_phases[SATBFiltering]->get(i) +
-        _gc_par_phases[UpdateRS]->get(i) +
-        _gc_par_phases[ScanRS]->get(i) +
-        _gc_par_phases[CodeRoots]->get(i) +
-        _gc_par_phases[ObjCopy]->get(i) +
-        _gc_par_phases[Termination]->get(i);
+      double worker_known_time =
+          worker_time(ExtRootScan, i)
+          + worker_time(SATBFiltering, i)
+          + worker_time(UpdateRS, i)
+          + worker_time(ScanRS, i)
+          + worker_time(CodeRoots, i)
+          + worker_time(ObjCopy, i)
+          + worker_time(Termination, i);
 
-    record_time_secs(Other, i, worker_time - worker_known_time);
-  }
-
-  for (int i = 0; i < GCParPhasesSentinel; i++) {
-    if (_gc_par_phases[i] != NULL) {
-      _gc_par_phases[i]->verify(_active_gc_threads);
+      record_time_secs(Other, i, total_worker_time - worker_known_time);
+    } else {
+      // Make sure all slots are uninitialized since this thread did not seem to have been started
+      ASSERT_PHASE_UNINITIALIZED(GCWorkerEnd);
+      ASSERT_PHASE_UNINITIALIZED(ExtRootScan);
+      ASSERT_PHASE_UNINITIALIZED(SATBFiltering);
+      ASSERT_PHASE_UNINITIALIZED(UpdateRS);
+      ASSERT_PHASE_UNINITIALIZED(ScanRS);
+      ASSERT_PHASE_UNINITIALIZED(CodeRoots);
+      ASSERT_PHASE_UNINITIALIZED(ObjCopy);
+      ASSERT_PHASE_UNINITIALIZED(Termination);
     }
   }
 }
 
+#undef ASSERT_PHASE_UNINITIALIZED
+
 // record the time a phase took in seconds
 void G1GCPhaseTimes::record_time_secs(GCParPhases phase, uint worker_i, double secs) {
   _gc_par_phases[phase]->set(worker_i, secs);
@@ -150,12 +171,12 @@
 
 // return the average time for a phase in milliseconds
 double G1GCPhaseTimes::average_time_ms(GCParPhases phase) {
-  return _gc_par_phases[phase]->average(_active_gc_threads) * 1000.0;
+  return _gc_par_phases[phase]->average() * 1000.0;
 }
 
 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(_active_gc_threads);
+  return _gc_par_phases[phase]->thread_work_items()->sum();
 }
 
 template <class T>
@@ -164,19 +185,19 @@
   if (log.is_level(LogLevel::Trace)) {
     outputStream* trace_out = log.trace_stream();
     trace_out->print("%s", indent);
-    phase->print_details_on(trace_out, _active_gc_threads);
+    phase->print_details_on(trace_out);
   }
 }
 
 void G1GCPhaseTimes::log_phase(WorkerDataArray<double>* phase, uint indent, outputStream* out, bool print_sum) {
   out->print("%s", Indents[indent]);
-  phase->print_summary_on(out, _active_gc_threads, print_sum);
+  phase->print_summary_on(out, print_sum);
   details(phase, Indents[indent]);
 
   WorkerDataArray<size_t>* work_items = phase->thread_work_items();
   if (work_items != NULL) {
     out->print("%s", Indents[indent + 1]);
-    work_items->print_summary_on(out, _active_gc_threads, true);
+    work_items->print_summary_on(out, true);
     details(work_items, Indents[indent + 1]);
   }
 }

--- a/hotspot/src/share/vm/gc/g1/g1GCPhaseTimes.hpp	Thu Mar 31 11:21:20 2016 +0000
+++ b/hotspot/src/share/vm/gc/g1/g1GCPhaseTimes.hpp	Thu Mar 31 17:05:29 2016 +0200
@@ -32,7 +32,6 @@
 template <class T> class WorkerDataArray;
 
 class G1GCPhaseTimes : public CHeapObj<mtGC> {
-  uint _active_gc_threads;
   uint _max_gc_threads;
   jlong _gc_start_counter;
   double _gc_pause_time_ms;
@@ -123,6 +122,7 @@
   double _cur_verify_before_time_ms;
   double _cur_verify_after_time_ms;
 
+  double worker_time(GCParPhases phase, uint worker);
   void note_gc_end();
 
   template <class T>
@@ -133,7 +133,7 @@
 
  public:
   G1GCPhaseTimes(uint max_gc_threads);
-  void note_gc_start(uint active_gc_threads);
+  void note_gc_start();
   void print();
 
   // record the time a phase took in seconds

--- a/hotspot/src/share/vm/gc/g1/workerDataArray.cpp	Thu Mar 31 11:21:20 2016 +0000
+++ b/hotspot/src/share/vm/gc/g1/workerDataArray.cpp	Thu Mar 31 17:05:29 2016 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2016, 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
@@ -27,67 +27,178 @@
 #include "utilities/ostream.hpp"
 
 template <>
-void WorkerDataArray<double>::WDAPrinter::summary(outputStream* out, const char* title, double min, double avg, double max, double diff, double sum, bool print_sum) {
-  out->print("%-25s Min: %4.1lf, Avg: %4.1lf, Max: %4.1lf, Diff: %4.1lf", title, min * MILLIUNITS, avg * MILLIUNITS, max * MILLIUNITS, diff* MILLIUNITS);
+size_t WorkerDataArray<size_t>::uninitialized() {
+  return (size_t)-1;
+}
+
+template <>
+double WorkerDataArray<double>::uninitialized() {
+  return -1.0;
+}
+
+template <>
+void WorkerDataArray<double>::WDAPrinter::summary(outputStream* out, double min, double avg, double max, double diff, double sum, bool print_sum) {
+  out->print(" Min: %4.1lf, Avg: %4.1lf, Max: %4.1lf, Diff: %4.1lf", min * MILLIUNITS, avg * MILLIUNITS, max * MILLIUNITS, diff* MILLIUNITS);
   if (print_sum) {
-    out->print_cr(", Sum: %4.1lf", sum * MILLIUNITS);
-  } else {
-    out->cr();
+    out->print(", Sum: %4.1lf", sum * MILLIUNITS);
   }
 }
 
 template <>
-void WorkerDataArray<size_t>::WDAPrinter::summary(outputStream* out, const char* title, size_t min, double avg, size_t max, size_t diff, size_t sum, bool print_sum) {
-  out->print("%-25s Min: " SIZE_FORMAT ", Avg: %4.1lf, Max: " SIZE_FORMAT ", Diff: " SIZE_FORMAT, title, min, avg, max, diff);
+void WorkerDataArray<size_t>::WDAPrinter::summary(outputStream* out, size_t min, double avg, size_t max, size_t diff, size_t sum, bool print_sum) {
+  out->print(" Min: " SIZE_FORMAT ", Avg: %4.1lf, Max: " SIZE_FORMAT ", Diff: " SIZE_FORMAT, min, avg, max, diff);
   if (print_sum) {
-    out->print_cr(", Sum: " SIZE_FORMAT, sum);
-  } else {
-    out->cr();
+    out->print(", Sum: " SIZE_FORMAT, sum);
   }
 }
 
 template <>
-void WorkerDataArray<double>::WDAPrinter::details(const WorkerDataArray<double>* phase, outputStream* out, uint active_threads) {
+void WorkerDataArray<double>::WDAPrinter::details(const WorkerDataArray<double>* phase, outputStream* out) {
   out->print("%-25s", "");
-  for (uint i = 0; i < active_threads; ++i) {
-    out->print(" %4.1lf", phase->get(i) * 1000.0);
+  for (uint i = 0; i < phase->_length; ++i) {
+    double value = phase->get(i);
+    if (value != phase->uninitialized()) {
+      out->print(" %4.1lf", phase->get(i) * 1000.0);
+    } else {
+      out->print(" -");
+    }
   }
   out->cr();
 }
 
 template <>
-void WorkerDataArray<size_t>::WDAPrinter::details(const WorkerDataArray<size_t>* phase, outputStream* out, uint active_threads) {
+void WorkerDataArray<size_t>::WDAPrinter::details(const WorkerDataArray<size_t>* phase, outputStream* out) {
   out->print("%-25s", "");
-  for (uint i = 0; i < active_threads; ++i) {
-    out->print("  " SIZE_FORMAT, phase->get(i));
+  for (uint i = 0; i < phase->_length; ++i) {
+    size_t value = phase->get(i);
+    if (value != phase->uninitialized()) {
+      out->print("  " SIZE_FORMAT, phase->get(i));
+    } else {
+      out->print(" -");
+    }
   }
   out->cr();
 }
 
 #ifndef PRODUCT
-void WorkerDataArray_test() {
-  const uint length = 3;
-  const char* title = "Test array";
+
+#include "memory/resourceArea.hpp"
+
+void WorkerDataArray_test_verify_string(const char* expected_string, const char* actual_string) {
+  const size_t expected_len = strlen(expected_string);
+
+  assert(expected_len == strlen(actual_string),
+      "Wrong string length, expected " SIZE_FORMAT " but got " SIZE_FORMAT "(Expected '%s' but got: '%s')",
+      expected_len, strlen(actual_string), expected_string, actual_string);
 
-  WorkerDataArray<size_t> array(length, title);
-  assert(strncmp(array.title(), title, strlen(title)) == 0 , "Expected titles to match");
+  // Can't use strncmp here because floating point values use different decimal points for different locales.
+  // Allow strings to differ in "." vs. "," only. This should still catch most errors.
+  for (size_t i = 0; i < expected_len; i++) {
+    char e = expected_string[i];
+    char a = actual_string[i];
+    if (e != a) {
+      if ((e == '.' || e == ',') && (a == '.' || a == ',')) {
+        // Most likely just a difference in locale
+      } else {
+        assert(false, "Expected '%s' but got: '%s'", expected_string, actual_string);
+      }
+    }
+  }
+}
+
+void WorkerDataArray_test_verify_array(WorkerDataArray<size_t>& array, size_t expected_sum, double expected_avg, const char* expected_summary, const char* exected_details) {
+  const double epsilon = 0.0001;
+  assert(array.sum() == expected_sum, "Wrong sum, expected: " SIZE_FORMAT " but got: " SIZE_FORMAT, expected_sum, array.sum());
+  assert(fabs(array.average() - expected_avg) < epsilon, "Wrong average, expected: %f but got: %f", expected_avg, array.average());
 
-  const size_t expected[length] = {5, 3, 7};
-  for (uint i = 0; i < length; i++) {
-    array.set(i, expected[i]);
-  }
-  for (uint i = 0; i < length; i++) {
-    assert(array.get(i) == expected[i], "Expected elements to match");
+  ResourceMark rm;
+  stringStream out;
+  array.print_summary_on(&out);
+  WorkerDataArray_test_verify_string(expected_summary, out.as_string());
+  out.reset();
+  array.print_details_on(&out);
+  WorkerDataArray_test_verify_string(exected_details, out.as_string());
+}
+
+void WorkerDataArray_test_verify_array(WorkerDataArray<double>& array, double expected_sum, double expected_avg, const char* expected_summary, const char* exected_details) {
+  const double epsilon = 0.0001;
+  assert(fabs(array.sum() - expected_sum) < epsilon, "Wrong sum, expected: %f but got: %f", expected_sum, array.sum());
+  assert(fabs(array.average() - expected_avg) < epsilon, "Wrong average, expected: %f but got: %f", expected_avg, array.average());
+
+  ResourceMark rm;
+  stringStream out;
+  array.print_summary_on(&out);
+  WorkerDataArray_test_verify_string(expected_summary, out.as_string());
+  out.reset();
+  array.print_details_on(&out);
+  WorkerDataArray_test_verify_string(exected_details, out.as_string());
+}
+
+void WorkerDataArray_test_basic() {
+  WorkerDataArray<size_t> array(3, "Test array");
+  array.set(0, 5);
+  array.set(1, 3);
+  array.set(2, 7);
+
+  WorkerDataArray_test_verify_array(array, 15, 5.0,
+      "Test array                Min: 3, Avg:  5.0, Max: 7, Diff: 4, Sum: 15, Workers: 3\n",
+      "                           5  3  7\n" );
+}
+
+void WorkerDataArray_test_add() {
+  WorkerDataArray<size_t> array(3, "Test array");
+  array.set(0, 5);
+  array.set(1, 3);
+  array.set(2, 7);
+
+  for (uint i = 0; i < 3; i++) {
+    array.add(i, 1);
   }
 
-  assert(array.sum(length) == (5 + 3 + 7), "Expected sums to match");
-  assert(array.average(length) == 5.0, "Expected averages to match");
+  WorkerDataArray_test_verify_array(array, 18, 6.0,
+      "Test array                Min: 4, Avg:  6.0, Max: 8, Diff: 4, Sum: 18, Workers: 3\n",
+      "                           6  4  8\n" );
+}
+
+void WorkerDataArray_test_with_uninitialized() {
+  WorkerDataArray<size_t> array(3, "Test array");
+  array.set(0, 5);
+  array.set(1, WorkerDataArray<size_t>::uninitialized());
+  array.set(2, 7);
+
+  WorkerDataArray_test_verify_array(array, 12, 6,
+      "Test array                Min: 5, Avg:  6.0, Max: 7, Diff: 2, Sum: 12, Workers: 2\n",
+      "                           5 -  7\n" );
+}
+
+void WorkerDataArray_test_uninitialized() {
+  WorkerDataArray<size_t> array(3, "Test array");
+  array.set(0, WorkerDataArray<size_t>::uninitialized());
+  array.set(1, WorkerDataArray<size_t>::uninitialized());
+  array.set(2, WorkerDataArray<size_t>::uninitialized());
 
-  for (uint i = 0; i < length; i++) {
-    array.add(i, 1);
-  }
-  for (uint i = 0; i < length; i++) {
-    assert(array.get(i) == expected[i] + 1, "Expected add to increment values");
-  }
+  WorkerDataArray_test_verify_array(array, 0, 0.0,
+      "Test array                skipped\n",
+      "                          - - -\n" );
 }
+
+void WorkerDataArray_test_double_with_uninitialized() {
+  WorkerDataArray<double> array(3, "Test array");
+  array.set(0, 5.1 / MILLIUNITS);
+  array.set(1, WorkerDataArray<double>::uninitialized());
+  array.set(2, 7.2 / MILLIUNITS);
+
+  WorkerDataArray_test_verify_array(array, 12.3 / MILLIUNITS, 6.15 / MILLIUNITS,
+      "Test array                Min:  5.1, Avg:  6.1, Max:  7.2, Diff:  2.1, Sum: 12.3, Workers: 2\n",
+      "                           5.1 -  7.2\n" );
+}
+
+void WorkerDataArray_test() {
+  WorkerDataArray_test_basic();
+  WorkerDataArray_test_add();
+  WorkerDataArray_test_with_uninitialized();
+  WorkerDataArray_test_uninitialized();
+  WorkerDataArray_test_double_with_uninitialized();
+}
+
 #endif

--- a/hotspot/src/share/vm/gc/g1/workerDataArray.hpp	Thu Mar 31 11:21:20 2016 +0000
+++ b/hotspot/src/share/vm/gc/g1/workerDataArray.hpp	Thu Mar 31 17:05:29 2016 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2016, 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
@@ -32,16 +32,13 @@
 
 template <class T>
 class WorkerDataArray  : public CHeapObj<mtGC> {
+  friend class WDAPrinter;
   T*          _data;
   uint        _length;
   const char* _title;
 
   WorkerDataArray<size_t>* _thread_work_items;
 
-  NOT_PRODUCT(inline T uninitialized() const;)
-
-  void set_all(T value);
-
  public:
   WorkerDataArray(uint length, const char* title);
   ~WorkerDataArray();
@@ -52,37 +49,38 @@
     return _thread_work_items;
   }
 
+  static T uninitialized();
+
   void set(uint worker_i, T value);
   T get(uint worker_i) const;
 
   void add(uint worker_i, T value);
 
-  double average(uint active_threads) const;
-  T sum(uint active_threads) const;
+  // The sum() and average() methods below consider uninitialized slots to be 0.
+  double average() const;
+  T sum() const;
 
   const char* title() const {
     return _title;
   }
 
-  void clear();
-
-  void reset() PRODUCT_RETURN;
-  void verify(uint active_threads) const PRODUCT_RETURN;
+  void reset();
+  void set_all(T value);
 
 
  private:
   class WDAPrinter {
   public:
-    static void summary(outputStream* out, const char* title, double min, double avg, double max, double diff, double sum, bool print_sum);
-    static void summary(outputStream* out, const char* title, size_t min, double avg, size_t max, size_t diff, size_t sum, bool print_sum);
+    static void summary(outputStream* out, double min, double avg, double max, double diff, double sum, bool print_sum);
+    static void summary(outputStream* out, size_t min, double avg, size_t max, size_t diff, size_t sum, bool print_sum);
 
-    static void details(const WorkerDataArray<double>* phase, outputStream* out, uint active_threads);
-    static void details(const WorkerDataArray<size_t>* phase, outputStream* out, uint active_threads);
+    static void details(const WorkerDataArray<double>* phase, outputStream* out);
+    static void details(const WorkerDataArray<size_t>* phase, outputStream* out);
   };
 
  public:
-  void print_summary_on(outputStream* out, uint active_threads, bool print_sum = true) const;
-  void print_details_on(outputStream* out, uint active_threads) const;
+  void print_summary_on(outputStream* out, bool print_sum = true) const;
+  void print_details_on(outputStream* out) const;
 };
 
 #endif // SHARE_VM_GC_G1_WORKERDATAARRAY_HPP

--- a/hotspot/src/share/vm/gc/g1/workerDataArray.inline.hpp	Thu Mar 31 11:21:20 2016 +0000
+++ b/hotspot/src/share/vm/gc/g1/workerDataArray.inline.hpp	Thu Mar 31 17:05:29 2016 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2016, 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
@@ -50,7 +50,6 @@
 template <typename T>
 T WorkerDataArray<T>::get(uint worker_i) const {
   assert(worker_i < _length, "Worker %d is greater than max: %d", worker_i, _length);
-  assert(_data[worker_i] != uninitialized(), "No data added for worker %d", worker_i);
   return _data[worker_i];
 }
 
@@ -78,25 +77,31 @@
 }
 
 template <typename T>
-double WorkerDataArray<T>::average(uint active_threads) const {
-  return sum(active_threads) / (double) active_threads;
+double WorkerDataArray<T>::average() const {
+  uint contributing_threads = 0;
+  for (uint i = 0; i < _length; ++i) {
+    if (get(i) != uninitialized()) {
+      contributing_threads++;
+    }
+  }
+  if (contributing_threads == 0) {
+    return 0.0;
+  }
+  return sum() / (double) contributing_threads;
 }
 
 template <typename T>
-T WorkerDataArray<T>::sum(uint active_threads) const {
-  T s = get(0);
-  for (uint i = 1; i < active_threads; ++i) {
-    s += get(i);
+T WorkerDataArray<T>::sum() const {
+  T s = 0;
+  for (uint i = 0; i < _length; ++i) {
+    if (get(i) != uninitialized()) {
+      s += get(i);
+    }
   }
   return s;
 }
 
 template <typename T>
-void WorkerDataArray<T>::clear() {
-  set_all(0);
-}
-
-template <typename T>
 void WorkerDataArray<T>::set_all(T value) {
   for (uint i = 0; i < _length; i++) {
     _data[i] = value;
@@ -104,27 +109,42 @@
 }
 
 template <class T>
-void WorkerDataArray<T>::print_summary_on(outputStream* out, uint active_threads, bool print_sum) const {
-  T max = get(0);
-  T min = max;
-  T sum = 0;
-  for (uint i = 1; i < active_threads; ++i) {
-    T value = get(i);
-    max = MAX2(max, value);
-    min = MIN2(min, value);
-    sum += value;
+void WorkerDataArray<T>::print_summary_on(outputStream* out, bool print_sum) const {
+  out->print("%-25s", title());
+  uint start = 0;
+  while (start < _length && get(start) == uninitialized()) {
+    start++;
   }
-  T diff = max - min;
-  double avg = sum / (double) active_threads;
-  WDAPrinter::summary(out, title(), min, avg, max, diff, sum, print_sum);
+  if (start < _length) {
+    T min = get(start);
+    T max = min;
+    T sum = 0;
+    uint contributing_threads = 0;
+    for (uint i = start; i < _length; ++i) {
+      T value = get(i);
+      if (value != uninitialized()) {
+        max = MAX2(max, value);
+        min = MIN2(min, value);
+        sum += value;
+        contributing_threads++;
+      }
+    }
+    T diff = max - min;
+    assert(contributing_threads != 0, "Must be since we found a used value for the start index");
+    double avg = sum / (double) contributing_threads;
+    WDAPrinter::summary(out, min, avg, max, diff, sum, print_sum);
+    out->print_cr(", Workers: %d", contributing_threads);
+  } else {
+    // No data for this phase.
+    out->print_cr(" skipped");
+  }
 }
 
 template <class T>
-void WorkerDataArray<T>::print_details_on(outputStream* out, uint active_threads) const {
-  WDAPrinter::details(this, out, active_threads);
+void WorkerDataArray<T>::print_details_on(outputStream* out) const {
+  WDAPrinter::details(this, out);
 }
 
-#ifndef PRODUCT
 template <typename T>
 void WorkerDataArray<T>::reset() {
   set_all(uninitialized());
@@ -133,27 +153,4 @@
   }
 }
 
-template <typename T>
-void WorkerDataArray<T>::verify(uint active_threads) const {
-  assert(active_threads <= _length, "Wrong number of active threads");
-  for (uint i = 0; i < active_threads; i++) {
-    assert(_data[i] != uninitialized(),
-           "Invalid data for worker %u in '%s'", i, _title);
-  }
-  if (_thread_work_items != NULL) {
-    _thread_work_items->verify(active_threads);
-  }
-}
-
-template <>
-inline size_t WorkerDataArray<size_t>::uninitialized() const {
-  return (size_t)-1;
-}
-
-template <>
-inline double WorkerDataArray<double>::uninitialized() const {
-  return -1.0;
-}
-#endif
-
 #endif // SHARE_VM_GC_G1_WORKERDATAARRAY_INLINE_HPP

--- a/hotspot/src/share/vm/gc/shared/workgroup.hpp	Thu Mar 31 11:21:20 2016 +0000
+++ b/hotspot/src/share/vm/gc/shared/workgroup.hpp	Thu Mar 31 17:05:29 2016 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2002, 2015, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2002, 2016, 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
@@ -152,7 +152,7 @@
     _active_workers = MAX2(1U, _active_workers);
     assert(UseDynamicNumberOfGCThreads || _active_workers == _total_workers,
            "Unless dynamic should use total workers");
-    log_info(gc, task)("GC Workers: %d", _active_workers);
+    log_info(gc, task)("GC Workers: using %d out of %d", _active_workers, _total_workers);
   }
 
   // Return the Ith worker.