--- a/hotspot/src/share/vm/gc/g1/collectionSetChooser.cpp	Fri Sep 11 10:02:35 2015 +0200
+++ b/hotspot/src/share/vm/gc/g1/collectionSetChooser.cpp	Fri Sep 11 13:20:05 2015 +0200
@@ -83,7 +83,7 @@
   _regions((ResourceObj::set_allocation_type((address) &_regions,
                                              ResourceObj::C_HEAP),
                   100), true /* C_Heap */),
-    _curr_index(0), _length(0), _first_par_unreserved_idx(0),
+    _front(0), _end(0), _first_par_unreserved_idx(0),
     _region_live_threshold_bytes(0), _remaining_reclaimable_bytes(0) {
   _region_live_threshold_bytes =
     HeapRegion::GrainBytes * (size_t) G1MixedGCLiveThresholdPercent / 100;
@@ -91,19 +91,19 @@
 
 #ifndef PRODUCT
 void CollectionSetChooser::verify() {
-  guarantee(_length <= regions_length(),
-         err_msg("_length: %u regions length: %u", _length, regions_length()));
-  guarantee(_curr_index <= _length,
-            err_msg("_curr_index: %u _length: %u", _curr_index, _length));
+  guarantee(_end <= regions_length(),
+         err_msg("_end: %u regions length: %u", _end, regions_length()));
+  guarantee(_front <= _end,
+            err_msg("_front: %u _end: %u", _front, _end));
   uint index = 0;
   size_t sum_of_reclaimable_bytes = 0;
-  while (index < _curr_index) {
+  while (index < _front) {
     guarantee(regions_at(index) == NULL,
-              "all entries before _curr_index should be NULL");
+              "all entries before _front should be NULL");
     index += 1;
   }
   HeapRegion *prev = NULL;
-  while (index < _length) {
+  while (index < _end) {
     HeapRegion *curr = regions_at(index++);
     guarantee(curr != NULL, "Regions in _regions array cannot be NULL");
     guarantee(!curr->is_young(), "should not be young!");
@@ -132,15 +132,15 @@
     regions_trunc_to(_first_par_unreserved_idx);
   }
   _regions.sort(order_regions);
-  assert(_length <= regions_length(), "Requirement");
+  assert(_end <= regions_length(), "Requirement");
 #ifdef ASSERT
-  for (uint i = 0; i < _length; i++) {
+  for (uint i = 0; i < _end; i++) {
     assert(regions_at(i) != NULL, "Should be true by sorting!");
   }
 #endif // ASSERT
   if (G1PrintRegionLivenessInfo) {
     G1PrintRegionLivenessInfoClosure cl(gclog_or_tty, "Post-Sorting");
-    for (uint i = 0; i < _length; ++i) {
+    for (uint i = 0; i < _end; ++i) {
       HeapRegion* r = regions_at(i);
       cl.doHeapRegion(r);
     }
@@ -154,11 +154,19 @@
          err_msg("Pinned region shouldn't be added to the collection set (index %u)", hr->hrm_index()));
   assert(!hr->is_young(), "should not be young!");
   _regions.append(hr);
-  _length++;
+  _end++;
   _remaining_reclaimable_bytes += hr->reclaimable_bytes();
   hr->calc_gc_efficiency();
 }
 
+void CollectionSetChooser::push(HeapRegion* hr) {
+  assert(hr != NULL, "Can't put back a NULL region");
+  assert(_front >= 1, "Too many regions have been put back");
+  _front--;
+  regions_at_put(_front, hr);
+  _remaining_reclaimable_bytes += hr->reclaimable_bytes();
+}
+
 void CollectionSetChooser::prepare_for_par_region_addition(uint n_threads,
                                                            uint n_regions,
                                                            uint chunk_size) {
@@ -193,7 +201,7 @@
     // We could have just used atomics instead of taking the
     // lock. However, we currently don't have an atomic add for size_t.
     MutexLockerEx x(ParGCRareEvent_lock, Mutex::_no_safepoint_check_flag);
-    _length += region_num;
+    _end += region_num;
     _remaining_reclaimable_bytes += reclaimable_bytes;
   } else {
     assert(reclaimable_bytes == 0, "invariant");
@@ -202,7 +210,7 @@
 
 void CollectionSetChooser::clear() {
   _regions.clear();
-  _curr_index = 0;
-  _length = 0;
+  _front = 0;
+  _end = 0;
   _remaining_reclaimable_bytes = 0;
 };

--- a/hotspot/src/share/vm/gc/g1/collectionSetChooser.hpp	Fri Sep 11 10:02:35 2015 +0200
+++ b/hotspot/src/share/vm/gc/g1/collectionSetChooser.hpp	Fri Sep 11 13:20:05 2015 +0200
@@ -48,12 +48,10 @@
 
   // The index of the next candidate old region to be considered for
   // addition to the CSet.
-  uint _curr_index;
+  uint _front;
 
-  // The number of candidate old regions added to the CSet chooser.
-  // Note: this is not updated when removing a region using
-  // remove_and_move_to_next() below.
-  uint _length;
+  // The index of the last candidate old region
+  uint _end;
 
   // Keeps track of the start of the next array chunk to be claimed by
   // parallel GC workers.
@@ -73,31 +71,33 @@
   // collection without removing it from the CSet chooser.
   HeapRegion* peek() {
     HeapRegion* res = NULL;
-    if (_curr_index < _length) {
-      res = regions_at(_curr_index);
+    if (_front < _end) {
+      res = regions_at(_front);
       assert(res != NULL,
              err_msg("Unexpected NULL hr in _regions at index %u",
-                     _curr_index));
+                     _front));
     }
     return res;
   }
 
   // Remove the given region from the CSet chooser and move to the
-  // next one. The given region should be the current candidate region
-  // in the CSet chooser.
-  void remove_and_move_to_next(HeapRegion* hr) {
+  // next one.
+  HeapRegion* pop() {
+    HeapRegion* hr = regions_at(_front);
     assert(hr != NULL, "pre-condition");
-    assert(_curr_index < _length, "pre-condition");
-    assert(regions_at(_curr_index) == hr, "pre-condition");
-    regions_at_put(_curr_index, NULL);
+    assert(_front < _end, "pre-condition");
+    regions_at_put(_front, NULL);
     assert(hr->reclaimable_bytes() <= _remaining_reclaimable_bytes,
            err_msg("remaining reclaimable bytes inconsistent "
                    "from region: " SIZE_FORMAT " remaining: " SIZE_FORMAT,
                    hr->reclaimable_bytes(), _remaining_reclaimable_bytes));
     _remaining_reclaimable_bytes -= hr->reclaimable_bytes();
-    _curr_index += 1;
+    _front += 1;
+    return hr;
   }
 
+  void push(HeapRegion* hr);
+
   CollectionSetChooser();
 
   void sort_regions();
@@ -113,7 +113,7 @@
   }
 
   // Returns the number candidate old regions added
-  uint length() { return _length; }
+  uint length() { return _end; }
 
   // Serial version.
   void add_region(HeapRegion *hr);
@@ -135,7 +135,7 @@
   void clear();
 
   // Return the number of candidate regions that remain to be collected.
-  uint remaining_regions() { return _length - _curr_index; }
+  uint remaining_regions() { return _end - _front; }
 
   // Determine whether the CSet chooser has more candidate regions or not.
   bool is_empty() { return remaining_regions() == 0; }

--- a/hotspot/src/share/vm/gc/g1/g1CollectorPolicy.cpp	Fri Sep 11 10:02:35 2015 +0200
+++ b/hotspot/src/share/vm/gc/g1/g1CollectorPolicy.cpp	Fri Sep 11 13:20:05 2015 +0200
@@ -2037,7 +2037,7 @@
       // We will add this region to the CSet.
       time_remaining_ms = MAX2(time_remaining_ms - predicted_time_ms, 0.0);
       predicted_old_time_ms += predicted_time_ms;
-      cset_chooser->remove_and_move_to_next(hr);
+      cset_chooser->pop(); // already have region via peek()
       _g1->old_set_remove(hr);
       add_old_region_to_cset(hr);