--- a/src/hotspot/share/gc/g1/collectionSetChooser.cpp	Mon Mar 26 16:51:41 2018 +0200
+++ b/src/hotspot/share/gc/g1/collectionSetChooser.cpp	Mon Mar 26 16:51:43 2018 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2018, 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
@@ -83,8 +83,7 @@
                   100), true /* C_Heap */),
     _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;
+  _region_live_threshold_bytes = mixed_gc_live_threshold_bytes();
 }
 
 #ifndef PRODUCT
@@ -203,6 +202,16 @@
   }
 }
 
+void CollectionSetChooser::iterate(HeapRegionClosure* cl) {
+  for (uint i = _front; i < _end; i++) {
+    HeapRegion* r = regions_at(i);
+    if (cl->do_heap_region(r)) {
+      cl->set_incomplete();
+      break;
+    }
+  }
+}
+
 void CollectionSetChooser::clear() {
   _regions.clear();
   _front = 0;
@@ -259,6 +268,17 @@
   return MAX2(n_regions / (n_workers * overpartition_factor), min_chunk_size);
 }
 
+bool CollectionSetChooser::region_occupancy_low_enough_for_evac(size_t live_bytes) {
+  return live_bytes < mixed_gc_live_threshold_bytes();
+}
+
+bool CollectionSetChooser::should_add(HeapRegion* hr) const {
+  assert(hr->is_marked(), "pre-condition");
+  assert(!hr->is_young(), "should never consider young regions");
+  return !hr->is_pinned() &&
+         region_occupancy_low_enough_for_evac(hr->live_bytes());
+}
+
 void CollectionSetChooser::rebuild(WorkGang* workers, uint n_regions) {
   clear();
 

--- a/src/hotspot/share/gc/g1/collectionSetChooser.hpp	Mon Mar 26 16:51:41 2018 +0200
+++ b/src/hotspot/share/gc/g1/collectionSetChooser.hpp	Mon Mar 26 16:51:43 2018 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2016, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2018, 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
@@ -101,17 +101,18 @@
 
   CollectionSetChooser();
 
+  static size_t mixed_gc_live_threshold_bytes() {
+    return HeapRegion::GrainBytes * (size_t) G1MixedGCLiveThresholdPercent / 100;
+  }
+
+  static bool region_occupancy_low_enough_for_evac(size_t live_bytes);
+
   void sort_regions();
 
   // Determine whether to add the given region to the CSet chooser or
   // not. Currently, we skip pinned regions and regions whose live
   // bytes are over the threshold. Humongous regions may be reclaimed during cleanup.
-  bool should_add(HeapRegion* hr) {
-    assert(hr->is_marked(), "pre-condition");
-    assert(!hr->is_young(), "should never consider young regions");
-    return !hr->is_pinned() &&
-            hr->live_bytes() < _region_live_threshold_bytes;
-  }
+  bool should_add(HeapRegion* hr) const;
 
   // Returns the number candidate old regions added
   uint length() { return _end; }
@@ -133,6 +134,9 @@
   // and the amount of reclaimable bytes by reclaimable_bytes.
   void update_totals(uint region_num, size_t reclaimable_bytes);
 
+  // Iterate over all collection set candidate regions.
+  void iterate(HeapRegionClosure* cl);
+
   void clear();
 
   void rebuild(WorkGang* workers, uint n_regions);

--- a/src/hotspot/share/gc/g1/heapRegion.hpp	Mon Mar 26 16:51:41 2018 +0200
+++ b/src/hotspot/share/gc/g1/heapRegion.hpp	Mon Mar 26 16:51:43 2018 +0200
@@ -713,6 +713,7 @@
 class HeapRegionClosure : public StackObj {
   friend class HeapRegionManager;
   friend class G1CollectionSet;
+  friend class CollectionSetChooser;
 
   bool _is_complete;
   void set_incomplete() { _is_complete = false; }