8267836: Separate eager reclaim remembered set threshold from G1RSetSparseRegionEntries

Reviewed-by: sjohanss, lkorinth

src/hotspot/share/gc/g1/g1Arguments.cpp

@@ -49,7 +49,14 @@ void G1Arguments::initialize_alignments() {
   // size, but the heap size should be aligned with the region size. To get
   // around this we use the unaligned values for the heap.
   HeapRegion::setup_heap_region_size(MaxHeapSize);
+
+  // The remembered set needs the heap regions set up.
   HeapRegionRemSet::setup_remset_size();
+  // Needs remembered set initialization as the ergonomics are based
+  // on it.
+  if (FLAG_IS_DEFAULT(G1EagerReclaimRemSetThreshold)) {
+    FLAG_SET_ERGO(G1EagerReclaimRemSetThreshold, G1RSetSparseRegionEntries);
+  }
 
   SpaceAlignment = HeapRegion::GrainBytes;
   HeapAlignment = calculate_heap_alignment(SpaceAlignment);

src/hotspot/share/gc/g1/g1CollectedHeap.cpp

@@ -2659,7 +2659,7 @@ bool G1CollectedHeap::is_potential_eager_reclaim_candidate(HeapRegion* r) const
   HeapRegionRemSet* rem_set = r->rem_set();
 
   return G1EagerReclaimHumongousObjectsWithStaleRefs ?
-         rem_set->occupancy_less_or_equal_than(G1RSetSparseRegionEntries) :
+         rem_set->occupancy_less_or_equal_than(G1EagerReclaimRemSetThreshold) :
          G1EagerReclaimHumongousObjects && rem_set->is_empty();
 }
 

src/hotspot/share/gc/g1/g1RemSet.cpp

@@ -1284,7 +1284,7 @@ class G1MergeHeapRootsTask : public AbstractGangTask {
         return false;
       }
 
-      guarantee(r->rem_set()->occupancy_less_or_equal_than(G1RSetSparseRegionEntries),
+      guarantee(r->rem_set()->occupancy_less_or_equal_than(G1EagerReclaimRemSetThreshold),
                 "Found a not-small remembered set here. This is inconsistent with previous assumptions.");
 
       _cl.dump_rem_set_for_region(r);

src/hotspot/share/gc/g1/g1_globals.hpp

@@ -240,6 +240,11 @@
           "Try to reclaim dead large objects that have a few stale "        \
           "references at every young GC.")                                  \
                                                                             \
+  product(uint, G1EagerReclaimRemSetThreshold, 0, EXPERIMENTAL,             \
+          "Maximum number of remembered set entries a humongous region "    \
+          "otherwise eligible for eager reclaim may have to be a candidate "\
+          "for eager reclaim. Will be selected ergonomically by default.")  \
+                                                                            \
   product(size_t, G1RebuildRemSetChunkSize, 256 * K, EXPERIMENTAL,          \
           "Chunk size used for rebuilding the remembered set.")             \
           range(4 * K, 32 * M)                                              \