8371791: G1: Improve accuracy of G1CollectedHeap::non_young_occupancy_after_allocation()

Reviewed-by: ayang, iwalulya

Author: Thomas Schatzl

src/hotspot/share/gc/g1/g1Allocator.cpp

@@ -123,6 +123,14 @@ void G1Allocator::reuse_retained_old_region(G1EvacInfo* evacuation_info,
   }
 }
 
+size_t G1Allocator::free_bytes_in_retained_old_region() const {
+  if (_retained_old_gc_alloc_region == nullptr) {
+    return 0;
+  } else {
+    return _retained_old_gc_alloc_region->free();
+  }
+}
+
 void G1Allocator::init_gc_alloc_regions(G1EvacInfo* evacuation_info) {
   assert_at_safepoint_on_vm_thread();
 

src/hotspot/share/gc/g1/g1Allocator.hpp

@@ -103,7 +103,10 @@ class G1Allocator : public CHeapObj<mtGC> {
   void init_gc_alloc_regions(G1EvacInfo* evacuation_info);
   void release_gc_alloc_regions(G1EvacInfo* evacuation_info);
   void abandon_gc_alloc_regions();
+
   bool is_retained_old_region(G1HeapRegion* hr);
+  // Return the amount of free bytes in the current retained old region.
+  size_t free_bytes_in_retained_old_region() const;
 
   // Node index of current thread.
   inline uint current_node_index() const;

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

@@ -2964,8 +2964,8 @@ void G1CollectedHeap::abandon_collection_set() {
 }
 
 size_t G1CollectedHeap::non_young_occupancy_after_allocation(size_t allocation_word_size) {
-  // For simplicity, just count whole regions.
-  const size_t cur_occupancy = (old_regions_count() + humongous_regions_count()) * G1HeapRegion::GrainBytes;
+  const size_t cur_occupancy = (old_regions_count() + humongous_regions_count()) * G1HeapRegion::GrainBytes -
+                               _allocator->free_bytes_in_retained_old_region();
   // Humongous allocations will always be assigned to non-young heap, so consider
   // that allocation in the result as well. Otherwise the allocation will always
   // be in young gen, so there is no need to account it here.