From 3812c7d0d3db94920df028ed5890886c073d1ade Mon Sep 17 00:00:00 2001
From: Thomas Schatzl <thomas.schatzl@oracle.com>
Date: Thu, 13 Nov 2025 14:59:29 +0100
Subject: [PATCH] 8371791

Hi all,

  please review this change that improves the accuracy of `G1CollectedHeap::non_young_occupancy_after_allocation()` by considering the free space in the old gen retained region.

Testing: tier1-3, gha

Thanks,
  Thomas
---
 src/hotspot/share/gc/g1/g1Allocator.cpp     | 8 ++++++++
 src/hotspot/share/gc/g1/g1Allocator.hpp     | 3 +++
 src/hotspot/share/gc/g1/g1CollectedHeap.cpp | 4 ++--
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/src/hotspot/share/gc/g1/g1Allocator.cpp b/src/hotspot/share/gc/g1/g1Allocator.cpp
index 713bafd478268..78710084ee37e 100644
--- a/src/hotspot/share/gc/g1/g1Allocator.cpp
+++ b/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();
 
diff --git a/src/hotspot/share/gc/g1/g1Allocator.hpp b/src/hotspot/share/gc/g1/g1Allocator.hpp
index 19b19c06e9249..9a7e62f5cc6b9 100644
--- a/src/hotspot/share/gc/g1/g1Allocator.hpp
+++ b/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;
diff --git a/src/hotspot/share/gc/g1/g1CollectedHeap.cpp b/src/hotspot/share/gc/g1/g1CollectedHeap.cpp
index f04658a1415e9..d18f61ff50770 100644
--- a/src/hotspot/share/gc/g1/g1CollectedHeap.cpp
+++ b/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.