From 57c25426e61920cb31eb7c4baafbd7ca396a67d3 Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Wed, 9 Apr 2025 08:41:46 +0200
Subject: [PATCH 01/14] Max tlab size should be reported in words

---
 src/hotspot/share/gc/z/zCollectedHeap.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/hotspot/share/gc/z/zCollectedHeap.cpp b/src/hotspot/share/gc/z/zCollectedHeap.cpp
index 642ad42a1d7bb..172dfaf680c6b 100644
--- a/src/hotspot/share/gc/z/zCollectedHeap.cpp
+++ b/src/hotspot/share/gc/z/zCollectedHeap.cpp
@@ -238,7 +238,7 @@ size_t ZCollectedHeap::tlab_used(Thread* ignored) const {
 }
 
 size_t ZCollectedHeap::max_tlab_size() const {
-  return _heap.max_tlab_size();
+  return _heap.max_tlab_size() / HeapWordSize;
 }
 
 size_t ZCollectedHeap::unsafe_max_tlab_alloc(Thread* ignored) const {

From 554a0dee37c8f1b3047fd576928e3818706cec78 Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Thu, 17 Apr 2025 12:57:19 +0200
Subject: [PATCH 02/14] Add class to keep track of TLAB usage

---
 src/hotspot/share/gc/z/zGeneration.cpp |  3 ++
 src/hotspot/share/gc/z/zHeap.cpp       | 17 +++++++++--
 src/hotspot/share/gc/z/zHeap.hpp       |  3 ++
 src/hotspot/share/gc/z/zTLABUsage.cpp  | 39 +++++++++++++++++++++++++
 src/hotspot/share/gc/z/zTLABUsage.hpp  | 40 ++++++++++++++++++++++++++
 5 files changed, 100 insertions(+), 2 deletions(-)
 create mode 100644 src/hotspot/share/gc/z/zTLABUsage.cpp
 create mode 100644 src/hotspot/share/gc/z/zTLABUsage.hpp

diff --git a/src/hotspot/share/gc/z/zGeneration.cpp b/src/hotspot/share/gc/z/zGeneration.cpp
index e74d7a8e9f3e0..ad4e59057702f 100644
--- a/src/hotspot/share/gc/z/zGeneration.cpp
+++ b/src/hotspot/share/gc/z/zGeneration.cpp
@@ -837,6 +837,9 @@ void ZGenerationYoung::mark_start() {
   // Change good colors
   flip_mark_start();
 
+  // Update TLAB usage
+  ZHeap::heap()->update_tlab_usage(ZAllocator::eden()->tlab_used());
+
   // Retire allocating pages
   ZAllocator::eden()->retire_pages();
   for (ZPageAge i = ZPageAge::survivor1; i <= ZPageAge::survivor14; i = static_cast<ZPageAge>(static_cast<uint>(i) + 1)) {
diff --git a/src/hotspot/share/gc/z/zHeap.cpp b/src/hotspot/share/gc/z/zHeap.cpp
index 90f8a86713532..1e072fb6ac43c 100644
--- a/src/hotspot/share/gc/z/zHeap.cpp
+++ b/src/hotspot/share/gc/z/zHeap.cpp
@@ -62,6 +62,7 @@ ZHeap::ZHeap()
     _serviceability(InitialHeapSize, min_capacity(), max_capacity()),
     _old(&_page_table, &_page_allocator),
     _young(&_page_table, _old.forwarding_table(), &_page_allocator),
+    _tlab_usage(),
     _initialized(false) {
 
   // Install global heap instance
@@ -131,11 +132,11 @@ size_t ZHeap::unused() const {
 }
 
 size_t ZHeap::tlab_capacity() const {
-  return capacity();
+  return _tlab_usage.capacity();
 }
 
 size_t ZHeap::tlab_used() const {
-  return _allocator_eden.tlab_used();
+  return _tlab_usage.used();
 }
 
 size_t ZHeap::max_tlab_size() const {
@@ -157,6 +158,18 @@ size_t ZHeap::unsafe_max_tlab_alloc() const {
   return MIN2(size, max_tlab_size());
 }
 
+void ZHeap::update_tlab_usage(size_t current_used) {
+  const size_t old_used = _tlab_usage.used();
+  const size_t old_capacity = _tlab_usage.capacity();
+
+  _tlab_usage.update(current_used);
+  log_debug(gc, tlab)("TLAB Usage update: used %zuM -> %zuM, capacity: %zuM -> %zuM",
+                      old_used / M,
+                      _tlab_usage.used() / M,
+                      old_capacity / M,
+                      _tlab_usage.capacity() / M);
+}
+
 bool ZHeap::is_in(uintptr_t addr) const {
   if (addr == 0) {
     // Null isn't in the heap.
diff --git a/src/hotspot/share/gc/z/zHeap.hpp b/src/hotspot/share/gc/z/zHeap.hpp
index 823fc009b2ca0..8dbc53f0ac506 100644
--- a/src/hotspot/share/gc/z/zHeap.hpp
+++ b/src/hotspot/share/gc/z/zHeap.hpp
@@ -33,6 +33,7 @@
 #include "gc/z/zPageTable.hpp"
 #include "gc/z/zPageType.hpp"
 #include "gc/z/zServiceability.hpp"
+#include "gc/z/zTLABUsage.hpp"
 
 class OopFieldClosure;
 
@@ -54,6 +55,7 @@ class ZHeap {
 
   ZGenerationOld          _old;
   ZGenerationYoung        _young;
+  ZTLABUsage              _tlab_usage;
 
   bool                    _initialized;
 
@@ -81,6 +83,7 @@ class ZHeap {
   size_t tlab_used() const;
   size_t max_tlab_size() const;
   size_t unsafe_max_tlab_alloc() const;
+  void update_tlab_usage(size_t current_used);
 
   bool is_in(uintptr_t addr) const;
   bool is_in_page_relaxed(const ZPage* page, zaddress addr) const;
diff --git a/src/hotspot/share/gc/z/zTLABUsage.cpp b/src/hotspot/share/gc/z/zTLABUsage.cpp
new file mode 100644
index 0000000000000..5f2b8184fa679
--- /dev/null
+++ b/src/hotspot/share/gc/z/zTLABUsage.cpp
@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2025, 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+ #include "gc/z/zTLABUsage.hpp"
+
+ZTLABUsage::ZTLABUsage()
+  : _used_history() { }
+
+void ZTLABUsage::update(size_t used) {
+  _used_history.add(used);
+}
+
+size_t ZTLABUsage::used() const {
+  return _used_history.last();
+}
+
+size_t ZTLABUsage::capacity() const {
+  return _used_history.avg();
+}
diff --git a/src/hotspot/share/gc/z/zTLABUsage.hpp b/src/hotspot/share/gc/z/zTLABUsage.hpp
new file mode 100644
index 0000000000000..82ea5c3879934
--- /dev/null
+++ b/src/hotspot/share/gc/z/zTLABUsage.hpp
@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2025, 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+#ifndef SHARE_GC_Z_ZTLABUSAGE_HPP
+#define SHARE_GC_Z_ZTLABUSAGE_HPP
+
+#include "utilities/numberSeq.hpp"
+
+class ZTLABUsage {
+private:
+  TruncatedSeq _used_history;
+
+public:
+  ZTLABUsage();
+  void update(size_t used);
+  size_t used() const;
+  size_t capacity() const;
+};
+
+#endif // SHARE_GC_Z_ZTLABUSAGE_HPP

From 92769517d89eb9c2d83dec1126cdbb0e2f36c28c Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Thu, 10 Apr 2025 10:33:02 +0200
Subject: [PATCH 03/14] Track eden-usage in page allocator

---
 src/hotspot/share/gc/z/zAllocator.cpp       |  4 ---
 src/hotspot/share/gc/z/zAllocator.hpp       |  1 -
 src/hotspot/share/gc/z/zGeneration.cpp      |  2 +-
 src/hotspot/share/gc/z/zHeap.cpp            |  3 +-
 src/hotspot/share/gc/z/zObjectAllocator.cpp | 34 +--------------------
 src/hotspot/share/gc/z/zObjectAllocator.hpp |  3 --
 src/hotspot/share/gc/z/zPageAllocator.cpp   | 18 +++++++++++
 src/hotspot/share/gc/z/zPageAllocator.hpp   |  4 +++
 src/hotspot/share/gc/z/zTLABUsage.cpp       |  2 +-
 src/hotspot/share/gc/z/zTLABUsage.hpp       | 11 +++++++
 10 files changed, 38 insertions(+), 44 deletions(-)

diff --git a/src/hotspot/share/gc/z/zAllocator.cpp b/src/hotspot/share/gc/z/zAllocator.cpp
index 87e1f6f76f941..22f3b6ba11232 100644
--- a/src/hotspot/share/gc/z/zAllocator.cpp
+++ b/src/hotspot/share/gc/z/zAllocator.cpp
@@ -39,10 +39,6 @@ ZAllocatorEden::ZAllocatorEden()
   ZAllocator::_eden = this;
 }
 
-size_t ZAllocatorEden::tlab_used() const {
-  return _object_allocator.used();
-}
-
 size_t ZAllocatorEden::remaining() const {
   return _object_allocator.remaining();
 }
diff --git a/src/hotspot/share/gc/z/zAllocator.hpp b/src/hotspot/share/gc/z/zAllocator.hpp
index 21702800e6b03..45a70888f9d73 100644
--- a/src/hotspot/share/gc/z/zAllocator.hpp
+++ b/src/hotspot/share/gc/z/zAllocator.hpp
@@ -62,7 +62,6 @@ class ZAllocatorEden : public ZAllocator {
   zaddress alloc_object(size_t size);
 
   // Statistics
-  size_t tlab_used() const;
   size_t remaining() const;
 };
 
diff --git a/src/hotspot/share/gc/z/zGeneration.cpp b/src/hotspot/share/gc/z/zGeneration.cpp
index ad4e59057702f..0ae8ec07e7dd3 100644
--- a/src/hotspot/share/gc/z/zGeneration.cpp
+++ b/src/hotspot/share/gc/z/zGeneration.cpp
@@ -838,7 +838,7 @@ void ZGenerationYoung::mark_start() {
   flip_mark_start();
 
   // Update TLAB usage
-  ZHeap::heap()->update_tlab_usage(ZAllocator::eden()->tlab_used());
+  ZHeap::heap()->update_tlab_usage(_page_allocator->used_eden());
 
   // Retire allocating pages
   ZAllocator::eden()->retire_pages();
diff --git a/src/hotspot/share/gc/z/zHeap.cpp b/src/hotspot/share/gc/z/zHeap.cpp
index 1e072fb6ac43c..9eefdd3d56c01 100644
--- a/src/hotspot/share/gc/z/zHeap.cpp
+++ b/src/hotspot/share/gc/z/zHeap.cpp
@@ -163,7 +163,8 @@ void ZHeap::update_tlab_usage(size_t current_used) {
   const size_t old_capacity = _tlab_usage.capacity();
 
   _tlab_usage.update(current_used);
-  log_debug(gc, tlab)("TLAB Usage update: used %zuM -> %zuM, capacity: %zuM -> %zuM",
+
+  log_debug(gc, tlab)("TLAB usage update: used %zuM -> %zuM, capacity: %zuM -> %zuM",
                       old_used / M,
                       _tlab_usage.used() / M,
                       old_capacity / M,
diff --git a/src/hotspot/share/gc/z/zObjectAllocator.cpp b/src/hotspot/share/gc/z/zObjectAllocator.cpp
index 54724c1b48ed4..b92828e24a683 100644
--- a/src/hotspot/share/gc/z/zObjectAllocator.cpp
+++ b/src/hotspot/share/gc/z/zObjectAllocator.cpp
@@ -43,8 +43,6 @@ static const ZStatCounter ZCounterUndoObjectAllocationFailed("Memory", "Undo Obj
 ZObjectAllocator::ZObjectAllocator(ZPageAge age)
   : _age(age),
     _use_per_cpu_shared_small_pages(ZHeuristics::use_per_cpu_shared_small_pages()),
-    _used(0),
-    _undone(0),
     _shared_small_page(nullptr),
     _shared_medium_page(nullptr),
     _medium_page_alloc_lock() {}
@@ -58,13 +56,7 @@ ZPage* const* ZObjectAllocator::shared_small_page_addr() const {
 }
 
 ZPage* ZObjectAllocator::alloc_page(ZPageType type, size_t size, ZAllocationFlags flags) {
-  ZPage* const page = ZHeap::heap()->alloc_page(type, size, flags, _age);
-  if (page != nullptr) {
-    // Increment used bytes
-    Atomic::add(_used.addr(), size);
-  }
-
-  return page;
+  return ZHeap::heap()->alloc_page(type, size, flags, _age);
 }
 
 ZPage* ZObjectAllocator::alloc_page_for_relocation(ZPageType type, size_t size, ZAllocationFlags flags) {
@@ -72,9 +64,6 @@ ZPage* ZObjectAllocator::alloc_page_for_relocation(ZPageType type, size_t size,
 }
 
 void ZObjectAllocator::undo_alloc_page(ZPage* page) {
-  // Increment undone bytes
-  Atomic::add(_undone.addr(), page->size());
-
   ZHeap::heap()->undo_alloc_page(page);
 }
 
@@ -229,23 +218,6 @@ ZPageAge ZObjectAllocator::age() const {
   return _age;
 }
 
-size_t ZObjectAllocator::used() const {
-  size_t total_used = 0;
-  size_t total_undone = 0;
-
-  ZPerCPUConstIterator<size_t> iter_used(&_used);
-  for (const size_t* cpu_used; iter_used.next(&cpu_used);) {
-    total_used += *cpu_used;
-  }
-
-  ZPerCPUConstIterator<size_t> iter_undone(&_undone);
-  for (const size_t* cpu_undone; iter_undone.next(&cpu_undone);) {
-    total_undone += *cpu_undone;
-  }
-
-  return total_used - total_undone;
-}
-
 size_t ZObjectAllocator::remaining() const {
   assert(Thread::current()->is_Java_thread(), "Should be a Java thread");
 
@@ -260,10 +232,6 @@ size_t ZObjectAllocator::remaining() const {
 void ZObjectAllocator::retire_pages() {
   assert(SafepointSynchronize::is_at_safepoint(), "Should be at safepoint");
 
-  // Reset used and undone bytes
-  _used.set_all(0);
-  _undone.set_all(0);
-
   // Reset allocation pages
   _shared_medium_page.set(nullptr);
   _shared_small_page.set_all(nullptr);
diff --git a/src/hotspot/share/gc/z/zObjectAllocator.hpp b/src/hotspot/share/gc/z/zObjectAllocator.hpp
index a350c4f876a7d..3a747afecbe7f 100644
--- a/src/hotspot/share/gc/z/zObjectAllocator.hpp
+++ b/src/hotspot/share/gc/z/zObjectAllocator.hpp
@@ -38,8 +38,6 @@ class ZObjectAllocator {
 private:
   ZPageAge           _age;
   const bool         _use_per_cpu_shared_small_pages;
-  ZPerCPU<size_t>    _used;
-  ZPerCPU<size_t>    _undone;
   ZPerCPU<ZPage*>    _shared_small_page;
   ZContended<ZPage*> _shared_medium_page;
   ZLock              _medium_page_alloc_lock;
@@ -80,7 +78,6 @@ class ZObjectAllocator {
 
   ZPageAge age() const;
 
-  size_t used() const;
   size_t remaining() const;
 
   void retire_pages();
diff --git a/src/hotspot/share/gc/z/zPageAllocator.cpp b/src/hotspot/share/gc/z/zPageAllocator.cpp
index 7bb1dcdcf81d4..c051f0816ec39 100644
--- a/src/hotspot/share/gc/z/zPageAllocator.cpp
+++ b/src/hotspot/share/gc/z/zPageAllocator.cpp
@@ -1264,6 +1264,7 @@ ZPageAllocator::ZPageAllocator(size_t min_capacity,
     _min_capacity(min_capacity),
     _max_capacity(max_capacity),
     _used(0),
+    _used_eden(0),
     _used_generations{0,0},
     _collection_stats{{0, 0},{0, 0}},
     _partitions(ZValueIdTagType{}, this),
@@ -1354,6 +1355,10 @@ size_t ZPageAllocator::used() const {
   return Atomic::load(&_used);
 }
 
+size_t ZPageAllocator::used_eden() const {
+  return Atomic::load(&_used_eden);
+}
+
 size_t ZPageAllocator::used_generation(ZGenerationId id) const {
   return Atomic::load(&_used_generations[(int)id]);
 }
@@ -1415,6 +1420,7 @@ ZPageAllocatorStats ZPageAllocator::update_and_stats(ZGeneration* generation) {
   ZLocker<ZLock> locker(&_lock);
 
   update_collection_stats(generation->id());
+  reset_used_eden();
   return stats_inner(generation);
 }
 
@@ -1570,6 +1576,7 @@ bool ZPageAllocator::claim_capacity_or_stall(ZPageAllocation* allocation) {
     if (claim_capacity(allocation)) {
       // Keep track of usage
       increase_used(allocation->size());
+      increase_used_eden(allocation);
 
       return true;
     }
@@ -2202,6 +2209,7 @@ void ZPageAllocator::satisfy_stalled() {
 
     // Keep track of usage
     increase_used(allocation->size());
+    increase_used_eden(allocation);
 
     // Allocation succeeded, dequeue and satisfy allocation request.
     // Note that we must dequeue the allocation request first, since
@@ -2262,6 +2270,16 @@ void ZPageAllocator::decrease_used(size_t size) {
   }
 }
 
+void ZPageAllocator::increase_used_eden(ZPageAllocation* allocation) {
+  if (allocation->age() == ZPageAge::eden) {
+    Atomic::add(&_used_eden, allocation->size());
+  }
+}
+
+void ZPageAllocator::reset_used_eden() {
+  Atomic::store(&_used_eden, (size_t) 0);
+}
+
 void ZPageAllocator::safe_destroy_page(ZPage* page) {
   // Destroy page safely
   _safe_destroy.schedule_delete(page);
diff --git a/src/hotspot/share/gc/z/zPageAllocator.hpp b/src/hotspot/share/gc/z/zPageAllocator.hpp
index 5cd08aee94cda..79f316b26807d 100644
--- a/src/hotspot/share/gc/z/zPageAllocator.hpp
+++ b/src/hotspot/share/gc/z/zPageAllocator.hpp
@@ -159,6 +159,7 @@ class ZPageAllocator {
   const size_t                _min_capacity;
   const size_t                _max_capacity;
   volatile size_t             _used;
+  volatile size_t             _used_eden;
   volatile size_t             _used_generations[2];
   struct {
     size_t _used_high;
@@ -231,6 +232,8 @@ class ZPageAllocator {
 
   void increase_used(size_t size);
   void decrease_used(size_t size);
+  void increase_used_eden(ZPageAllocation* allocation);
+  void reset_used_eden();
 
   void notify_out_of_memory();
   void restart_gc() const;
@@ -256,6 +259,7 @@ class ZPageAllocator {
   size_t current_max_capacity() const;
   size_t capacity() const;
   size_t used() const;
+  size_t used_eden() const;
   size_t used_generation(ZGenerationId id) const;
   size_t unused() const;
 
diff --git a/src/hotspot/share/gc/z/zTLABUsage.cpp b/src/hotspot/share/gc/z/zTLABUsage.cpp
index 5f2b8184fa679..60e324d4700ac 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.cpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.cpp
@@ -35,5 +35,5 @@ size_t ZTLABUsage::used() const {
 }
 
 size_t ZTLABUsage::capacity() const {
-  return _used_history.avg();
+  return _used_history.davg();
 }
diff --git a/src/hotspot/share/gc/z/zTLABUsage.hpp b/src/hotspot/share/gc/z/zTLABUsage.hpp
index 82ea5c3879934..b049725a501cd 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.hpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.hpp
@@ -26,6 +26,17 @@
 
 #include "utilities/numberSeq.hpp"
 
+// ZGC is retiring TLABs concurrently with the application running when
+// processing the stack watermarks. For the common TLAB heuristic to work we
+// need to return consistent TLAB usage information when a TLAB is retired.
+// We snapshot the TLAB usage in the mark start pause for the young generation
+// and use this information until the next garbage collection cycle.
+//
+// ZGC does not have set generation sizes like most other GCs and because of
+// this there is no fixed TLAB capacity. For the common TLAB sizing heuristic
+// to work properly ZGC estimates the current capacity by using a weighted
+// average of the last 10 used values.
+
 class ZTLABUsage {
 private:
   TruncatedSeq _used_history;

From 7421569b8d2e2d4c9a063e30de9a4a3df69d5b1b Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Tue, 22 Apr 2025 15:53:33 +0200
Subject: [PATCH 04/14] Fixes for TLABUsage

---
 src/hotspot/share/gc/z/zHeap.cpp | 4 ++--
 src/hotspot/share/gc/z/zHeap.hpp | 3 ++-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/hotspot/share/gc/z/zHeap.cpp b/src/hotspot/share/gc/z/zHeap.cpp
index 9eefdd3d56c01..f8cd36289f1ab 100644
--- a/src/hotspot/share/gc/z/zHeap.cpp
+++ b/src/hotspot/share/gc/z/zHeap.cpp
@@ -158,11 +158,11 @@ size_t ZHeap::unsafe_max_tlab_alloc() const {
   return MIN2(size, max_tlab_size());
 }
 
-void ZHeap::update_tlab_usage(size_t current_used) {
+void ZHeap::update_tlab_usage(size_t used) {
   const size_t old_used = _tlab_usage.used();
   const size_t old_capacity = _tlab_usage.capacity();
 
-  _tlab_usage.update(current_used);
+  _tlab_usage.update(used);
 
   log_debug(gc, tlab)("TLAB usage update: used %zuM -> %zuM, capacity: %zuM -> %zuM",
                       old_used / M,
diff --git a/src/hotspot/share/gc/z/zHeap.hpp b/src/hotspot/share/gc/z/zHeap.hpp
index 8dbc53f0ac506..4f2f15c52425c 100644
--- a/src/hotspot/share/gc/z/zHeap.hpp
+++ b/src/hotspot/share/gc/z/zHeap.hpp
@@ -55,6 +55,7 @@ class ZHeap {
 
   ZGenerationOld          _old;
   ZGenerationYoung        _young;
+
   ZTLABUsage              _tlab_usage;
 
   bool                    _initialized;
@@ -83,7 +84,7 @@ class ZHeap {
   size_t tlab_used() const;
   size_t max_tlab_size() const;
   size_t unsafe_max_tlab_alloc() const;
-  void update_tlab_usage(size_t current_used);
+  void update_tlab_usage(size_t used);
 
   bool is_in(uintptr_t addr) const;
   bool is_in_page_relaxed(const ZPage* page, zaddress addr) const;

From 1b132bfc18e1ef3d9b023aad25688c659b211ae9 Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Tue, 22 Apr 2025 16:10:39 +0200
Subject: [PATCH 05/14] Fixes for eden tracking

---
 src/hotspot/share/gc/z/zPageAllocator.cpp | 31 ++++++++++++++---------
 src/hotspot/share/gc/z/zPageAllocator.hpp |  3 ++-
 2 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/src/hotspot/share/gc/z/zPageAllocator.cpp b/src/hotspot/share/gc/z/zPageAllocator.cpp
index c051f0816ec39..93161a0ba2760 100644
--- a/src/hotspot/share/gc/z/zPageAllocator.cpp
+++ b/src/hotspot/share/gc/z/zPageAllocator.cpp
@@ -1419,8 +1419,13 @@ ZPageAllocatorStats ZPageAllocator::stats(ZGeneration* generation) const {
 ZPageAllocatorStats ZPageAllocator::update_and_stats(ZGeneration* generation) {
   ZLocker<ZLock> locker(&_lock);
 
+  // Update per-collection statistics
   update_collection_stats(generation->id());
+
+  // Clear eden statistics
   reset_used_eden();
+
+  // Return the collected statistics
   return stats_inner(generation);
 }
 
@@ -1576,7 +1581,6 @@ bool ZPageAllocator::claim_capacity_or_stall(ZPageAllocation* allocation) {
     if (claim_capacity(allocation)) {
       // Keep track of usage
       increase_used(allocation->size());
-      increase_used_eden(allocation);
 
       return true;
     }
@@ -2084,18 +2088,22 @@ void ZPageAllocator::free_memory_alloc_failed(ZMemoryAllocation* allocation) {
 }
 
 ZPage* ZPageAllocator::create_page(ZPageAllocation* allocation, const ZVirtualMemory& vmem) {
+  const size_t size = allocation->size();
+  const ZPageType type = allocation->type();
+  const ZPageAge age = allocation->age();
+  const ZGenerationId id = age == ZPageAge::old ? ZGenerationId::old : ZGenerationId::young;
+
   // We don't track generation usage when claiming capacity, because this page
   // could have been allocated by a thread that satisfies a stalling allocation.
   // The stalled thread can wake up and potentially realize that the page alloc
   // should be undone. If the alloc and the undo gets separated by a safepoint,
-  // the generation statistics could se a decreasing used value between mark
+  // the generation statistics could see a decreasing used value between mark
   // start and mark end. At this point an allocation will be successful, so we
   // update the generation usage.
-  const ZGenerationId id = allocation->age() == ZPageAge::old ? ZGenerationId::old : ZGenerationId::young;
-  increase_used_generation(id, allocation->size());
-
-  const ZPageType type = allocation->type();
-  const ZPageAge age = allocation->age();
+  increase_used_generation(id, size);
+  if (age == ZPageAge::eden) {
+    increase_used_eden(size);
+  }
 
   if (allocation->is_multi_partition()) {
     const ZMultiPartitionAllocation* const multi_partition_allocation = allocation->multi_partition_allocation();
@@ -2209,7 +2217,6 @@ void ZPageAllocator::satisfy_stalled() {
 
     // Keep track of usage
     increase_used(allocation->size());
-    increase_used_eden(allocation);
 
     // Allocation succeeded, dequeue and satisfy allocation request.
     // Note that we must dequeue the allocation request first, since
@@ -2270,13 +2277,13 @@ void ZPageAllocator::decrease_used(size_t size) {
   }
 }
 
-void ZPageAllocator::increase_used_eden(ZPageAllocation* allocation) {
-  if (allocation->age() == ZPageAge::eden) {
-    Atomic::add(&_used_eden, allocation->size());
-  }
+void ZPageAllocator::increase_used_eden(size_t size) {
+  // Update atomically since we have concurrent readers and writers
+  Atomic::add(&_used_eden, size);
 }
 
 void ZPageAllocator::reset_used_eden() {
+  // Reset atomically since we have concurrent readers and writers
   Atomic::store(&_used_eden, (size_t) 0);
 }
 
diff --git a/src/hotspot/share/gc/z/zPageAllocator.hpp b/src/hotspot/share/gc/z/zPageAllocator.hpp
index 79f316b26807d..fa03da01d76df 100644
--- a/src/hotspot/share/gc/z/zPageAllocator.hpp
+++ b/src/hotspot/share/gc/z/zPageAllocator.hpp
@@ -232,7 +232,8 @@ class ZPageAllocator {
 
   void increase_used(size_t size);
   void decrease_used(size_t size);
-  void increase_used_eden(ZPageAllocation* allocation);
+
+  void increase_used_eden(size_t size);
   void reset_used_eden();
 
   void notify_out_of_memory();

From 4e7ea16a220a3e38a0af7b72352770d71e827738 Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Tue, 22 Apr 2025 16:59:33 +0200
Subject: [PATCH 06/14] Junba-space

---
 src/hotspot/share/gc/z/zTLABUsage.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/hotspot/share/gc/z/zTLABUsage.cpp b/src/hotspot/share/gc/z/zTLABUsage.cpp
index 60e324d4700ac..663dd18301dec 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.cpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.cpp
@@ -21,7 +21,7 @@
  * questions.
  */
 
- #include "gc/z/zTLABUsage.hpp"
+#include "gc/z/zTLABUsage.hpp"
 
 ZTLABUsage::ZTLABUsage()
   : _used_history() { }

From c96c5a83e775258a217b8f9737de0d47f3333fd0 Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Tue, 22 Apr 2025 21:53:22 +0200
Subject: [PATCH 07/14] More TLABUsage fixes

---
 src/hotspot/share/gc/z/zTLABUsage.cpp | 2 +-
 src/hotspot/share/gc/z/zTLABUsage.hpp | 5 ++++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/hotspot/share/gc/z/zTLABUsage.cpp b/src/hotspot/share/gc/z/zTLABUsage.cpp
index 663dd18301dec..c2a0758d75ea8 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.cpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.cpp
@@ -24,7 +24,7 @@
 #include "gc/z/zTLABUsage.hpp"
 
 ZTLABUsage::ZTLABUsage()
-  : _used_history() { }
+  : _used_history() {}
 
 void ZTLABUsage::update(size_t used) {
   _used_history.add(used);
diff --git a/src/hotspot/share/gc/z/zTLABUsage.hpp b/src/hotspot/share/gc/z/zTLABUsage.hpp
index b049725a501cd..5b4480345fe66 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.hpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.hpp
@@ -24,6 +24,7 @@
 #ifndef SHARE_GC_Z_ZTLABUSAGE_HPP
 #define SHARE_GC_Z_ZTLABUSAGE_HPP
 
+#include "utilities/globalDefinitions.hpp"
 #include "utilities/numberSeq.hpp"
 
 // ZGC is retiring TLABs concurrently with the application running when
@@ -32,7 +33,7 @@
 // We snapshot the TLAB usage in the mark start pause for the young generation
 // and use this information until the next garbage collection cycle.
 //
-// ZGC does not have set generation sizes like most other GCs and because of
+// ZGC does not have set generation sizes unlike most other GCs and because of
 // this there is no fixed TLAB capacity. For the common TLAB sizing heuristic
 // to work properly ZGC estimates the current capacity by using a weighted
 // average of the last 10 used values.
@@ -43,7 +44,9 @@ class ZTLABUsage {
 
 public:
   ZTLABUsage();
+
   void update(size_t used);
+
   size_t used() const;
   size_t capacity() const;
 };

From 0c1f6eed32903f87ff8a73d2ed22d52ff044866b Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Wed, 23 Apr 2025 09:02:51 +0200
Subject: [PATCH 08/14] Change memory order to relaxed

---
 src/hotspot/share/gc/z/zPageAllocator.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/hotspot/share/gc/z/zPageAllocator.cpp b/src/hotspot/share/gc/z/zPageAllocator.cpp
index 93161a0ba2760..58e707eafc465 100644
--- a/src/hotspot/share/gc/z/zPageAllocator.cpp
+++ b/src/hotspot/share/gc/z/zPageAllocator.cpp
@@ -2279,7 +2279,7 @@ void ZPageAllocator::decrease_used(size_t size) {
 
 void ZPageAllocator::increase_used_eden(size_t size) {
   // Update atomically since we have concurrent readers and writers
-  Atomic::add(&_used_eden, size);
+  Atomic::add(&_used_eden, size, memory_order_relaxed);
 }
 
 void ZPageAllocator::reset_used_eden() {

From 76c79f5c7da260300473fa518c4183d6da727409 Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Wed, 23 Apr 2025 15:53:33 +0200
Subject: [PATCH 09/14] Default TLAB size of 8k, avoid 0 updates and reasonable
 starting values

---
 src/hotspot/share/gc/z/zArguments.cpp |  5 +++++
 src/hotspot/share/gc/z/zHeap.cpp      |  2 +-
 src/hotspot/share/gc/z/zTLABUsage.cpp | 12 ++++++++++--
 src/hotspot/share/gc/z/zTLABUsage.hpp |  2 +-
 4 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/src/hotspot/share/gc/z/zArguments.cpp b/src/hotspot/share/gc/z/zArguments.cpp
index 67b9f6f0bb918..b54fb503d2e11 100644
--- a/src/hotspot/share/gc/z/zArguments.cpp
+++ b/src/hotspot/share/gc/z/zArguments.cpp
@@ -133,6 +133,11 @@ void ZArguments::initialize() {
     FLAG_SET_ERGO_IF_DEFAULT(ZCollectionIntervalMajor, ZCollectionInterval);
   }
 
+  // Set a small initial TLAB size to avoid depending on the current capacity
+  if (FLAG_IS_DEFAULT(TLABSize)) {
+    FLAG_SET_DEFAULT(TLABSize, 8*K);
+  }
+
   // Set medium page size here because MaxTenuringThreshold may use it.
   ZHeuristics::set_medium_page_size();
 
diff --git a/src/hotspot/share/gc/z/zHeap.cpp b/src/hotspot/share/gc/z/zHeap.cpp
index f8cd36289f1ab..56cff7bae2e17 100644
--- a/src/hotspot/share/gc/z/zHeap.cpp
+++ b/src/hotspot/share/gc/z/zHeap.cpp
@@ -62,7 +62,7 @@ ZHeap::ZHeap()
     _serviceability(InitialHeapSize, min_capacity(), max_capacity()),
     _old(&_page_table, &_page_allocator),
     _young(&_page_table, _old.forwarding_table(), &_page_allocator),
-    _tlab_usage(),
+    _tlab_usage(InitialHeapSize),
     _initialized(false) {
 
   // Install global heap instance
diff --git a/src/hotspot/share/gc/z/zTLABUsage.cpp b/src/hotspot/share/gc/z/zTLABUsage.cpp
index c2a0758d75ea8..bee4be4e38bd0 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.cpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.cpp
@@ -23,10 +23,18 @@
 
 #include "gc/z/zTLABUsage.hpp"
 
-ZTLABUsage::ZTLABUsage()
-  : _used_history() {}
+ZTLABUsage::ZTLABUsage(size_t initial_heap_size)
+  : _used_history() {
+    // To get reasonable values for used and capacity until the first GC
+    // we populate the used history with the initial heap size.
+    _used_history.add(initial_heap_size);
+  }
 
 void ZTLABUsage::update(size_t used) {
+  if (used == 0) {
+    // Avoid updates for the second young generation collection of a SystemGC
+    return;
+  }
   _used_history.add(used);
 }
 
diff --git a/src/hotspot/share/gc/z/zTLABUsage.hpp b/src/hotspot/share/gc/z/zTLABUsage.hpp
index 5b4480345fe66..92ee319afd0f5 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.hpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.hpp
@@ -43,7 +43,7 @@ class ZTLABUsage {
   TruncatedSeq _used_history;
 
 public:
-  ZTLABUsage();
+  ZTLABUsage(size_t initial_heap_size);
 
   void update(size_t used);
 

From 4889af76064c39ea5a967817a3f388392f97d333 Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Tue, 6 May 2025 12:44:17 +0200
Subject: [PATCH 10/14] Revert initial value for TLABUsage

---
 src/hotspot/share/gc/z/zHeap.cpp      | 2 +-
 src/hotspot/share/gc/z/zTLABUsage.cpp | 8 ++------
 src/hotspot/share/gc/z/zTLABUsage.hpp | 2 +-
 3 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/src/hotspot/share/gc/z/zHeap.cpp b/src/hotspot/share/gc/z/zHeap.cpp
index 56cff7bae2e17..f8cd36289f1ab 100644
--- a/src/hotspot/share/gc/z/zHeap.cpp
+++ b/src/hotspot/share/gc/z/zHeap.cpp
@@ -62,7 +62,7 @@ ZHeap::ZHeap()
     _serviceability(InitialHeapSize, min_capacity(), max_capacity()),
     _old(&_page_table, &_page_allocator),
     _young(&_page_table, _old.forwarding_table(), &_page_allocator),
-    _tlab_usage(InitialHeapSize),
+    _tlab_usage(),
     _initialized(false) {
 
   // Install global heap instance
diff --git a/src/hotspot/share/gc/z/zTLABUsage.cpp b/src/hotspot/share/gc/z/zTLABUsage.cpp
index bee4be4e38bd0..569f46a4352eb 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.cpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.cpp
@@ -23,12 +23,8 @@
 
 #include "gc/z/zTLABUsage.hpp"
 
-ZTLABUsage::ZTLABUsage(size_t initial_heap_size)
-  : _used_history() {
-    // To get reasonable values for used and capacity until the first GC
-    // we populate the used history with the initial heap size.
-    _used_history.add(initial_heap_size);
-  }
+ZTLABUsage::ZTLABUsage()
+  : _used_history() {}
 
 void ZTLABUsage::update(size_t used) {
   if (used == 0) {
diff --git a/src/hotspot/share/gc/z/zTLABUsage.hpp b/src/hotspot/share/gc/z/zTLABUsage.hpp
index 92ee319afd0f5..5b4480345fe66 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.hpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.hpp
@@ -43,7 +43,7 @@ class ZTLABUsage {
   TruncatedSeq _used_history;
 
 public:
-  ZTLABUsage(size_t initial_heap_size);
+  ZTLABUsage();
 
   void update(size_t used);
 

From 10d455f21b3ffcacff90bdf984d327bca8df672b Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Wed, 7 May 2025 09:11:07 +0200
Subject: [PATCH 11/14] Keep all TLAB tracking in TLABUsage

---
 src/hotspot/share/gc/z/zGeneration.cpp      |  4 +--
 src/hotspot/share/gc/z/zHeap.cpp            | 15 +++------
 src/hotspot/share/gc/z/zHeap.hpp            |  4 ++-
 src/hotspot/share/gc/z/zObjectAllocator.cpp |  6 ++++
 src/hotspot/share/gc/z/zPageAllocator.cpp   | 37 ++++-----------------
 src/hotspot/share/gc/z/zPageAllocator.hpp   |  5 ---
 src/hotspot/share/gc/z/zTLABUsage.cpp       | 34 +++++++++++++++----
 src/hotspot/share/gc/z/zTLABUsage.hpp       |  6 ++--
 8 files changed, 54 insertions(+), 57 deletions(-)

diff --git a/src/hotspot/share/gc/z/zGeneration.cpp b/src/hotspot/share/gc/z/zGeneration.cpp
index 0ae8ec07e7dd3..78860bbe4cb36 100644
--- a/src/hotspot/share/gc/z/zGeneration.cpp
+++ b/src/hotspot/share/gc/z/zGeneration.cpp
@@ -837,8 +837,8 @@ void ZGenerationYoung::mark_start() {
   // Change good colors
   flip_mark_start();
 
-  // Update TLAB usage
-  ZHeap::heap()->update_tlab_usage(_page_allocator->used_eden());
+  // Reset TLAB usage
+  ZHeap::heap()->reset_tlab_used();
 
   // Retire allocating pages
   ZAllocator::eden()->retire_pages();
diff --git a/src/hotspot/share/gc/z/zHeap.cpp b/src/hotspot/share/gc/z/zHeap.cpp
index f8cd36289f1ab..90f151a6039f7 100644
--- a/src/hotspot/share/gc/z/zHeap.cpp
+++ b/src/hotspot/share/gc/z/zHeap.cpp
@@ -158,17 +158,12 @@ size_t ZHeap::unsafe_max_tlab_alloc() const {
   return MIN2(size, max_tlab_size());
 }
 
-void ZHeap::update_tlab_usage(size_t used) {
-  const size_t old_used = _tlab_usage.used();
-  const size_t old_capacity = _tlab_usage.capacity();
-
-  _tlab_usage.update(used);
+void ZHeap::increment_tlab_used() {
+  _tlab_usage.add(ZPageSizeSmall);
+}
 
-  log_debug(gc, tlab)("TLAB usage update: used %zuM -> %zuM, capacity: %zuM -> %zuM",
-                      old_used / M,
-                      _tlab_usage.used() / M,
-                      old_capacity / M,
-                      _tlab_usage.capacity() / M);
+void ZHeap::reset_tlab_used() {
+  _tlab_usage.reset();
 }
 
 bool ZHeap::is_in(uintptr_t addr) const {
diff --git a/src/hotspot/share/gc/z/zHeap.hpp b/src/hotspot/share/gc/z/zHeap.hpp
index 4f2f15c52425c..cee8b0d301190 100644
--- a/src/hotspot/share/gc/z/zHeap.hpp
+++ b/src/hotspot/share/gc/z/zHeap.hpp
@@ -84,7 +84,9 @@ class ZHeap {
   size_t tlab_used() const;
   size_t max_tlab_size() const;
   size_t unsafe_max_tlab_alloc() const;
-  void update_tlab_usage(size_t used);
+
+  void increment_tlab_used();
+  void reset_tlab_used();
 
   bool is_in(uintptr_t addr) const;
   bool is_in_page_relaxed(const ZPage* page, zaddress addr) const;
diff --git a/src/hotspot/share/gc/z/zObjectAllocator.cpp b/src/hotspot/share/gc/z/zObjectAllocator.cpp
index b92828e24a683..fc04057ba9223 100644
--- a/src/hotspot/share/gc/z/zObjectAllocator.cpp
+++ b/src/hotspot/share/gc/z/zObjectAllocator.cpp
@@ -109,6 +109,12 @@ zaddress ZObjectAllocator::alloc_object_in_shared_page(ZPage** shared_page,
 
         // Undo new page allocation
         undo_alloc_page(new_page);
+      } else {
+        // Page successfully installed
+        if (_age == ZPageAge::eden && page_type == ZPageType::small) {
+          // Increment TLAB usage for small eden pages
+          ZHeap::heap()->increment_tlab_used();
+        }
       }
     }
   }
diff --git a/src/hotspot/share/gc/z/zPageAllocator.cpp b/src/hotspot/share/gc/z/zPageAllocator.cpp
index 58e707eafc465..7bb1dcdcf81d4 100644
--- a/src/hotspot/share/gc/z/zPageAllocator.cpp
+++ b/src/hotspot/share/gc/z/zPageAllocator.cpp
@@ -1264,7 +1264,6 @@ ZPageAllocator::ZPageAllocator(size_t min_capacity,
     _min_capacity(min_capacity),
     _max_capacity(max_capacity),
     _used(0),
-    _used_eden(0),
     _used_generations{0,0},
     _collection_stats{{0, 0},{0, 0}},
     _partitions(ZValueIdTagType{}, this),
@@ -1355,10 +1354,6 @@ size_t ZPageAllocator::used() const {
   return Atomic::load(&_used);
 }
 
-size_t ZPageAllocator::used_eden() const {
-  return Atomic::load(&_used_eden);
-}
-
 size_t ZPageAllocator::used_generation(ZGenerationId id) const {
   return Atomic::load(&_used_generations[(int)id]);
 }
@@ -1419,13 +1414,7 @@ ZPageAllocatorStats ZPageAllocator::stats(ZGeneration* generation) const {
 ZPageAllocatorStats ZPageAllocator::update_and_stats(ZGeneration* generation) {
   ZLocker<ZLock> locker(&_lock);
 
-  // Update per-collection statistics
   update_collection_stats(generation->id());
-
-  // Clear eden statistics
-  reset_used_eden();
-
-  // Return the collected statistics
   return stats_inner(generation);
 }
 
@@ -2088,22 +2077,18 @@ void ZPageAllocator::free_memory_alloc_failed(ZMemoryAllocation* allocation) {
 }
 
 ZPage* ZPageAllocator::create_page(ZPageAllocation* allocation, const ZVirtualMemory& vmem) {
-  const size_t size = allocation->size();
-  const ZPageType type = allocation->type();
-  const ZPageAge age = allocation->age();
-  const ZGenerationId id = age == ZPageAge::old ? ZGenerationId::old : ZGenerationId::young;
-
   // We don't track generation usage when claiming capacity, because this page
   // could have been allocated by a thread that satisfies a stalling allocation.
   // The stalled thread can wake up and potentially realize that the page alloc
   // should be undone. If the alloc and the undo gets separated by a safepoint,
-  // the generation statistics could see a decreasing used value between mark
+  // the generation statistics could se a decreasing used value between mark
   // start and mark end. At this point an allocation will be successful, so we
   // update the generation usage.
-  increase_used_generation(id, size);
-  if (age == ZPageAge::eden) {
-    increase_used_eden(size);
-  }
+  const ZGenerationId id = allocation->age() == ZPageAge::old ? ZGenerationId::old : ZGenerationId::young;
+  increase_used_generation(id, allocation->size());
+
+  const ZPageType type = allocation->type();
+  const ZPageAge age = allocation->age();
 
   if (allocation->is_multi_partition()) {
     const ZMultiPartitionAllocation* const multi_partition_allocation = allocation->multi_partition_allocation();
@@ -2277,16 +2262,6 @@ void ZPageAllocator::decrease_used(size_t size) {
   }
 }
 
-void ZPageAllocator::increase_used_eden(size_t size) {
-  // Update atomically since we have concurrent readers and writers
-  Atomic::add(&_used_eden, size, memory_order_relaxed);
-}
-
-void ZPageAllocator::reset_used_eden() {
-  // Reset atomically since we have concurrent readers and writers
-  Atomic::store(&_used_eden, (size_t) 0);
-}
-
 void ZPageAllocator::safe_destroy_page(ZPage* page) {
   // Destroy page safely
   _safe_destroy.schedule_delete(page);
diff --git a/src/hotspot/share/gc/z/zPageAllocator.hpp b/src/hotspot/share/gc/z/zPageAllocator.hpp
index fa03da01d76df..5cd08aee94cda 100644
--- a/src/hotspot/share/gc/z/zPageAllocator.hpp
+++ b/src/hotspot/share/gc/z/zPageAllocator.hpp
@@ -159,7 +159,6 @@ class ZPageAllocator {
   const size_t                _min_capacity;
   const size_t                _max_capacity;
   volatile size_t             _used;
-  volatile size_t             _used_eden;
   volatile size_t             _used_generations[2];
   struct {
     size_t _used_high;
@@ -233,9 +232,6 @@ class ZPageAllocator {
   void increase_used(size_t size);
   void decrease_used(size_t size);
 
-  void increase_used_eden(size_t size);
-  void reset_used_eden();
-
   void notify_out_of_memory();
   void restart_gc() const;
 
@@ -260,7 +256,6 @@ class ZPageAllocator {
   size_t current_max_capacity() const;
   size_t capacity() const;
   size_t used() const;
-  size_t used_eden() const;
   size_t used_generation(ZGenerationId id) const;
   size_t unused() const;
 
diff --git a/src/hotspot/share/gc/z/zTLABUsage.cpp b/src/hotspot/share/gc/z/zTLABUsage.cpp
index 569f46a4352eb..9c042360c5318 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.cpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.cpp
@@ -22,17 +22,39 @@
  */
 
 #include "gc/z/zTLABUsage.hpp"
+#include "logging/log.hpp"
+#include "runtime/atomic.hpp"
 
 ZTLABUsage::ZTLABUsage()
-  : _used_history() {}
+  : _used(0),
+    _used_history() {}
 
-void ZTLABUsage::update(size_t used) {
-  if (used == 0) {
-    // Avoid updates for the second young generation collection of a SystemGC
+
+void ZTLABUsage::add(size_t size) {
+  Atomic::add(&_used, size, memory_order_relaxed);
+}
+
+void ZTLABUsage::reset() {
+  size_t current_used = Atomic::xchg(&_used, (size_t) 0);
+
+  // Avoid updates for the second young generation collection of a SystemGC
+  if (current_used == 0) {
     return;
   }
-  _used_history.add(used);
-}
+
+  // Save the old values for logging
+  size_t old_used = used();
+  size_t old_capacity = capacity();
+
+  // Update the usage history with the current value
+  _used_history.add(current_used);
+
+  log_debug(gc, tlab)("TLAB usage update: used %zuM -> %zuM, capacity: %zuM -> %zuM",
+                      old_used / M,
+                      used() / M,
+                      old_capacity / M,
+                      capacity() / M);
+  }
 
 size_t ZTLABUsage::used() const {
   return _used_history.last();
diff --git a/src/hotspot/share/gc/z/zTLABUsage.hpp b/src/hotspot/share/gc/z/zTLABUsage.hpp
index 5b4480345fe66..3b6635809b3f3 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.hpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.hpp
@@ -40,12 +40,14 @@
 
 class ZTLABUsage {
 private:
-  TruncatedSeq _used_history;
+  volatile size_t _used;
+  TruncatedSeq    _used_history;
 
 public:
   ZTLABUsage();
 
-  void update(size_t used);
+  void add(size_t size);
+  void reset();
 
   size_t used() const;
   size_t capacity() const;

From f361fc5d4588090652c8daa5ef3dc54657b2a873 Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Tue, 6 May 2025 21:45:55 +0200
Subject: [PATCH 12/14] Problemlist heap sampling test

---
 test/hotspot/jtreg/ProblemList-zgc.txt | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/test/hotspot/jtreg/ProblemList-zgc.txt b/test/hotspot/jtreg/ProblemList-zgc.txt
index 9571c717641a1..2706e1a3e63be 100644
--- a/test/hotspot/jtreg/ProblemList-zgc.txt
+++ b/test/hotspot/jtreg/ProblemList-zgc.txt
@@ -116,6 +116,9 @@ serviceability/sa/sadebugd/PmapOnDebugdTest.java              8307393   generic-
 serviceability/sa/sadebugd/RunCommandOnServerTest.java        8307393   generic-all
 serviceability/sa/sadebugd/SADebugDTest.java                  8307393   generic-all
 
+serviceability/jvmti/HeapMonitor/MyPackage/HeapMonitorInterpreterObjectTest.java                8356372 generic-all
+serviceability/jvmti/HeapMonitor/MyPackage/HeapMonitorStatObjectCorrectnessTest.java	          8356372 generic-all
+
 vmTestbase/gc/gctests/MemoryEaterMT/MemoryEaterMT.java        8289582   windows-x64
 
 compiler/jvmci/jdk.vm.ci.code.test/src/jdk/vm/ci/code/test/DataPatchTest.java                   8343233 generic-all

From ba7cb6735ded9c6dd6f1ba0b3be0ce357b808f5a Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Thu, 8 May 2025 09:07:42 +0200
Subject: [PATCH 13/14] Handle inc and dec in alloc/undo

---
 src/hotspot/share/gc/z/zArguments.cpp       |  4 +--
 src/hotspot/share/gc/z/zHeap.cpp            | 37 ++++++++++++++++-----
 src/hotspot/share/gc/z/zHeap.hpp            |  6 ++--
 src/hotspot/share/gc/z/zObjectAllocator.cpp |  6 ----
 src/hotspot/share/gc/z/zTLABUsage.cpp       | 13 +++++---
 src/hotspot/share/gc/z/zTLABUsage.hpp       |  3 +-
 6 files changed, 46 insertions(+), 23 deletions(-)

diff --git a/src/hotspot/share/gc/z/zArguments.cpp b/src/hotspot/share/gc/z/zArguments.cpp
index b54fb503d2e11..0de80765dd5d1 100644
--- a/src/hotspot/share/gc/z/zArguments.cpp
+++ b/src/hotspot/share/gc/z/zArguments.cpp
@@ -133,9 +133,9 @@ void ZArguments::initialize() {
     FLAG_SET_ERGO_IF_DEFAULT(ZCollectionIntervalMajor, ZCollectionInterval);
   }
 
-  // Set a small initial TLAB size to avoid depending on the current capacity
+  // Set an initial TLAB size to avoid depending on the current capacity
   if (FLAG_IS_DEFAULT(TLABSize)) {
-    FLAG_SET_DEFAULT(TLABSize, 8*K);
+    FLAG_SET_DEFAULT(TLABSize, 256*K);
   }
 
   // Set medium page size here because MaxTenuringThreshold may use it.
diff --git a/src/hotspot/share/gc/z/zHeap.cpp b/src/hotspot/share/gc/z/zHeap.cpp
index 90f151a6039f7..e99e32db8a51b 100644
--- a/src/hotspot/share/gc/z/zHeap.cpp
+++ b/src/hotspot/share/gc/z/zHeap.cpp
@@ -157,11 +157,6 @@ size_t ZHeap::unsafe_max_tlab_alloc() const {
 
   return MIN2(size, max_tlab_size());
 }
-
-void ZHeap::increment_tlab_used() {
-  _tlab_usage.add(ZPageSizeSmall);
-}
-
 void ZHeap::reset_tlab_used() {
   _tlab_usage.reset();
 }
@@ -228,11 +223,38 @@ void ZHeap::out_of_memory() {
   log_info(gc)("Out Of Memory (%s)", Thread::current()->name());
 }
 
+static bool is_small_eden_page(ZPage* page) {
+  return page->type() == ZPageType::small && page->age() == ZPageAge::eden;
+}
+
+void ZHeap::account_alloc_page(ZPage* page) {
+  // Do TLAB accounting for small eden pages
+  if (is_small_eden_page(page)) {
+    _tlab_usage.increase_used(page->size());
+  }
+}
+
+void ZHeap::account_undo_alloc_page(ZPage* page) {
+  // Increase the undo counter
+  ZStatInc(ZCounterUndoPageAllocation);
+
+  // Undo TLAB accounting for small eden pages
+  if (is_small_eden_page(page)) {
+    _tlab_usage.decrease_used(page->size());
+  }
+
+  log_trace(gc)("Undo page allocation, thread: " PTR_FORMAT " (%s), page: " PTR_FORMAT ", size: %zu",
+                p2i(Thread::current()), ZUtils::thread_name(), p2i(page), page->size());
+}
+
 ZPage* ZHeap::alloc_page(ZPageType type, size_t size, ZAllocationFlags flags, ZPageAge age) {
   ZPage* const page = _page_allocator.alloc_page(type, size, flags, age);
   if (page != nullptr) {
     // Insert page table entry
     _page_table.insert(page);
+
+    // Do accounting for the allocated page
+    account_alloc_page(page);
   }
 
   return page;
@@ -241,9 +263,8 @@ ZPage* ZHeap::alloc_page(ZPageType type, size_t size, ZAllocationFlags flags, ZP
 void ZHeap::undo_alloc_page(ZPage* page) {
   assert(page->is_allocating(), "Invalid page state");
 
-  ZStatInc(ZCounterUndoPageAllocation);
-  log_trace(gc)("Undo page allocation, thread: " PTR_FORMAT " (%s), page: " PTR_FORMAT ", size: %zu",
-                p2i(Thread::current()), ZUtils::thread_name(), p2i(page), page->size());
+  // Undo accounting for the page being freed
+  account_undo_alloc_page(page);
 
   free_page(page);
 }
diff --git a/src/hotspot/share/gc/z/zHeap.hpp b/src/hotspot/share/gc/z/zHeap.hpp
index cee8b0d301190..60a40d5722ed9 100644
--- a/src/hotspot/share/gc/z/zHeap.hpp
+++ b/src/hotspot/share/gc/z/zHeap.hpp
@@ -60,6 +60,10 @@ class ZHeap {
 
   bool                    _initialized;
 
+  // Page allocation accounting
+  void account_alloc_page(ZPage* page);
+  void account_undo_alloc_page(ZPage* page);
+
 public:
   static ZHeap* heap();
 
@@ -84,8 +88,6 @@ class ZHeap {
   size_t tlab_used() const;
   size_t max_tlab_size() const;
   size_t unsafe_max_tlab_alloc() const;
-
-  void increment_tlab_used();
   void reset_tlab_used();
 
   bool is_in(uintptr_t addr) const;
diff --git a/src/hotspot/share/gc/z/zObjectAllocator.cpp b/src/hotspot/share/gc/z/zObjectAllocator.cpp
index fc04057ba9223..b92828e24a683 100644
--- a/src/hotspot/share/gc/z/zObjectAllocator.cpp
+++ b/src/hotspot/share/gc/z/zObjectAllocator.cpp
@@ -109,12 +109,6 @@ zaddress ZObjectAllocator::alloc_object_in_shared_page(ZPage** shared_page,
 
         // Undo new page allocation
         undo_alloc_page(new_page);
-      } else {
-        // Page successfully installed
-        if (_age == ZPageAge::eden && page_type == ZPageType::small) {
-          // Increment TLAB usage for small eden pages
-          ZHeap::heap()->increment_tlab_used();
-        }
       }
     }
   }
diff --git a/src/hotspot/share/gc/z/zTLABUsage.cpp b/src/hotspot/share/gc/z/zTLABUsage.cpp
index 9c042360c5318..30f2851fb1380 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.cpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.cpp
@@ -30,12 +30,17 @@ ZTLABUsage::ZTLABUsage()
     _used_history() {}
 
 
-void ZTLABUsage::add(size_t size) {
+void ZTLABUsage::increase_used(size_t size) {
   Atomic::add(&_used, size, memory_order_relaxed);
 }
 
+void ZTLABUsage::decrease_used(size_t size) {
+  precond(size <= _used);
+  Atomic::sub(&_used, size, memory_order_relaxed);
+}
+
 void ZTLABUsage::reset() {
-  size_t current_used = Atomic::xchg(&_used, (size_t) 0);
+  const size_t current_used = Atomic::xchg(&_used, (size_t) 0);
 
   // Avoid updates for the second young generation collection of a SystemGC
   if (current_used == 0) {
@@ -43,8 +48,8 @@ void ZTLABUsage::reset() {
   }
 
   // Save the old values for logging
-  size_t old_used = used();
-  size_t old_capacity = capacity();
+  const size_t old_used = used();
+  const size_t old_capacity = capacity();
 
   // Update the usage history with the current value
   _used_history.add(current_used);
diff --git a/src/hotspot/share/gc/z/zTLABUsage.hpp b/src/hotspot/share/gc/z/zTLABUsage.hpp
index 3b6635809b3f3..bdfb8c4ef7d20 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.hpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.hpp
@@ -46,7 +46,8 @@ class ZTLABUsage {
 public:
   ZTLABUsage();
 
-  void add(size_t size);
+  void increase_used(size_t size);
+  void decrease_used(size_t size);
   void reset();
 
   size_t used() const;

From 2f5742fce16673a8da90ebab8681c0c37d943bca Mon Sep 17 00:00:00 2001
From: Stefan Johansson <stefan.johansson@oracle.com>
Date: Fri, 9 May 2025 09:36:02 +0200
Subject: [PATCH 14/14] StefanK review

---
 src/hotspot/share/gc/z/zHeap.cpp      |  4 ++--
 src/hotspot/share/gc/z/zTLABUsage.cpp | 26 +++++++++++++-------------
 src/hotspot/share/gc/z/zTLABUsage.hpp |  9 ++++++---
 3 files changed, 21 insertions(+), 18 deletions(-)

diff --git a/src/hotspot/share/gc/z/zHeap.cpp b/src/hotspot/share/gc/z/zHeap.cpp
index e99e32db8a51b..96679d0723b6c 100644
--- a/src/hotspot/share/gc/z/zHeap.cpp
+++ b/src/hotspot/share/gc/z/zHeap.cpp
@@ -132,11 +132,11 @@ size_t ZHeap::unused() const {
 }
 
 size_t ZHeap::tlab_capacity() const {
-  return _tlab_usage.capacity();
+  return _tlab_usage.tlab_capacity();
 }
 
 size_t ZHeap::tlab_used() const {
-  return _tlab_usage.used();
+  return _tlab_usage.tlab_used();
 }
 
 size_t ZHeap::max_tlab_size() const {
diff --git a/src/hotspot/share/gc/z/zTLABUsage.cpp b/src/hotspot/share/gc/z/zTLABUsage.cpp
index 30f2851fb1380..cd8141e47ffbb 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.cpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.cpp
@@ -29,42 +29,42 @@ ZTLABUsage::ZTLABUsage()
   : _used(0),
     _used_history() {}
 
-
 void ZTLABUsage::increase_used(size_t size) {
   Atomic::add(&_used, size, memory_order_relaxed);
 }
 
 void ZTLABUsage::decrease_used(size_t size) {
   precond(size <= _used);
+
   Atomic::sub(&_used, size, memory_order_relaxed);
 }
 
 void ZTLABUsage::reset() {
-  const size_t current_used = Atomic::xchg(&_used, (size_t) 0);
+  const size_t used = Atomic::xchg(&_used, (size_t) 0);
 
-  // Avoid updates for the second young generation collection of a SystemGC
-  if (current_used == 0) {
+  // Avoid updates when nothing has been allocated since the last YC
+  if (used == 0) {
     return;
   }
 
   // Save the old values for logging
-  const size_t old_used = used();
-  const size_t old_capacity = capacity();
+  const size_t old_tlab_used = tlab_used();
+  const size_t old_tlab_capacity = tlab_capacity();
 
   // Update the usage history with the current value
-  _used_history.add(current_used);
+  _used_history.add(used);
 
   log_debug(gc, tlab)("TLAB usage update: used %zuM -> %zuM, capacity: %zuM -> %zuM",
-                      old_used / M,
-                      used() / M,
-                      old_capacity / M,
-                      capacity() / M);
+                      old_tlab_used / M,
+                      tlab_used() / M,
+                      old_tlab_capacity / M,
+                      tlab_capacity() / M);
   }
 
-size_t ZTLABUsage::used() const {
+size_t ZTLABUsage::tlab_used() const {
   return _used_history.last();
 }
 
-size_t ZTLABUsage::capacity() const {
+size_t ZTLABUsage::tlab_capacity() const {
   return _used_history.davg();
 }
diff --git a/src/hotspot/share/gc/z/zTLABUsage.hpp b/src/hotspot/share/gc/z/zTLABUsage.hpp
index bdfb8c4ef7d20..3d1b084fe161c 100644
--- a/src/hotspot/share/gc/z/zTLABUsage.hpp
+++ b/src/hotspot/share/gc/z/zTLABUsage.hpp
@@ -36,11 +36,14 @@
 // ZGC does not have set generation sizes unlike most other GCs and because of
 // this there is no fixed TLAB capacity. For the common TLAB sizing heuristic
 // to work properly ZGC estimates the current capacity by using a weighted
-// average of the last 10 used values.
+// average of the last 10 used values. ZGC uses the last snapshotted value as
+// the value returned as tlab_used().
 
 class ZTLABUsage {
 private:
+  // Accounting TLAB used until the next GC cycle
   volatile size_t _used;
+  // Sequence of historic used values
   TruncatedSeq    _used_history;
 
 public:
@@ -50,8 +53,8 @@ class ZTLABUsage {
   void decrease_used(size_t size);
   void reset();
 
-  size_t used() const;
-  size_t capacity() const;
+  size_t tlab_used() const;
+  size_t tlab_capacity() const;
 };
 
 #endif // SHARE_GC_Z_ZTLABUSAGE_HPP