8238686: G1 may waste lots of space or fail to uncommit when observing MinHeapFreeRatio during sizing after full gc

Reviewed-by: tschatzl, sjohanss

Author: walulyai

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2026, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2017, Red Hat, Inc. and/or its affiliates.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
@@ -209,6 +209,17 @@ void G1Arguments::initialize() {
     FLAG_SET_DEFAULT(GCTimeRatio, 24);
   }
 
+  // Do not interfere with GC-Pressure driven heap resizing unless the user
+  // explicitly sets otherwise. G1 heap sizing should be free to grow or shrink
+  // the heap based on GC pressure, rather than being forced to satisfy
+  // MinHeapFreeRatio or MaxHeapFreeRatio defaults that the user did not set.
+  if (FLAG_IS_DEFAULT(MinHeapFreeRatio)) {
+    FLAG_SET_DEFAULT(MinHeapFreeRatio, 0);
+  }
+  if (FLAG_IS_DEFAULT(MaxHeapFreeRatio)) {
+    FLAG_SET_DEFAULT(MaxHeapFreeRatio, 100);
+  }
+
   // Below, we might need to calculate the pause time interval based on
   // the pause target. When we do so we are going to give G1 maximum
   // flexibility and allow it to do pauses when it needs to. So, we'll