diff -r 9a7135d0a309 -r 7386b3a385ac src/share/vm/memory/generation.cpp
--- a/src/share/vm/memory/generation.cpp	Tue Dec 17 05:07:06 2019 +0000
+++ b/src/share/vm/memory/generation.cpp	Tue Dec 17 05:26:57 2019 +0000
@@ -68,6 +68,12 @@
   return gch->_gen_specs[level()];
 }
 
+// This is for CMS. It returns stable monotonic used space size.
+// Remove this when CMS is removed.
+size_t Generation::used_stable() const {
+  return used();
+}
+
 size_t Generation::max_capacity() const {
   return reserved().byte_size();
 }