--- a/src/hotspot/share/gc/shared/collectedHeap.hpp	Thu Apr 25 08:55:49 2019 +0200
+++ b/src/hotspot/share/gc/shared/collectedHeap.hpp	Thu Apr 25 08:55:49 2019 +0200
@@ -210,6 +210,9 @@
   virtual size_t capacity() const = 0;
   virtual size_t used() const = 0;
 
+  // Returns unused capacity.
+  virtual size_t unused() const;
+
   // Return "true" if the part of the heap that allocates Java
   // objects has reached the maximal committed limit that it can
   // reach, without a garbage collection.