--- a/src/hotspot/share/gc/shared/collectedHeap.cpp	Thu Apr 25 08:55:49 2019 +0200
+++ b/src/hotspot/share/gc/shared/collectedHeap.cpp	Thu Apr 25 08:55:49 2019 +0200
@@ -81,6 +81,11 @@
   st.print_cr("}");
 }
 
+size_t CollectedHeap::unused() const {
+  MutexLocker ml(Heap_lock);
+  return capacity() - used();
+}
+
 VirtualSpaceSummary CollectedHeap::create_heap_space_summary() {
   size_t capacity_in_words = capacity() / HeapWordSize;