--- old/src/hotspot/share/memory/heapInspection.cpp	2020-04-25 20:53:37.571321886 +0800
+++ new/src/hotspot/share/memory/heapInspection.cpp	2020-04-25 20:53:37.391322436 +0800
@@ -237,6 +237,42 @@
   return _size_of_instances_in_words;
 }
 
+// Return false if the entry could not be recorded on account
+// of running out of space required to create a new entry.
+bool KlassInfoTable::merge_entry(const KlassInfoEntry* cie) {
+  Klass*        k = cie->klass();
+  KlassInfoEntry* elt = lookup(k);
+  // elt may be NULL if it's a new klass for which we
+  // could not allocate space for a new entry in the hashtable.
+  if (elt != NULL) {
+    elt->set_count(elt->count() + cie->count());
+    elt->set_words(elt->words() + cie->words());
+    _size_of_instances_in_words += cie->words();
+    return true;
+  } else {
+    return false;
+  }
+}
+
+class KlassInfoTableMergeClosure : public KlassInfoClosure {
+private:
+  KlassInfoTable* _dest;
+  bool _success;
+public:
+  KlassInfoTableMergeClosure(KlassInfoTable* table) : _dest(table), _success(true) {}
+  void do_cinfo(KlassInfoEntry* cie) {
+    _success &= _dest->merge_entry(cie);
+  }
+  bool is_success() { return _success; }
+};
+
+// merge from table
+bool KlassInfoTable::merge(KlassInfoTable* table) {
+  KlassInfoTableMergeClosure closure(this);
+  table->iterate(&closure);
+  return closure.is_success();
+}
+
 int KlassInfoHisto::sort_helper(KlassInfoEntry** e1, KlassInfoEntry** e2) {
   return (*e1)->compare(*e1,*e2);
 }
@@ -504,21 +540,72 @@
   }
 };
 
-size_t HeapInspection::populate_table(KlassInfoTable* cit, BoolObjectClosure *filter) {
+// Heap inspection for every worker.
+// When native OOM hanppens for KlassInfoTable, set _success to false.
+// TODO(?) it seems atomically set/get _success is unnecessary becasue it
+// is set to true at constructor and can only be set to false here.
+// the only risk seems a worker may continue inspect heap when another
+// worker set _success to false, but this is OK because the current worker
+// doesn't change _success if everything is OK for it's inpection work, and
+// the _success will be false finally and serial heap inpection can be tried.
+void ParHeapInspectTask::work(uint worker_id) {
+  size_t missed_count = 0;
+  if (!_success) {
+    // other worker has failed on parallel iteration.
+    return;
+  }
+
+  KlassInfoTable cit(false);
+  if (!cit.allocation_failed()) {
+    RecordInstanceClosure ric(&cit, _filter);
+    _poi->object_iterate(&ric, worker_id);
+    missed_count = ric.missed_count();
+  } else {
+    // fail to allocate memory, stop parallel mode.
+    _success = false;
+    return;
+  }
+  {
+    MutexLocker x(&_mutex);
+
+    if (!_shared_cit->merge(&cit)) {
+      _success = false;
+      return;
+    }
+    _shared_missed_count += missed_count;
+  }
+}
+
+size_t HeapInspection::populate_table(KlassInfoTable* cit, BoolObjectClosure *filter, size_t parallel_thread_num) {
   ResourceMark rm;
 
+  // Try parallel first.
+  if (parallel_thread_num > 1) {
+    ParallelObjectIterator* poi = Universe::heap()->parallel_object_iterator(parallel_thread_num);
+    if (poi != NULL) {
+      ParHeapInspectTask task(poi, cit, filter);
+      Universe::heap()->run_task(&task);
+      delete poi;
+      if (task.success()) {
+        return task.missed_count();
+      }
+    }
+  }
+
+  // If no parallel iteration available, run serially.
   RecordInstanceClosure ric(cit, filter);
   Universe::heap()->object_iterate(&ric);
   return ric.missed_count();
 }
 
-void HeapInspection::heap_inspection(outputStream* st) {
+void HeapInspection::heap_inspection(outputStream* st, size_t parallel_thread_num) {
   ResourceMark rm;
 
   KlassInfoTable cit(false);
   if (!cit.allocation_failed()) {
+    size_t missed_count = 0;;
     // populate table with object allocation info
-    size_t missed_count = populate_table(&cit);
+    missed_count = populate_table(&cit, NULL, parallel_thread_num);
     if (missed_count != 0) {
       log_info(gc, classhisto)("WARNING: Ran out of C-heap; undercounted " SIZE_FORMAT
                                " total instances in data below",