// Failed allocation policy. Must be called from the VM thread, and
// only at a safepoint! Note that this method has policy for allocation
// flow, and NOT collection policy. So we do not check for gc collection
// time over limit here, that is the responsibility of the heap specific
// collection methods. This method decides where to attempt allocations,
// and when to attempt collections, but no collection specific policy.

HeapWord* ParallelScavengeHeap::failed_mem_allocate(size_t size, bool is_tlab) {

  {- -------------------------------------------
  (1) (assert)
      ---------------------------------------- -}

      assert(SafepointSynchronize::is_at_safepoint(), "should be at safepoint");
      assert(Thread::current() == (Thread*)VMThread::vm_thread(), "should be in vm thread");
      assert(!Universe::heap()->is_gc_active(), "not reentrant");
      assert(!Heap_lock->owned_by_self(), "this thread should not own the Heap_lock");

  {- -------------------------------------------
  (1) (PSScavenge 実行前の mark sweep 実行回数を記録しておく)
      ---------------------------------------- -}

      size_t mark_sweep_invocation_count = total_invocations();

  {- -------------------------------------------
  (1) まず PSScavenge::invoke() で PSScavenge による GC を行う.
      ---------------------------------------- -}

      // We assume (and assert!) that an allocation at this point will fail
      // unless we collect.

      // First level allocation failure, scavenge and allocate in young gen.
      GCCauseSetter gccs(this, GCCause::_allocation_failure);
      PSScavenge::invoke();

  {- -------------------------------------------
  (1) GC が終わったら, PSYoungGen::allocate() での確保を試みる.
      ---------------------------------------- -}

      HeapWord* result = young_gen()->allocate(size, is_tlab);

  {- -------------------------------------------
  (1) 確保が成功しなければ, invoke_full_gc() で Full GC を行った後, もう一度 PSYoungGen::allocate() での確保を試みる.
      (なお, PSScavenge 内で mark sweep が実行されることがあるため, total_invocations() の値を確認している.
       PSScavenge 実行前と値が違えば, 既に mark sweep が実行されているので (= もう一度 mark sweep しても結果は変わらないので) 
       ここでは何もしない.)
      ---------------------------------------- -}

      // Second level allocation failure.
      //   Mark sweep and allocate in young generation.
      if (result == NULL) {
        // There is some chance the scavenge method decided to invoke mark_sweep.
        // Don't mark sweep twice if so.
        if (mark_sweep_invocation_count == total_invocations()) {
          invoke_full_gc(false);
          result = young_gen()->allocate(size, is_tlab);
        }
      }

  {- -------------------------------------------
  (1) まだ確保が成功していなければ, PSOldGen::allocate() で Old Generation から確保を試みる.
      ---------------------------------------- -}

      // Third level allocation failure.
      //   After mark sweep and young generation allocation failure,
      //   allocate in old generation.
      if (result == NULL && !is_tlab) {
        result = old_gen()->allocate(size, is_tlab);
      }

  {- -------------------------------------------
  (1) まだ確保が成功していなければ, invoke_full_gc() で(今度は完璧に) Full GC を行い, 
      その後 PSYoungGen::allocate() での確保を試みる.
      ---------------------------------------- -}

      // Fourth level allocation failure. We're running out of memory.
      //   More complete mark sweep and allocate in young generation.
      if (result == NULL) {
        invoke_full_gc(true);
        result = young_gen()->allocate(size, is_tlab);
      }

  {- -------------------------------------------
  (1) それでもまだ確保が成功していなければ, PSOldGen::allocate() でもう一度 Old Generation から確保を試みる.
       (今度は完璧に Full GC した後なので, さっきとは結果が変わる可能性がある)
      ---------------------------------------- -}

      // Fifth level allocation failure.
      //   After more complete mark sweep, allocate in old generation.
      if (result == NULL && !is_tlab) {
        result = old_gen()->allocate(size, is_tlab);
      }

  {- -------------------------------------------
  (1) 結果をリターンする.
      ---------------------------------------- -}

      return result;
    }