void ConcurrentMark::checkpointRootsInitialPost() {

  {- -------------------------------------------
  (1) (変数宣言など)
      ---------------------------------------- -}

      G1CollectedHeap*   g1h = G1CollectedHeap::heap();

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

      // If we force an overflow during remark, the remark operation will
      // actually abort and we'll restart concurrent marking. If we always
      // force an oveflow during remark we'll never actually complete the
      // marking phase. So, we initilize this here, at the start of the
      // cycle, so that at the remaining overflow number will decrease at
      // every remark and we'll eventually not need to cause one.
      force_overflow_stw()->init();

  {- -------------------------------------------
  (1) これから concurrent marking 処理を行うので, 
      NoteStartOfMarkHRClosure を使って
      各 HeapRegion の next TAMS に現在の top 位置を記録しておく.
      ---------------------------------------- -}

      // For each region note start of marking.
      NoteStartOfMarkHRClosure startcl;
      g1h->heap_region_iterate(&startcl);

  {- -------------------------------------------
  (1) これから GC を行うので, ReferenceProcessor オブジェクトをリセットしておく.
      ---------------------------------------- -}

      // Start weak-reference discovery.
      ReferenceProcessor* rp = g1h->ref_processor();
      rp->verify_no_references_recorded();
      rp->enable_discovery(); // enable ("weak") refs discovery
      rp->setup_policy(false); // snapshot the soft ref policy to be used in this cycle

  {- -------------------------------------------
  (1) これから concurrent marking 処理を行うので, 
      SATBMarkQueueSet::set_active_all_threads() を呼んで
      write barrier を concurrent marking 時のモード(ポインタを記録するモード)にしておく.
      (See: [here](no2114EV0.html) for details)
      ---------------------------------------- -}

      SATBMarkQueueSet& satb_mq_set = JavaThread::satb_mark_queue_set();
      // This is the start of  the marking cycle, we're expected all
      // threads to have SATB queues with active set to false.
      satb_mq_set.set_active_all_threads(true, /* new active value */
                                         false /* expected_active */);

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

      // update_g1_committed() will be called at the end of an evac pause
      // when marking is on. So, it's also called at the end of the
      // initial-mark pause to update the heap end, if the heap expands
      // during it. No need to call it here.
    }