   0: producer wants to put item-0
   0: producer put item-0
   1: producer wants to put item-1
   1: producer put item-1
   2: producer wants to put item-2
   3: producer put item-2
   3: consumer got item-0
   4: producer wants to put item-3
   5: producer put item-3
   5: consumer got item-1
   6: producer wants to put item-4
   7: producer put item-4
   7: consumer got item-2
   9: consumer got item-3
  11: consumer got item-4
