n_gpu = 4
        if(n_gpu == 1):
            self._build_model()
            self._build_train_op()
        else:
            # 1 GPU for training, n-1 for playing
            if(self.id <= 3):
                gpu_id = 0
            else:
                gpu_id = (1 + (self.id%(n_gpu-1)))
            os.environ["CUDA_VISIBLE_DEVICES"]=str(gpu_id)
            self._build_model()
            self._build_train_op()