As the number of graph applications increases rapidly in many domains, new graph algorithms (or queries) have become more important than ever before. The current two-step approach to develop and test a graph algorithm is very expensive for trillion-scale graphs required in many industrial applications. In this paper, we propose a concept of graph processing simulation, a single-step approach that generates a graph and processes a graph algorithm simultaneously. It consists of a top-down graph upscaling method called V-Upscaler and a graph processing simulation method following the vertex-centric GAS model called T-GPS. Users can develop a graph algorithm and check its correctness and performance conveniently and cost-efficiently even for trillion-scale graphs. Through extensive experiments, we have demonstrated that our single-step approach of V-Upscaler and T-GPS significantly outperforms the conventional two-step approach, although ours uses only a single machine, while the conventional one uses a cluster of eleven machines.