I. Introduction
Computations on graph-based data structures are the basis of many applications in machine learning and data mining, enabling many important capabilities in modern computing (e.g., stereo matching [11], image segmentation [11], handwriting recognition [9], etc). Vertex-centric abstraction [5] [7] [8] is widely used for capturing such graph-based applications, which have arbitrary graph structures, data types, and graph update functions.