Before revision:  The sparsity of the graphs is exploited hierarchically by storing only non-empty tiles of the graphs and non-zero elements within each tile using a coordinate format and bitmaps, respectively. A new partition-based reordering algorithm is proposed for aggregating non-zero elements of the graphs into fewer but denser tiles in order to improve the efficiency of the sparse format.
After revision:  The sparsity of the graphs is exploited hierarchically by storing only non-empty tiles of the graphs and non-zero elements within each tile using bitmaps. We propose a new partition-based reordering algorithm is proposed for aggregating non-zero elements of the graphs into fewer but denser tiles in order to improve the efficiency of the sparse format.