MeNDA: a near-memory multi-way merge solution for sparse transposition and dataflows

Near-memory processing has been extensively studied to optimize memory intensive workloads. However, none of the proposed designs address sparse matrix transposition, an important building block in sparse linear algebra applications. Prior work shows that sparse matrix transposition does not scale as well as other sparse primitives such as sparse matrix vector multiplication (SpMV) and hence has become a growing bottleneck in common applications. Sparse matrix transposition is highly memory intensive but low in computational intensity, making it a promising candidate for near-memory processing. In this work, we propose MeNDA, a scalable near-DRAM multi-way merge accelerator that eliminates the off-chip memory interface bottleneck and exposes the high internal memory bandwidth to improve performance and reduce energy consumption for sparse matrix transposition. MeNDA adopts a merge sort based algorithm, exploiting spatial locality, and proposes a near-memory processing unit (PU) featuring a high-performance hardware merge tree. Because of the wide application of merge sort in sparse linear algebra, MeNDA is an extensible solution that can be easily adapted to support other sparse primitives such as SpMV. Techniques including seamless back-to-back merge sort, stall reducing prefetching and request coalescing are further explored to take full advantage of the increased system memory bandwidth. Compared to two state-of-the-art implementations of sparse matrix transposition on a CPU and a sparse library on a GPU, MeNDA is able to achieve a speedup of 19.1X, 12.0X, and 7.7x, respectively. MeNDA also shows an efficiency gain of 3.8x over a recent SpMV accelerator integrated with HBM. Incurring a power consumption of only 78.6 mW, a MeNDA PU can be easily accommodated by commodity DIMMs.