Leyuan Wang, a Ph.D. student in the UC Davis Department of Computer Science, presented one of only two “Distinguished Papers” of the 51 accepted at Euro-Par 2015. Euro-Par is a European conference devoted to all aspects of parallel and distributed processing held August 24-28 at Austria’s Vienna University of Technology.

Leyuan’s paper *Fast Parallel Suffix Array on the GPU*, co-authored by her advisor John Owens and Sean Baxter, a research scientist at New York’s DE Shaw Research, details their efforts to implement a linear-time suffix array construction algorithm on NVIDIA GPUs, resulting in algorithmic improvements and significant speedups over the existing state of the art.

Wang completed her master’s degree in electrical and computer engineering at UC Davis in October 2014, after having earned her undergraduate degree in electronics science and technology at China’s Zhejiang University.

**Brad: Can you talk a bit about your current research?**

**Leyuan Wang**: I work on high-performance string processing and graph processing algorithms, mostly in string and graph queries. My current research focus is on GPGPU (general-purpose computing on graphics processing units) and the benchmark I care about most is speed. I’ve been working on designing and improving parallel suffix array construction algorithms (SACAs) and incorporating the implementations in a Burrows-Wheeler transform-based lossless data compression (bzip2) and a parallel FM index for pattern searching. The suffix array (SA) of a string is the sorted set of all suffixes of the string. The inverse suffix array (ISA) is also the lexicographic ranks of suffixes.

The Burrows-Wheeler transform (BWT) of a string is generated by lexicographically sorting the cyclic shift of the string to form a string matrix and taking the last column of the matrix. The BWT groups repeated characters together by permuting the string; it is also reversible, which means the original string can be recovered. These two characteristics make BWT a popular choice for a compression pipeline stage (for instance, bzip2). It is directly related to the suffix array: the sorted rows in the matrix are essentially the sorted suffixes of the string and the first column of the matrix reflects a suffix array. Table 1 shows an example of the SA, ISA and BWT of the input string “banana$”

The suffix array data structure is a building block in a spectrum of applications, including data compression, bioinformatics, text indexing, etc. I’ve studied the taxonomy of all classes of SACAs and compared them in order to find the best candidate for the GPU. I revisited the previous conclusion that skew SACAs are best suited on the GPU by demonstrating that prefix-doubling SACAs are actually better both in theoretical analysis and experimental benchmarks. Our hybrid skew/prefix-doubling suffix array implementation (with our amazing research collaborator Sean Baxter, formerly of NVIDIA Research) using a Tesla K20 achieves a 7.9x speedup against the previous state-of-the-art skew implementation. Our optimized skew SACA implementation has been added as a primitive to CUDPP 2.2 (CUDA Data Parallel Primitives Library) and incorporated into the BWT and bzip2 data compression application, resulting in great speedups compared with bzip2 in CUDPP 2.1. Figure 1 shows pseudocode for our two approaches. Continue reading