M. A. O'Neil and M. Burtscher. Rethinking the Parallelization of Random-Restart Hill Climbing: A Case Study in Optimizing a 2-Opt TSP Solver for GPU Execution. Eighth Workshop on General Purpose Processing on Graphics Processing Units (GPGPU'15). San Francisco, CA. February 2015. [doi]

Random-restart hill climbing is a common approach to combinatorial optimization problems such as the traveling salesman problem (TSP). We present and evaluate an implementation of random-restart hill climbing with 2-opt local search applied to TSP. Our implementation is capable of addressing large problem sizes at high throughput. It is based on the key insight that the GPU's hierarchical hardware parallelism is best exploited with a hierarchical implementation strategy, where independent climbs are parallelized between blocks and the 2-opt evaluations are parallelized across the threads within a block. We analyze the performance impact of this and other optimizations on our heuristic TSP solver and compare its performance to existing GPU-based 2-opt TSP solvers as well as a parallel CPU implementation. Our code outperforms the existing implementations by up to 3X, evaluating up to 60 billion 2-opt moves per second on a single K40 GPU. It also outperforms an OpenMP implementation run on 20 CPU cores by up to 8X.


M. A. O'Neil and M. Burtscher. Microarchitectural Performance Characterization of Irregular GPU Kernels. The 2014 IEEE International Symposium on Workload Characterization (IISWC'14). Raleigh, NC. October 2014. [doi]

GPUs are increasingly being used to accelerate general-purpose applications, including applications with data-dependent, irregular memory access patterns and control flow. However, relatively little is known about the behavior of irregular GPU codes, and there has been minimal effort to quantify the ways in which they differ from regular GPGPU applications. We examine the behavior of a suite of optimized irregular CUDA applications on a cycle-accurate GPU simulator. We characterize the performance bottlenecks in each program and connect source code with microarchitectural characteristics. We also assess the impact of improvements in cache and DRAM bandwidth and latency and discuss the implications for GPU architecture design. We find that, while irregular graph codes exhibit significantly more underutilized execution cycles due to thread divergence, load imbalance, and synchronization overhead than regular programs, these factors contribute less to performance degradation than we expected. It appears that code optimizations are often able to effectively address these performance hurdles. Insufficient bandwidth and long memory latency are the biggest limiters of performance. Surprisingly, we find that applications with irregular memory access patterns are more sensitive to changes in L2 latency and bandwidth than DRAM latency and bandwidth.


M. A. O'Neil, D. Tamir, and M. Burtscher. A Parallel GPU Version of the Traveling Salesman Problem. The 2011 International Conference on Parallel and Distributed Processing Techniques and Applications (PDPTA'11). Las Vegas, NV. July 2011.

This paper describes and evaluates an implementation of iterative hill climbing with random restart for determining high-quality solutions to the traveling salesman problem. With 100,000 restarts, this algorithm finds the optimal solution for four out of five 100-city TSPLIB inputs and yields a tour that is only 0.07% longer than the optimum on the fifth input. The presented implementation is highly parallel and optimized for GPU-based execution. Running on a single GPU, it evaluates over 20 billion tour modifications per second. It takes 32 CPUs with 8 cores each (256 cores total) to match this performance.


M. A. O'Neil and M. Burtscher. Floating-Point Data Compression at 75 Gb/s on a GPU. Fourth Workshop on General Purpose Processing on Graphics Processing Units (GPGPU'11). Newport Beach, CA. March 2011. [doi]

Numeric simulations often generate large amounts of data that need to be stored or sent to other compute nodes. This paper investigates whether GPUs are powerful enough to make real-time data compression and decompression possible in such environments, that is, whether they can operate at the 32- or 40-Gb/s throughput of emerging network cards. The fastest parallel CPU-based floating-point data compression algorithm operates below 20 Gb/s on eight Xeon cores, which is significantly slower than the network speed and thus insufficient for compression to be practical in high-end networks. As a remedy, we have created the highly parallel GFC compression algorithm for double-precision floating-point data. This algorithm is specifically designed for GPUs. It compresses at a minimum of 75 Gb/s, decompresses at 90 Gb/s and above, and can therefore improve internode communication throughput on current and upcoming networks by fully saturating the interconnection links with compressed data.