University of North Carolina at Chapel Hill Graduate School
Doi
Abstract
In modern autonomous systems such as self-driving cars, sustained safe operation requires running complex software at rates possible only with the help of specialized computational accelerators. Graphics processing units (GPUs) remain a foremost example of such accelerators, due to their relative ease of use and the proficiency with which they can accelerate neural-network computations underlying modern computer-vision and artificial-intelligence algorithms. This means that ensuring GPU processing completes in a timely manner is essential---but doing so is not necessarily simple, especially when a single GPU is concurrently shared by many applications. Existing real-time research includes several techniques for improving timing characteristics of shared-GPU workloads, each with varying tradeoffs and practical limitations. In the world of timing correctness, however, one problem stands above all others: the lack of detailed information about how GPU hardware and software behaves. GPU manufacturers are usually willing to publish documentation sufficient for producing logically correct software, or guidance on tuning software to achieve "real-fast," high-throughput performance, but the same manufacturers neglect to provide details used when establishing temporal predictability. Techniques for improving the reliability of GPU software's temporal performance are only as good as the information upon which they are based, incentivising researchers to spend inordinate amounts of time learning foundational facts about existing hardware---facts that chip manufacturers must know, but are not willing to publish. This is both a continual inconvenience in established GPU research, and a high barrier to entry for newcomers. This dissertation addresses the "information problem" hindering real-time GPU research in several ways. First, it seeks to fight back against the monoculture that has arisen with respect to platform choice. Virtually all prior real-time GPU research is developed for and evaluated using GPUs manufactured by NVIDIA, but this dissertation provides details about an alternate platform: AMD GPUs. Second, this dissertation works towards establishing a model with which GPU performance can be predicted or controlled. To this end, it uses a series of experiments to discern the policy that governs the queuing behavior of concurrent GPU-sharing processes, on both NVIDIA and AMD GPUs. Finally, this dissertation addresses the novel problems for safety-critical systems caused by the changing landscape of the applications that run on GPUs. In particular, the advent of neural-network-based artificial-intelligence has catapulted GPU usage into safety-critical domains that are not prepared for the complexity of the new software or the fact that it cannot guarantee logical correctness. The lack of logical guarantees is unlikely to be "solved" in the near future, motivating a focus on increased throughput. Higher throughput increases the probability of producing a correct result within a fixed amount of time, but GPU-management efforts typically focus on worst-case performance, often at the expense of throughput. This dissertation's final chapter therefore evaluates existing GPU-management techniques' efficacy at managing neural-network applications, both from a throughput and worst-case perspective.Doctor of Philosoph
