Microarchitectural Performance Characterization of Irregular GPU Kernels

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 branch 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.