Batch-Aware Unified Memory Management in GPUs for Irregular Workloads
Session: SIMT--So many instructions, multiple tricks!
Authors: Hyojong Kim (Georgia Institute of Technology); Jaewoong Sim (Intel Labs); Prasun Gera (Georgia Institute of Technology); Ramyad Hadidi (Georgia Institute of Technology); Hyesoon Kim (Georgia Institute of Technology)
While unified virtual memory and demand paging in modern GPUs provide convenient abstractions to programmers for working with large-scale applications, they come at a significant performance cost. We provide the first comprehensive analysis of major inefficiencies that arise in page fault handling mechanisms employed in modern GPUs. To amortize the high costs in fault handling, the GPU runtime processes a large number of GPU page faults together. We observe that this batched processing of page faults introduces large-scale serialization that greatly hurts the GPU's execution throughput. We show real machine measurements that corroborate our findings. Our goal is to mitigate these inefficiencies and enable efficient demand paging for GPUs. To this end, we propose a GPU runtime software and hardware solution that (1) increases the batch size (i.e., the number of page faults handled together), thereby amortizing the \overheadName time, and reduces the number of batches by supporting CPU-like thread block context switching, and (2) takes page eviction off the critical path with no hardware changes by overlapping evictions with CPU-to-GPU page migrations. Our evaluation demonstrates that the proposed solution provides an average speedup of 2x over the state-of-the-art page prefetching. We show that our solution increases the batch size by 2.27x and reduces the total number of batches by 51% on average. We also show that the average batch processing time is reduced by 27%.