Evaluating and mitigating bandwidth bottlenecks across the memory hierarchy in GPUs

Abstract

GPUs are often limited by off-chip memory bandwidth. With the advent of general-purpose computing on GPUs, a cache hierarchy has been introduced to filter the bandwidth demand to the off-chip memory. However, the cache hierarchy presents its own bandwidth limitations in sustaining such high levels of memory traffic. In this paper, we characterize the bandwidth bottlenecks present across the memory hierarchy in GPUs for generalpurpose applications. We quantify the stalls throughout the memory hierarchy and identify the architectural parameters that play a pivotal role in leading to a congested memory system. We explore the architectural design space to mitigate the bandwidth bottlenecks and show that performance improvement achieved by mitigating the bandwidth bottleneck in the cache hierarchy can exceed the speedup obtained by a memory system with a baseline cache hierarchy and High Bandwidth Memory (HBM) DRAM. We also show that addressing the bandwidth bottleneck in isolation at specific levels can be sub-optimal and can even be counter-productive. Therefore, we show that it is imperative to resolve the bandwidth bottlenecks synergistically across different levels of the memory hierarchy. With the insights developed in this paper, we perform a cost-benefit analysis and identify costeffective configurations of the memory hierarchy that effectively mitigate the bandwidth bottlenecks. We show that our final configuration achieves a performance improvement of 29% on average with a minimal area overhead of 1.6%.