Analyzing Reverse Address Translation Overheads in Multi-GPU Scale-Up Pods

Abstract

Distributed ML workloads rely heavily on collective communication across multi-GPU, multi-node systems. Emerging scale-up fabrics, such as NVLink and UALink, enable direct memory access across nodes but introduce a critical destination-side translation step: translating Network Physical Addresses (NPAs) to System Physical Addresses (SPAs), which we term Reverse Translation (Reverse Address Translation). Despite its importance, the performance impact of Reverse Address Translation remains poorly understood. In this work, we present the first systematic study of Reverse Address Translation in large-scale GPU clusters. Using an extended ASTRA-sim framework with Omnet++ as the network backend, we model Link MMUs and Link TLBs and evaluate their effect on All-to-All collective communication across varying input sizes and GPU counts. Our analysis shows that cold TLB misses dominate latency for small, latency-sensitive collectives, causing up to 1.4x performance degradation, while larger collectives benefit from warmed caches and experience diminishing returns from over sized TLBs. Based on these observations, we propose two avenues for optimization: fused pre-translation kernels that overlap Reverse Address Translation with computation and software-guided TLB prefetching to proactively populate likely-needed entries. These techniques aim to hide translation latency, particularly for small collectives, improving throughput and scalability for inference workloads. Our study establishes a foundation for designing efficient destination-side translation mechanisms in large-scale multi-GPU systems.