In recent years, there has been a trend towards the use of accelerators and architectural specialization to continue scaling performance in spite of a slowing of Moore’s Law. GPUs have always relied on dedicated hardware for graphics workloads, but modern GPUs now also incorporate compute-domain accelerators such as NVIDIA’s Tensor Cores for machine learning. For these accelerators to be successfully integrated into a general-purpose programming language such as C++ or CUDA, there must be a forward- and backward-compatible API for the functionality they provide. To the extent that all of these accelerators interact with program threads through memory, they should be incorporated into the GPU’s memory consistency model. Unfortunately, the use of accelerators and/or special non-coherent paths into memory produces non-standard memory behavior that existing GPU memory models cannot capture.
In this work, we describe the “proxy” extensions added to version 7.5 of NVIDIA’s PTX ISA for GPUs. A proxy is an extra tag abstractly applied to every memory or fence operation. Proxies generalize the notion of address translation and specialized non-coherent cache hierarchies into an abstraction that cleanly describes the resulting non-standard behavior. The goal of proxies is to facilitate integration of these specialized memory accesses into the general-purpose PTX programming model in a fully composable manner. This paper characterizes the behaviors that proxies can capture, the microarchitectural intuition behind them, the necessary updates to the formal memory model, and the tooling that we built in order to ensure that the resulting model both is sound and meets the needs of business-critical workloads that they are designed to support.
Copyright by the Association for Computing Machinery, Inc. Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from Publications Dept, ACM Inc., fax +1 (212) 869-0481, or email@example.com. The definitive version of this paper can be found at ACM's Digital Library http://www.acm.org/dl/.