CUBA: an architecture for efficient CPU/co-processor data communication

Isaac Gelado, John H. Kelm, Shane Ryoo, Steven S. Lumetta, Nacho Navarro, Wen-mei W. Hwu

Abstract: Data-parallel co-processors have the potential to improve performance in highly parallel regions of code when coupled to a general-purpose CPU. However, applications often have to be modified in non-intuitive and complicated ways to mitigate the cost of data marshalling between the CPU and the co-processor. In some applications the overheads cannot be amortized and co-processors are unable to provide benefit. The additional effort and complexity of incorporating co-processors makes it difficult, if not impossible, to effectively utilize co-processors in large applications.This paper presents CUBA, an architecture model where co-processors encapsulated as function calls can efficiently access their input and output data structures through pointer parameters. The key idea is to map the data structures required by the co-processor to the co-processor local memory as opposed to the CPU's main memory. The mapping in CUBA preserves the original layout of the shared data structures hosted in the co-processor local memory. The mapping renders the data marshalling process unnecessary and reduces the need for code changes in order to use the co-processors. CUBA allows the CPU to cache hosted data structures with a selective write-through cache policy, allowing the CPU to access hosted data structures while supporting efficient communication with the co-processors. Benchmark simulation results show that a CUBA-based system can approach optimal transfer rates while requiring few changes to the code that executes on the CPU.