Custom-sized caches in application-specific memory hierarchies

Abstract

Developing FPGA implementations with an input specification in a high-level programming language such as C/C++ or OpenCL allows for a substantially shortened design cycle compared to a design entry at register transfer level. This work targets high-level synthesis (HLS) implementations that process large amounts of data and therefore require access to an off-chip memory. We leverage the customizability of the FPGA on-chip memory to automatically construct a multi-cache architecture in order to enhance the performance of the interface between parallel functional units of the HLS core and an external memory. Our focus is on automatic cache sizing. Firstly, our technique determines and uses up unused left-over block RAM resources for the construction of on-chip caches. Secondly, we devise a high-level cache performance estimation based on the memory access trace of the program. We use this memory trace to find a heterogeneous configuration of cache sizes, tailored to the application's memory access characteristic, that maximizes the performance of the multi-cache system subject to an on-chip memory resource constraint. We evaluate our technique with three benchmark implementations on an FPGA board and obtain a reduction in execution latency of up to 2× (1.5× on average) when compared to a one-size-fits-all cache sizing. We also quantify the impact of our automatically generated cache system on the overall energy consumption of the implementation.