A high-performance overlay architecture for pipelined execution of data flow graphs

Abstract

A major issue facing the widespread use of FPGAs as accelerators is their programmability wall: the difficulty of hardware design and the long synthesis times. Overlays-pre-synthesized FPGA circuits that are themselves reconfigurable - promise to tackle these challenges. We design and evaluate an overlay architecture, structured as a mesh of functional units, for pipelined execution of data-flow graphs (DFGs), a common abstraction for expressing parallelism in applications. We use data-driven execution based on elastic pipelines to balance pipeline latencies and achieve a high fMAX, scalability and maximum throughput. We prototype two overlays on a Stratix IV FPGA: a 355 MHz 24×16 integer overlay and a 312 MHz 18×16 floating-point overlay. We also design a tool that maps DFGs to overlays. We map 15 DFGs and show that the two overlays deliver throughputs of up to 35 GOPS and 22 GFLOPS, respectively. We also show that DFG mapping is fast, taking no more than 6 seconds for the largest DFG. Thus, our overlay architecture raises the level of abstraction of FPGA programming closer to that of software and avoids lengthy synthesis time, easing the use of these devices to accelerate applications.