High-Level Debugging and Verification for FPGA-Based Multicore Architectures

Simulators are key tools for computer architecture research. However, multicore architectures represent a highly complex challenge for software simulators, which may suffer from fidelity loss and long execution times. FPGAs can simulate multicore architectures with scalable performance and high accuracy, but the difficulty of debugging could hinder their adoption. In this paper we propose several techniques for inspection, debugging and verification of multicore architectures, both for software-based and FPGA-based simulations. These debugging extensions are cycle-accurate and unobtrusive. As a proof of concept, we have developed a 24-core RISC multiprocessor that runs the Linux Kernel, for which we provide three simulation modes: a fast, functional simulation; a detailed, cycle-accurate simulation; and a FPGA-based simulation. Our platform can run up to 24 cores and perform full-system verification at 17 million instructions per second.Peer ReviewedPostprint (author's final draft

An Experimental Study of Reduced-Voltage Operation in Modern FPGAs for Neural Network Acceleration

We empirically evaluate an undervolting technique, i.e., underscaling the circuit supply voltage below the nominal level, to improve the power-efficiency of Convolutional Neural Network (CNN) accelerators mapped to Field Programmable Gate Arrays (FPGAs). Undervolting below a safe voltage level can lead to timing faults due to excessive circuit latency increase. We evaluate the reliability-power trade-off for such accelerators. Specifically, we experimentally study the reduced-voltage operation of multiple components of real FPGAs, characterize the corresponding reliability behavior of CNN accelerators, propose techniques to minimize the drawbacks of reduced-voltage operation, and combine undervolting with architectural CNN optimization techniques, i.e., quantization and pruning. We investigate the effect of environmental temperature on the reliability-power trade-off of such accelerators. We perform experiments on three identical samples of modern Xilinx ZCU102 FPGA platforms with five state-of-the-art image classification CNN benchmarks. This approach allows us to study the effects of our undervolting technique for both software and hardware variability. We achieve more than 3X power-efficiency (GOPs/W) gain via undervolting. 2.6X of this gain is the result of eliminating the voltage guardband region, i.e., the safe voltage region below the nominal level that is set by FPGA vendor to ensure correct functionality in worst-case environmental and circuit conditions. 43% of the power-efficiency gain is due to further undervolting below the guardband, which comes at the cost of accuracy loss in the CNN accelerator. We evaluate an effective frequency underscaling technique that prevents this accuracy loss, and find that it reduces the power-efficiency gain from 43% to 25%.Comment: To appear at the DSN 2020 conferenc

Manticore: Hardware-Accelerated RTL Simulation with Static Bulk-Synchronous Parallelism

The demise of Moore's Law and Dennard Scaling has revived interest in specialized computer architectures and accelerators. Verification and testing of this hardware heavily uses cycle-accurate simulation of register-transfer-level (RTL) designs. The best software RTL simulators can simulate designs at 1--1000~kHz, i.e., more than three orders of magnitude slower than hardware. Faster simulation can increase productivity by speeding design iterations and permitting more exhaustive exploration. One possibility is to use parallelism as RTL exposes considerable fine-grain concurrency. However, state-of-the-art RTL simulators generally perform best when single-threaded since modern processors cannot effectively exploit fine-grain parallelism. This work presents Manticore: a parallel computer designed to accelerate RTL simulation. Manticore uses a static bulk-synchronous parallel (BSP) execution model to eliminate runtime synchronization barriers among many simple processors. Manticore relies entirely on its compiler to schedule resources and communication. Because RTL code is practically free of long divergent execution paths, static scheduling is feasible. Communication and synchronization no longer incur runtime overhead, enabling efficient fine-grain parallelism. Moreover, static scheduling dramatically simplifies the physical implementation, significantly increasing the potential parallelism on a chip. Our 225-core FPGA prototype running at 475 MHz outperforms a state-of-the-art RTL simulator on an Intel Xeon processor running at $\approx$ 3.3 GHz by up to 27.9$\times$ (geomean 5.3$\times$) in nine Verilog benchmarks

Integrating NVIDIA Deep Learning Accelerator (NVDLA) with RISC-V SoC on FireSim

NVDLA is an open-source deep neural network (DNN) accelerator which has received a lot of attention by the community since its introduction by Nvidia. It is a full-featured hardware IP and can serve as a good reference for conducting research and development of SoCs with integrated accelerators. However, an expensive FPGA board is required to do experiments with this IP in a real SoC. Moreover, since NVDLA is clocked at a lower frequency on an FPGA, it would be hard to do accurate performance analysis with such a setup. To overcome these limitations, we integrate NVDLA into a real RISC-V SoC on the Amazon cloud FPGA using FireSim, a cycle-exact FPGA-accelerated simulator. We then evaluate the performance of NVDLA by running YOLOv3 object-detection algorithm. Our results show that NVDLA can sustain 7.5 fps when running YOLOv3. We further analyze the performance by showing that sharing the last-level cache with NVDLA can result in up to 1.56x speedup. We then identify that sharing the memory system with the accelerator can result in unpredictable execution time for the real-time tasks running on this platform. We believe this is an important issue that must be addressed in order for on-chip DNN accelerators to be incorporated in real-time embedded systems.Comment: Presented at the 2nd Workshop on Energy Efficient Machine Learning and Cognitive Computing for Embedded Applications (EMC2'19