Many accelerators today process deep neural networks layer by layer. As a consequence of this processing style, every intermediate feature map incurs expensive off-chip transfers. Layer fusion eliminates off-chip transfers of intermediate results, leading to better latency and energy efficiency. Prior works have explored only subsets of the fused-layer design space, looking only at a particular choice of tiling, scheduling, and buffering strategy. Their architectural models are also tailored for their proposed datatlow. The lack of a unified, systematic representation of designs and a versatile evaluation method has prevented thorough exploration of the design space. To enable systematic exploration of this design space, we present LoopTree, a framework for describing and evaluating any design in our expanded fused-layer datatlow design space. With a case study, we explore new designs to show that exploring our larger design space uncovers more efficient designs, especially for recent workloads with diverse layer types. Our design achieves 2.5× speedup and 2× lower energy compared to an optimized layer-by-layer design. Compared to a state-of-the-art fused-layer design, we match latency and energy while using 25% less onchip buffer space.

Due to complex interactions among various deep neural network (DNN) optimization techniques, modern DNNs can have weights and activations that are dense or sparse with diverse sparsity degrees. To offer a good trade-off between accuracy and hardware performance, an ideal DNN accelerator should have high flexibility to efficiently translate DNN sparsity into reductions in energy and/or latency without incurring significant complexity overhead.

Sparse tensor algebra is a challenging class of workloads to accelerate due to low arithmetic intensity and varying sparsity patterns. Prior sparse tensor algebra accelerators have explored tiling sparse data to increase exploitable data reuse and improve throughput, but typically allocate tile size in a given buffer for the worst-case data occupancy. This severely limits the utilization of available memory resources and reduces data reuse. Other accelerators employ complex tiling during preprocessing or at runtime to determine the exact tile size based on its occupancy.

In recent years, many accelerators have been proposed to efficiently process sparse tensor algebra applications (e.g., sparse neural networks). However, these proposals are single points in a large and diverse design space. The lack of systematic description and modeling support for these sparse tensor accelerators impedes hardware designers from efficient and effective design space exploration. This paper first presents a unified taxonomy to systematically describe the diverse sparse tensor accelerator design space. Based on the proposed taxonomy, it then introduces Sparseloop, the first fast, accurate, and flexible analytical modeling framework to enable early-stage evaluation and exploration of sparse tensor accelerators. Sparseloop comprehends a large set of architecture specifications, including various dataflows and sparse acceleration features (e.g., elimination of zero-based compute). Using these specifications, Sparseloop evaluates a design’s processing speed and energy efficiency while accounting for data movement and compute incurred by the employed dataflow, including the savings and overhead introduced by the sparse acceleration features using stochastic density models. Across representative accelerator designs and workloads, Sparseloop achieves over 2000× faster modeling speed than cycle-level simulations, maintains relative performance trends, and achieves 0.1% to 8% average error. The paper also presents example use cases of Sparseloop in different accelerator design flows to reveal important design insights.

Due to the data and computation intensive nature of many popular data processing applications, e.g., deep neural networks (DNNs), a variety of accelerators have been proposed to improve performance and energy efficiency. As a result, computing systems have become increasingly heterogeneous, with application-specific processing offloaded from the CPU to specialized accelerators. To understand the energy efficiency of such systems, it is desirable to characterize holistically the energy consumption of the CPU, the accelerator, and the data transfers in between. We present a modularized architecture-level energy estimation framework that captures the energy breakdown across the various CPU and accelerator components with a unified energy estimation back-end that allows easy integration of accelerator modeling frameworks for emerging designs. Using DNN workloads as examples, we show that CPU-end preprocessing and data transfers to and from the accelerator can account for up to 45-50% of total energy when assessing the system as a whole. Related open-source code is available at https://accelergy.mit.edu.

This paper presents Sparseloop, the first infrastructure that implements an analytical design space exploration methodology for sparse tensor accelerators. Sparseloop comprehends a wide set of architecture specifications including various sparse optimization features such as compressed tensor storage. Using these specifications, Sparseloop can calculate a design's energy efficiency while accounting for both optimization savings and metadata overhead at each storage and compute level of the architecture using stochastic tensor density models. We validate Sparseloop on a well-known accelerator design and achieve ~99% accuracy in terms of runtime activities (e.g., compressed memory accesses). We also present a case study that highlights the key factors (e.g., uncompressed traffic, data density) that affect sparse optimization features' impact on energy efficiency. Tool available at: https://github.com/NVlabs/timeloop.

Processing-in-memory (PIM) deep neural network (DNN) accelerators, which aim to improve energy/area efficiency of DNN processing by integrating computation into data storage, have gained popularity in recent years. Therefore, it is attractive to have a generally applicable framework that is able to quickly provide insights into the various trade-offs involved in PIM accelerator designs. We present an architecture-level design estimation framework for PIM accelerators that allows easy representations of the designs with provided architecture templates and component design templates, performs analytical runtime simulations, and produces technology-dependent area and energy estimations. We show that the framework can be easily used to evaluate state of the art PIM accelerator designs; it achieves 95% accurate total energy estimations and reproduces exact area breakdowns of the components in the design. Related open-source code is available at http://accelergy.mit.edu/.

With Moore's law slowing down and Dennard scaling ended, energy-efficient domain-specific accelerators, such as deep neural network (DNN) processors for machine learning and programmable network switches for cloud applications, have become a promising way for hardware designers to continue bringing energy efficiency improvements to data and computation-intensive applications. To ensure the fast exploration of the accelerator design space, architecture-level energy estimators, which perform energy estimations without requiring complete hardware description of the designs, are critical to designers. However, it is difficult to use existing architecture-level energy estimators to obtain accurate estimates for accelerator designs, as accelerator designs are diverse and sensitive to data patterns. This paper presents Accelergy, a generally applicable energy estimation methodology for accelerators that allows design specifications comprised of user-defined high-level compound components and user-defined low-level primitive components, which can be characterized by third-party energy estimation plug-ins. An example with primitive and compound components for DNN accelerator designs is also provided as an application of the proposed methodology. Overall, Accelergy achieves 95% accuracy on Eyeriss, a well-known DNN accelerator design, and can correctly capture the energy breakdown of components at different granularities. The Accelergy code is available at http://accelergy.mit.edu.