SPARE: Spiking Neural Network Acceleration Using ROM-Embedded RAMs as In-Memory-Computation Primitives

From the little we know about the human brain, the inherent cognitive mechanism is very different from the de facto state-of-the-art computing platforms. The human brain uses distributed, yet integrated memory and computation units, unlike the physically separate memory and computation cores in typical von Neumann architectures. Despite huge success of artificial intelligence, hardware systems running these algorithms consume orders of magnitude higher energy compared to the human brain, mainly due to heavy data movements between the memory unit and the computation cores. Spiking neural networks (SNNs) built using bio-plausible neuron and synaptic models have emerged as the power efficient choice for designing cognitive applications. These algorithms involve several lookup-table (LUT) based function evaluations such as high-order polynomials and transcendental functions for solving complex neuro-synaptic models, that typically require additional storage and thus, bigger memories. To that effect, we propose `SPARE'-an in-memory, distributed processing architecture built on ROM-embedded RAM technology, for accelerating SNNs. ROM-embedded RAMs allow storage of LUTs (for neuro-synaptic models), embedded within a typical memory array, without additional area overhead. Our proposed architecture consists of a 2-D array of Processing Elements (PEs), wherein each PE has its own ROM-embedded RAM structure and executes part of the SNN computation. Since most of the computations (including multiple math-table evaluations) are done locally within each PE, unnecessary data transfers are restricted, thereby alleviating the problems arising due to physically separate remote memory unit and the computation core. SPARE thus leverages both, the hardware benefits of distributed, in-memory processing, and also the algorithmic benefits of SNNs. We evaluate SPARE for two different ROM-Embedded RAM structures-CMOS based ROM-Embedded SRAMs (R-SRAMs) and STT-MRAM based ROM-Embedded MRAMs (R-MRAMs). Moreover, we analyze trade-offs in terms of energy, area and performance, for using the two technologies on a range of image classification benchmarks. Furthermore, we leverage the additional storage density to implement complex neuro-synaptic functionalities. This enhances the utility of the proposed architecture by provisioning implementation of any neuron/synaptic behavior as necessitated by the application. Our results show up-to
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1.75×,
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1.95× and
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1.95× improvement in energy, iso-storage area, and iso-area performance, respectively, by using neural network accelerators built on ROM-embedded RAM primitives.