Session 8: Inference

Moderator: Sarah Bird


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Wed 7 April 15:20 - 15:40 PDT
Nimble: Efficiently Compiling Dynamic Neural Networks for Model Inference

Haichen Shen · Jared Roesch · Zhi Chen · Wei Chen · Yong Wu · Mu Li · Vin Sharma · Zachary Tatlock · Yida Wang

Modern deep neural networks increasingly make use of features such as control flow, dynamic data structures, and dynamic tensor shapes. Existing deep learning systems focus on optimizing and executing static neural networks which assume a pre-determined model architecture and input data shapes—assumptions that are violated by dynamic neural networks. Therefore, executing dynamic models with deep learning systems is currently both inflexible and sub-optimal, if not impossible. Optimizing dynamic neural networks is more challenging than static neural networks; optimizations must consider all possible execution paths and tensor shapes. This paper proposes Nimble, a high-performance and flexible system to optimize, compile, and execute dynamic neural networks on multiple platforms. Nimble handles model dynamism by introducing a dynamic type system, a set of dynamism-oriented optimizations, and a light-weight virtual machine runtime. Our evaluation demonstrates that Nimble outperforms existing solutions for dynamic neural networks by up to 20x on hardware platforms including Intel CPUs, ARM CPUs, and Nvidia GPUs.

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Wed 7 April 15:40 - 16:00 PDT
MicroRec: Efficient Recommendation Inference by Hardware and Data Structure Solutions

Wenqi Jiang · Zhenhao He · Shuai Zhang · Thomas B. Preußer · Kai Zeng · Liang Feng · Jiansong Zhang · Tongxuan Liu · Yong Li · Jingren Zhou · Ce Zhang · Gustavo Alonso

Deep neural networks are widely used in personalized recommendation systems. Unlike regular DNN inference workloads, recommendation inference is memory-bound due to the many random memory accesses needed to lookup the embedding tables. The inference is also heavily constrained in terms of latency because producing a recommendation for a user must be done in about tens of milliseconds. In this paper, we propose MicroRec, a high-performance inference engine for recommendation systems. MicroRec accelerates recommendation inference by (1) redesigning the data structures involved in the embeddings to reduce the number of lookups needed and (2) taking advantage of the availability of High-Bandwidth Memory (HBM) in FPGA accelerators to tackle the latency by enabling parallel lookups. We have implemented the resulting design on an FPGA board including the embedding lookup step as well as the complete inference process. Compared to the optimized CPU baseline (16 vCPU, AVX2-enabled), MicroRec achieves 13.8~14.7x speedup on embedding lookup alone and 2.5~5.4x speedup for the entire recommendation inference in terms of throughput. As for latency, CPU-based engines needs milliseconds for inferring a recommendation while MicroRec only takes microseconds, a significant advantage in real-time recommendation systems.

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Wed 7 April 16:00 - 16:20 PDT
VS-Quant: Per-vector Scaled Quantization for Accurate Low-Precision Neural Network Inference

Steve Dai · Rangha Venkatesan · Mark Ren · Brian Zimmer · William Dally · Brucek Khailany

Quantization enables efficient acceleration of deep neural networks by reducing model memory footprint and exploiting low-cost integer math hardware units. Quantization maps floating-point weights and activations in a trained model to low-bitwidth integer values using scale factors. Excessive quantization, reducing precision too aggressively, results in accuracy degradation. When scale factors are shared at a coarse granularity across many dimensions of each tensor, effective precision of individual elements within the tensor are limited. To reduce quantization-related accuracy loss, we propose using a separate scale factor for each small vector of (~16-64) elements within a single dimension of a tensor. To achieve an efficient hardware implementation, the per-vector scale factors can be implemented with low-bitwidth integers when calibrated using a two-level quantization scheme. We find that per-vector scaling consistently achieves better inference accuracy at low precision compared to conventional scaling techniques for popular neural networks without requiring retraining. We also modify a deep learning accelerator hardware design to study the area and energy overheads of per-vector scaling support. Our evaluation demonstrates that per-vector scaled quantization with 4-bit weights and activations achieves 69% energy saving and 36% area saving over an 8-bit baseline while maintaining over 75% accuracy for ResNet50 on ImageNet. 4-bit weights and 8-bit activations achieve near-full-precision accuracy for both BERT-base and BERT-large on SQuAD while reducing area by 28% compared to an 8-bit baseline.

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Wed 7 April 16:20 - 16:40 PDT
Accelerate Inference of CNNs for Video Analysis While Preserving Exactness Exploiting Activation Sparsity

Toshiaki Wakatsuki · Sekitoshi Kanai · Yasuhiro Fujiwara

This paper proposes a range-bound-aware convolution layer that accelerates the inference of rectified linear unit (ReLU)-based convolutional neural networks (CNNs) for analyzing video streams. Since video analysis systems require to process each video frame in real-time, the computational cost of inference of CNNs must be reduced. Several techniques heuristically skip the computation for the current frame and reuse the results of the previous frame when the current and previous frames are sufficiently similar. However, for critical applications such as surveillance systems, their accuracy can be unsatisfactory because they sacrifice accuracy for efficiency. In contrast, our method reduces the computational cost of convolution layers accompanied by ReLU while producing exactly the same inference results as an original model. We utilize both temporal similarity of video frames and activation sparsity in ReLU-based CNNs to guarantee to skip truly redundant computations. We experimentally confirm that our method can accelerate widely used pre-trained CNNs with both CPU and GPU implementations.

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Wed 7 April 16:40 - 17:00 PDT
sensAI: ConvNets Decomposition via Class Parallelism for Fast Inference on Live Data

Guanhua Wang · Zhuang Liu · Brandon Hsieh · Siyuan Zhuang · Joseph Gonzalez · Trevor Darrell · Ion Stoica

Convolutional Neural Networks (ConvNets) enable computers to excel on vision learning tasks such as image classification, object detection. Recently, real-time inference on live data is becoming more and more important. From a system perspective, it requires fast inference on each single, incoming data item (e.g. 1 image). Two main-stream distributed model serving paradigms – data parallelism and model parallelism – are not necessarily desirable here, because we cannot further split a single input data piece via data parallelism, and model parallelism introduces huge communication overhead. To achieve live data inference with low latency, we propose sensAI, a novel and generic approach that decouples a CNN model into disconnected subnets, each is responsible for predicting certain class(es). We call this new model distribution paradigm as class parallelism. Experimental results show that, sensAI achieves up to 18x faster inference on single input data item with no or negligible accuracy loss on CIFAR-10, CIFAR-100 and ImageNet-1K datasets.

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