# Introduction

To accelerate CNN inference, existing deep learning frameworks focus on optimizing intra-operator parallelization. However, a single operator can no longer fully utilize the available parallelism given the rapid advances in high-performance hardware, resulting in a large gap between the peak performance and the real performance. This performance gap is more severe under smaller batch sizes. In this work, we extensively study the parallelism between operators and propose Inter-Operator Scheduler (IOS) to automatically schedule multiple operators’ parallel execution through a novel dynamic programming algorithm. IOS consistently outperforms state-of-the-art libraries (e.g., TensorRT) by 1.1 to 1.5x on modern CNN benchmarks.

# Inter-Operator Scheduler (IOS)

IOS explores the inter-operator parallelization schedule space exhaustively through dynamic programming algorithm.

For each subset of operators $$S$$ (initially, $$S$$ contains all operators in the model):

• Enumerate a subset $$S'$$ of $$S$$, taking it as the last stage of $$S$$.
• Convert the original problem that finds the optimal schedule for $$S$$, into a sub-problem that finds the optimal schedule for $$S-S'$$.

IOS would record the optimal choice of $$S'$$ for each S and use it to construct the optimal schedule. IOS reuses the optimal schedule for each subgraph.

We prove that its time complexity is $$(n/d+1)^{2d}$$, where $$n$$ is the number of operators and $$d$$ is the maximum number of parallelizable operators. The time complexity is only exponential in $$d$$, that is usually small, making it feasible to explore the schedule space exhasutively.

## Parallelization Strategies in IOS

IOS consider two strategies to parallelize operators in a stage S’: concurrent execution and operator merge.

• Concurrent Execution: We can launch all kernels concurrently on the accelerator (e.g., GPU).
• Operator Merge: If we can find a single kernel to do all computation of operators in the stage, we can directly use this kernel.

IOS is a profile-based scheduler and would measure the execution time of both strategies for each proposed stage and select the better one.

## Results

Citation

@inproceedings{ding2021ios,
author={Yaoyao Ding and Ligeng Zhu and Zhihao Jia and Gennady Pekhimenko and Song Han},
booktitle = {Proceedings of Machine Learning and Systems},
title = {{IOS: Inter-Operator Scheduler for CNN Acceleration}},
volume = {3},
year = {2021}


Acknowledgments

We want to thank Xiaodan (Serina) Tan for NVIDIA GPU related issues and constructive discussion.