Session

Parallel and Distributed Systems 2: Communication

Ballroom C

Moderator: Christopher De Sa

Abstract:

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Ballroom B - Position 4
Cupcake: A Compression Scheduler for Scalable Communication-Efficient Distributed Training

Zhuang Wang · Xinyu Wu · Zhaozhuo Xu · T. S. Eugene Ng

Data-parallel distributed training (DDT) is the de facto way to accelerate deep learning on multiple GPUs.In DDT, communication for gradient synchronization is the major efficiency bottleneck.Many gradient compression (GC) algorithms have been proposed to address this communication bottleneck by reducing the amount of communicated data.Unfortunately, it has been observed that GC only achieves moderate performance improvement in DDT, or even harms the performance.In this paper, we argue that the current way of deploying GC in a layer-wise fashion reduces communication time at the cost of non-negligible compression overheads.To address this problem, we propose Cupcake, a compression optimizer to fully unleash GC algorithms' advantages in accelerating DDT. It applies GC algorithms in a fusion fashion and determines the provably optimal fusion strategy to maximize the training throughput of compression-enabled DDT jobs.Experimental evaluations show that GC algorithms with Cupcake can achieve up to 2.03x speedup in training throughput over training without GC, and up to 1.79x speedup over the state-of-the-art approaches of applying GC to DDT in a layer-wise fashion.


Ballroom B - Position 5
Communication-Efficient Graph Neural Networks with Probabilistic Neighborhood Expansion Analysis and Caching

Tim Kaler · Alexandros Iliopoulos · Philip Murzynowski · Tao Schardl · Charles E. Leiserson · Jie Chen

Training and inference with graph neural networks (GNNs) on massive graphs in a distributed environment has been actively studied since the inception of GNNs, owing to the widespread use and success of GNNs in applications such as recommendation systems and financial forensics. This paper is concerned with minibatch training and inference with GNNs in distributed settings, where the necessary partitioning of vertex features across distributed storage causes feature communication to become a major bottleneck that hampers scalability.To significantly reduce the communication volume without compromising prediction accuracy, we propose a policy for caching data associated with frequently accessed vertices in remote partitions. The proposed policy is based on an analysis of vertex-wise inclusion probabilities (VIP) during multi-hop neighborhood sampling, which may expand the neighborhood far beyond the partition boundary of the graph. The VIP analysis not only enables the elimination of the communication bottleneck, but also offers a means to organize in-memory data by prioritizing GPU storage for the most frequently accessed vertex features. We present SALIENT++, which extends the prior state-of-the-art SALIENT system to work with partitioned feature data and leverages the VIP-driven caching policy. SALIENT++ retains the local training efficiency and scalability of SALIENT by using a deep pipeline and drastically reducing communication volume while consuming only a fraction of the storage required by SALIENT. We demonstrate experiments on the Open Graph Benchmark data sets and show that training a 3-layer GraphSAGE model with SALIENT++ on 8 single-GPU machines is 7.1x faster than with SALIENT on 1 single-GPU machine, and 12.7x faster than with DistDGL on 8 single-GPU machines.


Ballroom B - Position 6
Adaptive Message Quantization and Parallelization for Distributed Full-graph GNN Training

Borui Wan · Juntao Zhao · Chuan Wu

Distributed full-graph training of Graph Neural Networks (GNNs) over large graphs is bandwidth-demanding and time-consuming. Frequent exchanges of node features, embeddings and embedding gradients (all referred to as messages) across devices bring significant communication overhead for nodes with remote neighbors on other devices (marginal nodes) and unnecessary waiting time for nodes without remote neighbors (central nodes) in the training graph. This paper proposes an efficient GNN training system, AdaQP, to expedite distributed full-graph GNN training. We stochastically quantize messages transferred across devices to lower-precision integers for communication traffic reduction and advocate communication-computation parallelization between marginal nodes and central nodes. We provide theoretical analysis to prove fast training convergence (at the rate of O(T^{-1}) with T being the total number of training epochs) and design an adaptive quantization bit-width assignment scheme for each message based on the analysis, targeting a good trade-off between training convergence and efficiency. Extensive experiments on mainstream graph datasets show that AdaQP substantially improves distributed full-graph training's throughput (up to 3.01 X) with negligible accuracy drop (at most 0.30%) or even accuracy improvement (up to 0.19%) in most cases, showing significant advantages over the state-of-the-art works.


Ballroom B - Position 7
On Optimizing the Communication of Model Parallelism

Yonghao Zhuang · Hexu Zhao · Lianmin Zheng · Zhuohan Li · Eric Xing · Qirong Ho · Joseph Gonzalez · Ion Stoica · Hao Zhang · Hexu Zhao

We study a novel and important communication pattern in large-scale model-parallel deep learning (DL), which we call cross-mesh resharding. This pattern emerges when the two paradigms of model parallelism – intra-operator and inter-operator parallelism – are combined to support large models on large clusters. In cross-mesh resharding, a sharded tensor needs to be sent from a source device mesh to a destination device mesh, on which the tensor may be distributed with the same or different layouts. We formalize this as a many-to-many multicast communication problem, and show that existing approaches either are sub-optimal or do not generalize to different network topologies or tensor layouts, which result from different model architectures and parallelism strategies. We then propose two contributions to address cross-mesh resharding: an efficient broadcast-based communication system, and an “overlapping-friendly" pipeline schedule. On microbenchmarks, our overall system outperforms existing ones by up to 10x across various tensor and mesh layouts. On end-to-end training of two large models, GPT-3 and U-Transformer, we improve throughput by 10% and 50%, respectively.