Tackling Over-Smoothing for General Graph Convolutional Networks
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Increasing the depth of Graph Convolutional Networks (GCN), which in principal can permit more expressivity, is shown to incur detriment to the performance especially on node classification. The main cause of this issue lies in over-smoothing. As its name implies, over-smoothing drives the output of GCN with the increase in network depth towards a space that contains limited distinguished information among nodes, leading to poor trainability and expressivity. Several works on refining the architecture of deep GCN have been proposed, but the improvement in performance is still marginal and it is still unknown in theory whether or not these refinements are able to relieve over-smoothing. In this paper, we first theoretically analyze the over-smoothing issue for a general family of prevailing GCNs, including generic GCN, GCN with bias, ResGCN, and APPNP. We prove that the over-smoothing of all these models is characterized by an universal process, i.e. all nodes converging to a cuboid of specific structure. Upon this universal theorem, we further propose DropEdge, a novel and flexible technique to alleviate over-smoothing. At its core, DropEdge randomly removes a certain number of edges from the input graph at each training epoch, acting like a data augmenter and also a message passing reducer. Furthermore, we theoretically demonstrate that DropEdge either reduces the convergence speed of over-smoothing for general GCNs or relieves the information loss caused by it. One group of experimental evaluations on simulated dataset has visualized the difference of over-smoothing between different GCNs as well as verifying the validity of our proposed theorems. Moreover, extensive experiments on several real benchmarks support that DropEdge consistently improves the performance on a variety of both shallow and deep GCNs.
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