Paper:
The Graph Convolutional Network (GCN) and its variants are powerful models for graph representation learning and have recently achieved great success on many graph-based applications. However, most of them target on shallow models (e.g. 2 layers) on relatively small graphs. Very recently, although many acceleration methods have been developed for GCNs training, it still remains a severe challenge how to scale GCN-like models to larger graphs and deeper layers due to the over-expansion of neighborhoods across layers. In this paper, to address the above challenge, we propose a novel layer-wise sampling strategy, which samples the nodes layer by layer conditionally based on the factors of the bi-directional diffusion between layers. In this way, we potentially restrict the time complexity linear to the number of layers, and construct a mini-batch of nodes with high local bi-directional influence (correlation). Further, we apply the self-attention mechanism to flexibly learn suitable weights for the sampled nodes, which allows the model to be able to incorporate both the first-order and higher-order proximities during a single layer propagation process without extra recursive propagation or skip connection. Extensive experiments on three large benchmark graphs demonstrate the effectiveness and efficiency of the proposed model.

Review:
This paper aims at improving the computational efficiency of GCNs to effectively capture information from the larger multi-hop neighborhood. Conventionally, GCNs use information from all the neighbors up to a certain depth; in which case, with consideration of each further hop, the neighborhood size increases exponentially. To avoid the exponentially increasing memory and computational footprints of GCNs as a result of an exponential neighborhood expansion, this paper proposes a (hop) layer-wise sampling procedure that reduces the complexity to a linear factor. The sampling of nodes at a layer, ‘l’ is based on the transmission probabilities of the nodes at layer ‘l’ and their immediate neighbors sampled earlier in layer ‘l+1’ from both directions of diffusion. The proposed model is based on Graph ATtention Network (GAT) which is adopted here to aggregates neighborhood information only over the nodes sampled with their bi-diffusion sampler. 
Strengths of the paper: The paper intuitively suggests that some of the popular sampling-based scaling approaches for GCN may not be powerful enough as they don’t consider bi-directional influences. 
Weaknesses of this paper:
- Novelty: The idea is incremental. The paper is similar to the layer-wise sampling model, AS-GCN where instead of the base GCN model this paper uses GAT coupled with its proposed bi-diffusion sampler. 
- Experimental results: 
    - (a) Inconsistent baseline results: The performance of baselines reported here on standard train/test/val splits are significantly lower than the ones reported in the original papers. For ex: with FastGCN the original papers report 0.88 and 0.937 on PPI and Reddit which is ~0.03 more than what is reported here. With the case of AS-GCN, the original performance scores are superior to the proposed model in the paper, however here they are reported ~0.04 scores lower. Since the codes for all these baselines are available, it is only fair to use the original implementation; if not, it is important to replicate the original results before using a different implementation. 
    - (b) Variance and statistical significance results are missing
    - (c) Cluster-GCN though discussed, an experimental comparison with it is missing. Reported results from Cluster-GCN paper on Reddit and PPI suggests a superior performance over BLS-GAT. 
    - (d) BLS-GCN missing. This would be a fair comparison to FastGCN and AS-GCN. 
    - (e) Experimental comparison with Jumping Neural network (Xu et al) is missing to understand how the proposed solution improves over existing solutions for over-smoothing. It would be helpful to even couple it with Fast-GCN/AS-GCN sampler to better understand the benefits of this paper. 
- Writing:
    - The paper is not well written. Though there are only minor grammatical mistakes, multiple sections of the paper are not clear and are hard to read because of complex sentences and long paragraphs. 
    - Some of the terminologies used are not clearly described and are not explained prior to the usage. Some of them are neighbor-explosion, over-expansion, the width of neighborhood expansion, local correlations, etc, In some places, over-expansion is used to refer only neighbourhood explosion or only over-smoothing and both. It will be comprehensive if it is grounded. 
    - Numerous claims/ideas put forth in this paper are abstract and intuitive. The intuitions should be backed with proper support. Some of the major concerns are:-
        - (a) proof/arguments to show that layer-wise sampling may lead to sparse mini-batches and how does that in-turn impact over-smoothing 
        - (b) how does the proposed model avoid over-smoothing?
        - (c) why sub-graph methods are not effective ? .. etc
    - It is true that using a fixed neighborhood weightage function as with GCNs may not be optimal. However, the discussion made on GCN and its lack of an appropriate normalization/ neighbourhood weightage function is incorrect. GCNs aggregate information from further neighborhoods according to the respective higher-order diffusion laplacian matrix entries. You can see that by simply removing the nonlinearity+weights and recursively expanding the GCN equation. 
- Other comments:
    - Provide the complexity of the proposed model (GCN + sampler) and compare it with other sampling approaches. 
    - The connectivity structure + signal on the nodes of the graphs is the data that is being convolved and they are not the filters. The weights being learned are the filters. 
    - In Eqn: 2, I believe you are providing an equation for GS-GCN. In which case the fraction should be N(v)+1/ (N_s(V)+1) to match the original model/implementation for GraphSAGE (GS) paper. 
    - I think the summation in the denominator for AS-GCN following Eqn: 3 should run over V instead of V_l.
    - It will be helpful to run the model on directed datasets to see improved benefits of bi-diffusion sampling. 
    - Need more discussion about AS-GCN, Cluster-GCN and Jumping Neural networks (Xu 2018b)