HIVE Year 1 Report: Executive Summary

This report is located online at the following URL: https://gunrock.github.io/docs/hive_year1_summary.html.

Herein UC Davis produces the following three deliverables that it promised to deliver in Year 1:

  1. 7--9 kernels running on a single GPU on DGX-1. The PM had indicated that the application targets are the graph-specific kernels of larger applications, and that our effort should target these kernels. These kernels run on one GPU of the DGX-1. These kernels are in Gunrock's GitHub repository as standalone kernels. While we committed to delivering 7--9 kernels, as of the date of this addendum, we deliver all 11 v0 kernels.
  2. (High-level) performance analysis of these kernels. In this report we analyze the performance of these kernels.
  3. Separable communication benchmark predicting latency and throughput for a multi-GPU implementation. This report (and associated code, also in the Gunrock GitHub repository) analyzes the DGX-1's communication capabilities and projects how single-GPU benchmarks will scale on this machine to 8 GPUs.

Specific notes on applications and scaling follow:

Application Classification

Application Classification Application classification involves a number of dense-matrix operations, which did not make it an obvious candidate for implementation in Gunrock. However, our GPU implementation using the CUDA CUB library shows substantial speedups (10-50x) over the multi-threaded OpenMP implementations.

However, there are two neighbor reduce operations that may benefit from the kind of load balancing implemented in Gunrock. Thus, it would be useful to either expose lightweight wrappers of high-performance Gunrock primitives for easy intergration into outside projects or come up with a workflow inside of Gunrock that makes programming applications with lots of non-graph operations straightforward.

Geolocation

Geolocation Geolocation or geotagging is an interesting parallel problem, because it is among the few that exhibits the dynamic parallelism pattern within the compute. The pattern is as follows; there is parallel compute across nodes, each node has some serial work and within the serial work there are several parallel math operations. Even without leveraging dynamic parallelism within CUDA (kernel launches within a kernel), Geolocation performs well on the GPU environment because it mainly requires simple math operations, instead of complicated memory movement schemes.

However, the challenge within the application is load balancing this simple compute, such that each processor has roughly the same amount of work. Currently, in Gunrock, we map Geolocation using the ForAll() compute operator with optimizations to exit early (performing less work and fewer reads). Even without addressing load balancing issue with a complicated balancing scheme, on the HIVE datasets we achieve a 100x speedup with respect to the CPU reference code, implemented using C++ and OpenMP, and ~533x speedup with respect to the GTUSC implementation. We improve upon the algorithm by avoiding a global gather and a global synchronize, and using 3x less memory than the GTUSC reference implementation.

GraphSAGE

GraphSAGE The vertex embedding part of the GraphSAGE algorithm is implemented in the Gunrock framework using custom CUDA kernels, utilizing block-level parallelism, that allow a shorter running time. For the embedding part alone, the GPU implementation is 7.5X to 15X on P100, and 20X to 30X on V100, faster than an OpenMP implementation using 32 threads. The GPU hardware, especially the memory system, has high utilizations from these custom kernels. It is still unclear how to expose block-level parallelism for more general usage in other applications in Gunrock.

Connecting the vertex embedding with the neural network training part, and making the GraphSAGE workflow complete, would be an interesting task for year 2. Testing on the complete workflow for prediction accuracy and running speed will be more meaningful.

GraphSearch

GraphSearch Graph search is a relatively minor modification to Gunrock's random walk application, and was straightforward to implement. Though random walks are a "worst case scenario" for GPU memory bandwidth, we still achieve 3--5x speedup over a modified version of the OpenMP reference implementation.

The original OpenMP reference implementation actually ran slower with more threads -- we fixed the bugs, but the benchmarking experience highlights the need for performant and hardened CPU baselines.

Until recently, Gunrock did not support parallelism within the lambda functions run by the advance operator, so neighbor selection for a given step in the walk is done sequentially. Methods for exposing more parallelism to the programmer are currently being developed via parallel neighbor reduce functions.

In an end-to-end graph search application, we'd need to implement the scoring function as well as the graph walk component. For performance, we'd likely want to implement the scoring function on the GPU as well, which makes this a good example of a "Gunrock+X" app, where we'd need to integrate the high-performance graph processing component with arbitrary user code.

Community Detection (Louvain)

Community Detection (Louvain) The Gunrock implementation uses sort and segmented reduce to implement the Louvain algorithm, different from the commonly used hash table mapping. The GPU implementation is about ~1.5X faster than the OpenMP implementation, and also faster than previous GPU works. It is still unknown whether the sort and segmented reduce formulation map the problem better than hash table on the GPU. The modularities resulting from the GPU implementation are within small differences as the serial implementation, and are better when the graph is larger. A custom hash table can potentially improve the running time. The GPU Louvain implementation should have moderate scalability across multiple GPUs in an DGX-1.

Local Graph Clustering (LGC)

Local Graph Clustering (LGC) This variant of local graph clustering (L1 regularized PageRank via FISTA) is a natural fit for Gunrock's frontier-based programming paradigm. We observe speedups of 2-3 orders of magnitude over the HIVE reference implementation.

The reference implementation of the algorithm was not explicitly written as advance/filter/compute operations, but we were able to quickly determine how to map the operations by using a lightweight Python implementation of the Gunrock programming API as a development environment. Thus, LGC was a good exercise in implementing a non-trivial end-to-end application in Gunrock from scratch.

Graph Projections

Graph Projections Because it has a natural representation in terms of sparse matrix operations, graph projections gave us an opportunity to compare ease of implementation and performance between Gunrock and another UC-Davis project, GPU GraphBLAS.

Overall, we found that Gunrock was more flexible and more performant than GraphBLAS, likely due to better load balancing. However, in this case, the GraphBLAS application was substantially easier to program than Gunrock, and also allowed us to take advantage of some more sophisticated memory allocation methods available in the GraphBLAS cuSPARSE backend. These findings suggest that addition of certain commonly used API functions to Gunrock could be a fruitful direction for further work.

Scan Statistics

Scan Statistics Scan statistics applied to static graphs fits perfectly into the Gunrock framework. Using a combination of ForAll and Intersection operations, we outperform the parallel OpenMP CPU reference by up to 45.4 times speedup on the small Enron graph (provided as part of the HIVE workflows) and up to a 580 times speedup on larger graphs that feature enough computation to saturate the throughput of the GPU.

Seeded Graph Matching (SGM)

Seeded Graph Matching (SGM) SGM is a fruitful workflow to optimize, because the existing implementations were not written with performance in mind. By making minor modifications to the algorithm that allow use of sparse data structures, we enable scaling to larger datasets than previously possible.

The application involves solving a linear assignment problem (LSAP) as a subproblem. Solving these problems on the GPU is an active area of research -- though papers have been written describing high-performance parallel LSAP solvers, reference implementations are not available. We implement a GPU LSAP solver via Bertsekas' auction algorithm, and make it available as a standalone library.

SGM is an approximate algorithm that minimizes graph adjacency disagreements via the Frank-Wolfe algorithm. Certain uses of the auction algorithm can introduce additional approximation in the gradients of the Frank-Wolfe iterations. An interesting direction for future work would be a rigorous study of the effects of this kind of approximation on a variety of different graph tolopogies. Understanding of those dynamics could allow further scaling beyond what our current implementations can handle.

Sparse Fused Lasso

Sparse Fused Lasso The SFL problem is mainly divided into two parts, computing residual graphs from maxflow and renormalizing the weights of the vertices. Maxflow is performed with the parallelizable lock-free push-relabel algorithm. For renormalization: each vertex has to compute which communities it belongs to, and normalize the weights with other vertices in the same community. SFL iterates on maxflow and renormalization kernels with a global synchronization in between them until convergence. Current analysis show that maxflow is the bottleneck of the whole workflow, with over 90% of the runtime being spent on the maxflow kernels.

GPU SFL runs ~2 times slower than the CPU benchmark on the largest dataset provided. On smaller datasets, GPU SFL is much slower because there just isn't enough work to fill up a GPU and leverage the compute we have available. Analyzing the runs on the larger datasets, show that the parametric maxflow on the CPU converges much faster than our parallel push-relabel max flow algorithm on the GPU. Investigating the parallelization of parametric maxflow is an interesting research challenge.

Vertex Nomination

Vertex Nomination The term "vertex nomination" covers a variety of different node ranking schemes that fuse "content" and "context" information. The HIVE reference code implements a "multiple-source shortest path" context scoring function, but uses a very suboptimal algorithm. By using a more efficient algorithm, our serial CPU implementation achieves 1-2 orders of magnitude speedup over the HIVE implementation and our GPU implementation achieves another 1-2 orders of magnitude on top of that. Implementation was straightforward, involving only a small modification to the existing Gunrock SSSP app.

Scaling analysis for HIVE applications

Scaling analysis for HIVE applications Scaling summary:

Application Computation to communication ratio Scalability Impl. difficulty
Louvain $E/p : 2V$ Okay Hard
Graph SAGE $\sim CF : \min(C, 2p) \cdot 4$ Good Easy
Random walk Duplicated graph: infinity \linebreak Distributed graph: $1 : 24$ Perfect \linebreak Very poor Trivial \linebreak Easy
Graph search: Uniform $1 : 24$ Very poor Easy
Graph search: Greedy Straightforward: $d : 24$ \linebreak Pre-visit: $1:24$ Poor \linebreak Very poor Easy \linebreak Easy
Graph search: Stochastic greedy Straightforward: $d : 24$ \linebreak Pre-visit: $\log(d) : 24$ Poor \linebreak Very poor Easy \linebreak Easy
Geolocation Explicit movement: $25E/p : 4V$ \linebreak UVM or peer access: $25 : 1$ Okay \linebreak Good Easy \linebreak Easy
Vertex nomination $E : 8V \cdot \min(d, p)$ Okay Easy
Scan statistics Duplicated graph: infinity \linebreak Distributed graph: $\sim (d+a \cdot \log(d)):12$ Perfect \linebreak Okay Trivial \linebreak Easy
Sparse fused lasso $\sim a:8$ Less than okay Hard
Graph projection Duplicated graph : infinity \linebreak Distributed graph : $dE/p + E' : 6E'$ Perfect \linebreak Okay Easy \linebreak Easy
Local graph clustering $(6 + d)/p : 4$ Good Easy
Seeded graph matching
Application classification

Seeded graph matching and application classification are matrix-operation-based and not covered in this table.

From the scaling analysis, we can see these workflows can be roughly grouped into three categories, by their scalabilities:

Good scalability GraphSAGE, geolocation using UVM or peer accesses, and local graph clustering belong to this group. They share some algorithmic signatures: the whole graph needs to be visited at least once in every iteration, and visiting each edge involves nontrivial computation. The communication costs are roughly at the level of $V$. As a result, the computation vs. communication ratio is larger than $E : V$. PageRank is a standard graph algorithm that falls in this group.

Moderate scalability This group includes Louvain, geolocation using explicit movement, vertex nomination, scan statistics, and graph projection. They either only visit part of the graph in an iteration, have only trivial computation during an edge visit, or communicate a little more data than $V$. The computation vs. communication is less than $E : V$, but still larger than 1 (or 1 operation : 4 bytes). They are still scalable on the DGX-1 system, but not as well as the previous group. Single source shortest path (SSSP) is an typical example for this group.

Poor scalability Random walk, graph search, and sparse fused lasso belong to this group. They need to send out some data for each vertex or edge visit. As a result, the computation vs communication ratio is less than 1 (or 1 operation : 4 bytes). They are very hard to scale across multiple GPUs. Random walk is an typical example.