Many real world information networks consist of millions or billions of vertices representing heterogeneous entities and billions or trillions of edges representing heterogeneous types of relationships among entities.
For example, the crawled Web graph is estimated to have more than 20 billions of pages (vertices) with 160 billions hyperlinks (edges). Facebook user community exceeds 1 billion users (vertices) with more than 140 billion friendship relationships (edges) in 2012. The billion triple challenges from the Semantic Web community have put forward large collection of RDF datasets with hundreds of millions of vertices and billions of edges.
As the size and variety of information networks continue to grow in many science and engineering domains, graph computations often exceed the processing capacity of conventional hardware, software systems and tools for a number of reasons. First, graph data often exhibits higher data correlations through both direct and indirect edges and such high correlation tends to generate large size of intermediate results during graph computations. When the size of intermediate results exceeds the available memory, the out of memory problem is unavoidable. Second, the graph datasets are growing in volume, variety and velocity. The bigger the size of the graphs gets, the worse the performance will be for most of the graph computations. One open challenge in this space is how to effectively partition a large graph to enable efficient parallel processing of complex graph operations.
One of the papers to be presented at the ACM/IEEE SC13 conference, titled “Efficient data partitioning model for heterogeneous graphs in the Cloud”, proposes a flexible graph partitioning framework, called VB-partitioner. This work is co-authored by the doctorate student Kisung Lee and Prof. Dr. Ling Liu from the school of Computer Science at Georgia Institute of Technology. To make parallel graph computations highly efficient, an important design goal of VB-Partitioner is to devise graph partitioning techniques that can effectively minimize the inter-partition communication overhead and maximize the intra-partition computation capacity (local processing).
Concretely, the first prototype of the VB-Partitioner focuses on efficient processing of graph queries, namely finding all the subgraphs matching a given subgraph pattern. VB-Partitioner partitions a large graph in three steps.
- First, it constructs three types of Vertex Blocks (in-VBs, out-VBs and bi-VBs) to capture the general graph processing locality. Each vertex block has an anchor vertex.
- Second, it constructs three types of k-hop Extended Vertex Blocks (in-EVBs, out-EVBs and bi-EVBs) to distribute vertex blocks with better query locality. Each EVB has one anchor vertex. It achieves query locality by employing controlled edge replication. The setting of k is determined by the radius of frequent query graphs in order to ensure that most frequently requested queries can be processed in parallel at all partitions with minimized inter-partition communication overhead.
- Third, it partitions a graph by grouping its vertex blocks and EVBs to maximize parallelism in graph processing while ensuring load balance, controlled edge replication and fast grouping.
Four techniques are considered and compared in the context of grouping and placement of VBs and EVBs to partitions: random grouping, hash-based grouping, min-cut based grouping and high degree vertex-based grouping. As an integral part of the VB-Partitioner, a data partition-aware query partitioning model is also developed to handle the cases where the radius of a query is larger than k. The experimental results reported in the paper demonstrate the effectiveness of VB-Partitioner in terms of query processing efficiency, data loading time and partition distribution balance.
Graph computations can be broadly classified into two categories, graph queries that find matching subgraphs of a given pattern and iterative graph algorithms that find clusters, orderings, paths or correlation patterns. The former targets at subgraph matching problems over large static graphs and the later targets at graph inference kernels that traverse the graph by iteratively updating the weight of vertices or edges, such as PageRank, shortest path algorithms, spanning tree algorithms, topological sort, and so forth.
Although the first generation of the VB-Partitioner is tailored primarily for efficient parallel processing of graph queries, the ongoing work on VB-Partitioner includes exploring the feasibility and effectiveness of VB-Partitioner in the context of iterative graph algorithms. For example, to minimize inter-partition communications and maximize parallelism in graph computation, it is crucial to optimize the shared memory by minimizing parallel overhead of synchronization barriers. It is equally important to optimize the distributed memory by bounding message buffer sizes, bundling messages, overlapping communication with computation to amortize the overhead of barriers.
In addition to exploring parallel computation opportunities through graph partitioning using multi-threads, multi-cores, multiple workers, one can also exploit and combine with other performance optimization techniques to scale large graph analytics. Example techniques include
- Compression to provide compact storage and in-memory processing,
- Data placements on disk and in memory to balance computation with storage, and to maximize sequential access and minimize random access,
- Indexing at vertex and/or edge level to utilize sequential access and minimize unnecessary random access,
- Caching at vertex, edge or query level to gain performance for repeated access.
Please come hear more on Tuesday, November 19, 2013 10:30AM – 11:00AM (Location: Room 205/207)
About the Authors
Ling Liu is a Professor in the School of Computer Science at Georgia Institute of Technology. She directs the research programs in Distributed Data Intensive Systems Lab (DiSL), examining various aspects of large scale data intensive systems. Prof. Ling Liu is an internationally recognized expert in the areas of Cloud computing, Distributed Computing, Big Data technologies, Database systems and Service oriented computing. Prof. Liu is a recipient of IEEE Computer Society Technical Achievement Award in 2012. Currently Prof. Liu is the editor in chief of IEEE Transactions on Service Computing, and serves on the editorial board of half dozen international journals, including Journal of Parallel and Distributed Computing (JPDC), ACM Transactions on Internet Technology (TOIT), ACM Transactions on Web (TWEB). Dr. Liu’s current research is primarily sponsored by NSF, IBM, and Intel.
Kisung Lee is a Ph.D student in the School of Computer Science at Georgia Tech. He received his BS and MS degree in computer science from KAIST in 2005 and 2007 respectively. He had worked for ETRI as a researcher from 2007 to 2010. He is conducting research in distributed and parallel processing of big data in the Cloud, mobile computing and social network analysis.