Collective Communication on Hadoop

Harp: Collective Communication on Hadoop Bingjing Zhang, Yang Ruan, Judy Qiu Computer Science Department Indiana University Bloomington, IN, USA Abstract—Big data tools have evolved rapidly in recent years. MapReduce is very successful but not optimized for many important analytics; especially those involving iteration. In this regard, Iterative MapReduce frameworks improve performance of MapReduce job chains through caching. Further Pregel, Giraph and GraphLab abstract data as a graph and process it in iterations. However, all these tools are designed with fixed data abstraction and have limitations in communication support. In this paper, we introduce a collective communication layer which provides optimized communication operations on several important data abstractions such as arrays, key-values and graphs, and define a Map-Collective model which serves the diverse communication demands in different parallel applications. In addition, we design our enhancements as plug-ins to Hadoop so they can be used with the rich Apache Big Data Stack. Then for example, Hadoop can do in-memory communication between Map tasks without writing intermediate data to HDFS. With improved expressiveness and excellent performance on collective communication, we can simultaneously support various applications from HPC to Cloud systems together with a high performance Apache Big Data Stack. Fig. 1. Big Data Analysis Tools Keywords—Collective Communication; Big Data Processing; Hadoop Spark [5] also uses caching to accelerate iterative algorithms without restricting computation to a chain of MapReduce jobs. I. INTRODUCTION To process graph data, Google announced Pregel [6] and soon It is estimated that organizations with high-end computing open source versions Giraph [7] and Hama [8] emerged. infrastructures and data centers are doubling the amount of data Whatever their differences, all such programs are based on they archive every year. Sophisticated machine learning a kind of “top-down” design. The whole programming model, techniques aim to offer the best results to return to the user. It from data abstraction to computation and communication did not take long for these ideas to be applied to the full range pattern, is fixed. Users have to put their applications into the of scientific challenges. Many of the primary software tools model, which could cause performance inefficiency. For used to do large-scale data analysis are required by these example, in K-Means clustering [9] (Lloyd's algorithm [10]), applications to optimize their performance. There is a need to every successive task needs all the centroids generated in the integrate features of traditional high-performance computing, last iteration. Mahout [11] on Hadoop chooses to reduce the such as scientific libraries, communication and resource results of all the map tasks in one reduce task and store it on management middleware, with the rich set of capabilities found HDFS. This data is then read by all the map tasks in the job at in the commercial Big Data ecosystem such as Apache open the next iteration. The “reduce” stage can be parallelized by source software stack. The overarching question we will chunking intermediate data to partitions and using multiple address is “How should we design, analyze and implement a reduce tasks to compute each part in parallel. This type of unified and interoperable infrastructure to meet the “(multi-)reduce-gather-broadcast” strategy is also applied in requirements of a broad set of data intensive applications?” other frameworks through in-memory communication, e.g. The primary software tools used to do the large-scale data Twister and Spark. analysis has evolved rapidly as shown in Fig. 1. The last Regardless, “gather-broadcast” is not an efficient way to decade witnessed a huge computing shift derived from relocate the new centroids generated by reduce tasks, especially publication of the Google MapReduce paper [1]. Since then, its when the centroids data grows large in size. The time open source version Hadoop [2] has become the mainstream of complexity of “gather” is at least kdβ where k is the number of big data processing, with many other tools emerging to process centroids, d is the number of dimensions and β is the different big data problems, extending the original MapReduce communication time used to send each element in the centroids model to include iterative MapReduce. Tools such as Twister (communication initiation time α is neglected). Also the time [3] and HaLoop [4] can cache loop invariant data locally to complexity of “broadcast” is at least kdβ [12] [13]. Thus the avoid repeat input data loading in a MapReduce job chain. time complexity of “gather-broadcast” is about 2kdβ. But if we II. RELATED WORK use allgather bucket algorithm [14], it is reduced to kdβ. The landscape of big data tools grows bigger and more In iterative algorithms, communication participated in by complicated. Each tool has its own computation model with all the workers happens once or more per iteration. This makes related data abstraction and communication abstraction in order communication algorithm performance crucial to the efficiency to achieve optimization for different applications. Before the of the whole application. We call this “Collective MapReduce model, MPI was the main tool used for large-scale Communication”. Rather than fixing communication patterns, simulation, exclusive on expensive hardware such as HPC or we decided to separate this layer out and provide collective supercomputers. But MapReduce tries to use commodity communication abstraction. Collective communication features machines to solve big data problems. This model defines data in MPI [15], designed for HPC systems and supercomputers. abstraction as key-value pairs and computation flow as “map, While well defined, there are many limitations. It cannot shuffle and then reduce”. One advantage of this tool is that it support high level data abstractions other than arrays, objects, doesn’t rely on memory to load and process all the data, instead and related communication patterns on them, e.g. shuffling on using local disks. MapReduce model can solve problems where key-values or message passing along edges in graphs. Another the data size is too large to fit into the memory. Furthermore, it drawback is that it forces users to focus on every detail of a also provides fault tolerance, which is important to big data communication call. For example, users have to calculate the processing. The open source implementation Hadoop [2] is buffer size for data receiving, which is difficult to obtain in widely used nowadays in both industry and academia. many applications as the amount of sending data may be very MapReduce became popular for its simplicity and dynamic and unknown to the receivers. scalability, yet is still slow when running iterative algorithms. In response to these issues, we present Harp library. Harp The resultant chain of MapReduce jobs means repeat input data provides data abstractions and related communication loading occurs in each iteration. Several frameworks such as abstractions with optimized implementation. By plugging Harp Twister [3], HaLoop [4] and Spark [5] solve this problem by into Hadoop, we convert MapReduce model to Map-Collective caching intermediate data. Another model used for iterative model and enable efficient in-memory communication between computation is the Graph model, which abstracts data as map tasks across a variety of important data analysis vertices and edges. Here computation happens on vertices, and applications. The word “harp” symbolizes how parallel each worker caches vertices and related out-edges as graph processes cooperate through collective communication for partitions. The whole parallelization is BSP (Bulk Synchronous efficient data processing, just as strings in harps can make Parallel) style. There are two open source projects following concordant sound (see Fig. 2). The inclusion of collective Pregel’s design. One is Giraph [7] and another is Hama [8]. communication abstraction and Map-Collective model means Giraph exactly follows iterative BSP graph computation Harp is neither a replication of MPI nor an attempt to transplant pattern while Hama tries to build a general BSP computation MPI into the Hadoop system [16], as we will elaborate in model. By contrast, GraphLab [17] [18] abstracts data as a subsequent sections. “data graph” and uses consistency models to control vertex value update. GraphLab was later enhanced with PowerGraph In the rest of this paper, Section 2 discusses about related [19] abstraction to reduce the communication overhead. This work. Section 3 describes the abstraction of collective was also learned by GraphX [20]. communication. Section 4 shows how Map-Collective model works in Hadoop-Harp. Section 5 gives several applications The third model is DAG, which abstracts computation flow implemented in Harp. Section 6 shows the performance of as a directed acyclic graph. Each vertex in the graph is a Harp through benchmarking on the applications. process, and each edge is a communication channel between two processes. DAG model is helpful to those applications which have complicated parallel workflows. Dryad [21] is an example of a parallel engine using DAG. Tools employing this model are often used for query and stream data processing. Some attempts have been made to blend select features of these models. For example, Spark’s RDD data abstraction and transformation operations are very similar to MapReduce model. But it organizes computation tasks as DAGs. Stratosphere [22] and REEF [23] also try to support several different models in one framework. “Broadcast” operation from MPI is added in Hadoop by using distributed cache through a simple algorithm (one-by-one sending). And it is optimized by using BitTorrent technology in Spark [24] or using a pipeline-based chain algorithm in Twister [12] [13]. For all these tools, communication is still hidden and coupled with the computation flow. Though these programs [12] [13] [24] [25] try to add or improve collective communication operations, they are still limited in type and Fig. 2. Parallelism and Architecture of Harp constrained by the computation flow.

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