Section: New Results

Advanced data management on clouds

Benchmarking Spark and Flink

Participants : Ovidiu-Cristian Marcu, Alexandru Costan, Gabriel Antoniu.

Spark and Flink are two Apache-hosted data analytics frameworks that represent the state of the art in modern in-memory Map-Reduce processing. They facilitate the development of multi-step data pipelines using directly acyclic graph (DAG) patterns. In the framework of our BigStorage project, we performed a comparative study [23] which evaluates the performance of Spark versus Flink. The objective is to identify and explain the impact of the different architectural choices and the parameter configurations on the perceived end-to-end performance.

Based on empirical evidences, the study points out that in Big Data processing there is not a single framework for all data types, sizes and job patterns and emphasize a set of design choices that play an important role in the behaviour of a Big Data framework: memory management, pipelined execution, optimizations and parameter configuration easiness. What raises our attention is that a streaming engine (i.e., Flink) delivers in many benchmarks better performance than a batch-based engine (i.e., Spark), showing that a more general Big Data architecture (treating batches as finite sets of streamed data) is plausible and may subsume both streaming and batching use cases.

Collaboration.

This work was done in collaboration with María Pérez, UPM, Spain.

Geo-distributed graph processing

Participants : Chi Zhou, Shadi Ibrahim.

Graph processing is an emerging model adopted by a wide range of applications to easily parallelize the computations over graph data. Partitioning graph processing workloads to multiple machines is an important task for reducing the communication cost and improving the performance of graph processing jobs. Recently, many real-world applications store their data on multiple geographically distributed datacenters (DCs) to ensure flexible and low-latency services. Due to the limited Wide Area Network (WAN) bandwidths and the network heterogeneity of the geo-distributed DCs, existing graph partitioning methods need to be redesigned to improve the performance of graph processing jobs in geo-distributed DCs.

To address the above challenges, we propose a heterogeneity-aware graph partitioning method named G-Cut, which aims at minimizing the runtime of graph processing jobs in geo-distributed DCs while satisfying the WAN usage budget. G-Cut is a two-stage graph partitioning method. In the traffic-aware graph partitioning stage, we adopt the one-pass edge assignment to place edges into different partitions while minimizing the inter-DC data traffic size. In the network-aware partition refinement stage, we map the partitions obtained in the first stage onto different DCs in order to minimize the inter-DC data transfer time. We evaluate the effectiveness and efficiency of G-Cut using real-world graphs and the evaluation results show that G-Cut can achieve both lower WAN usage and shorter inter-DC data transfer time compared to state-of-the-art graph partitioning methods.

Collaboration.

This work was done in collaboration with Bingsheng He NUS, Singapore.

Fairness and scheduling

Participants : Orçun Yildiz, Shadi Ibrahim, Gabriel Antoniu.

Recently, Map-Reduce and its open-source implementation Hadoop have emerged as prevalent tools for big data analysis in the cloud. Fair resource allocation in-between jobs and users is an important issue, especially in multi-tenant environments such as clouds. Several scheduling policies have been developed to preserve fairness in multi-tenant Hadoop clusters. At the core of these schedulers, simple (non-) preemptive approaches are employed to free resources for tasks belonging to jobs with less share. For example, Hadoop Fair Scheduler is equipped with two approaches: wait and kill. While wait may introduce a serious violation in fairness, kill may result in a huge waste of resources. Yet, recently some work have introduced preemption approach in shared Hadoop clusters.

To this end, we closely examine three approaches including wait, kill and preemption when Hadoop Fair Scheduler is employed for ensuring fair execution between multiple concurrent jobs. We perform extensive experiments to assess the impact of these approaches on performance and resource utilization while ensuring fairness. Our experimental results bring out the differences between these approaches and illustrate that these approaches are only sub-optimal for different workloads and cluster configurations: the efficiency of achieving fairness and the overall performance varies with the workload composition, resource availability and the cost of the adopted preemption technique.

Stragglers in Map-Reduce

Participants : Tien-Dat Phan, Shadi Ibrahim.

Big Data systems (e.g., Map-Reduce, Hadoop, Spark) rely increasingly on speculative execution to mask slow tasks also known as stragglers because a job's execution time is dominated by the slowest task instance. Big Data systems typically identify stragglers and speculatively run copies of those tasks with the expectation a copy may complete faster to shorten job execution times.

There is a rich body of recent results on straggler mitigation in Map-Reduce. However, the majority of these do not consider the problem of accurately detecting stragglers. Instead, they adopt a particular straggler detection approach and then study its effectiveness in terms of performance, e.g., reduction in job completion time, or its efficiency, e.g., extra resource usage.

In this work, we consider a complete framework for straggler detection and mitigation. We start with a set of metrics that can be used to characterizes and detect stragglers such as Precision, Recall, Detection Latency, Undetected Time and Fake Positive. We then develop an architectural model by which these metrics can be linked to measures of performance including execution time and system energy overheads.

We further conduct a series of experiments to demonstrate which metrics and approaches are more effective in detecting stragglers and are also predictive of effectiveness in terms of performance and energy efficiency. For example, our results indicate that the default Hadoop straggler detector could be made more effective. In certain cases, precision is low and only 65 % of those detected are actual stragglers and recall, i.e., the proportion of stragglers which are actually detected, is also relatively low at 56 %. For the same case, the hierarchical approach (i.e., a green-driven detector based on the default one) achieves a precision of 98 % and a recall of 33 %.

Further, these increases in precision can be used to achieve lower execution time and energy consumption, and thus higher performance and energy efficiency. Compared to the default Hadoop mechanism, energy consumption is reduced by almost 30 %. These results demonstrate how our framework can offer useful insights and be applied in practical settings to characterize and design new straggler detection mechanisms for Map-Reduce systems.

Collaboration.

This work was carried out in collaboration with Guillaume Aupy and Padma Raghavan whilst they were affiliated with Vanderbilt University, USA.