Cluster caching systems increasingly store structured data objects in the columnar format. However, these systems routinely face the imbalanced load that significantly impairs the I/O performance. Existing load-balancing solutions, while effective for reading unstructured data objects, fall short in handling columnar data. Unlike unstructured data that can only be read through a full-object scan, columnar data supports direct query of specific columns with two distinct access patterns: (1) columns have the heavily skewed popularity, and (2) hot columns are likely accessed together in a query job. Based on these two access patterns, we propose an effective load-balancing solution for structured data. Our solution, which we call RepBun, groups hot columns into a bundle. It then copies multiple replicas of the column bundle and stores them uniformly across servers. We show that RepBun achieves improved load balancing with reduced memory overhead, while avoiding data shuffling between cache servers. We implemented RepBun atop Alluxio, a popular in-memory distributed storage, and evaluate its performance through EC2 deployment against the TPC-H benchmark work-load. Experimental results show that RepBun outperforms the existing load-balancing solutions with significantly shorter read latency and faster query completion.

Cluster schedulers routinely face data-parallel jobs with complex task dependencies expressed as DAGs (directed acyclic graphs). Understanding DAG structures and runtime characteristics in large production clusters hence plays a key role in scheduler design, which, however, remains an important missing piece in the literature. In this work, we present a comprehensive study of a recently released cluster trace in Alibaba. We examine the dependency structures of Alibaba jobs and find that their DAGs have sparsely connected vertices and can be approximately decomposed into multiple trees with bounded depth. We also characterize the runtime performance of DAGs and show that dependent tasks may have significant variability in resource usage and duration---even for recurring tasks. In both aspects, we compare the query jobs in the standard TPC benchmarks with the production DAGs and find the former inadequately representative. To better benchmark DAG schedulers at scale, we develop a workload generator that can faithfully synthesize task dependencies based on the production Alibaba trace. Extensive evaluations show that the synthesized DAGs have consistent statistical characteristics as the production DAGs, and the synthesized and real workloads yield similar scheduling results with various schedulers.