PDL Abstract

Addressing the Long-Lineage Bottleneck in Apache Spark

Carnegie Mellon University Parallel Data Lab Technical Report CMU-PDL-18-101, January 2018.

Haoran Wang*, Jinliang Wei*, Garth A. Gibson*†

* Carnegie Mellon University
† Vector Institute

Apache Spark employs lazy evaluation [11, 6]; that is, in Spark, a dataset is represented as Resilient Distributed Dataset (RDD), and a single-threaded application (driver) program simply describes transformations (RDD to RDD), referred to as lineage [7, 12], without performing distributed computation until output is requested. The lineage traces computation and dependency back to external (and assumed durable) data sources, allowing Spark to opportunistically cache intermediate RDDs, because it can recompute everything from external data sources. To initiate computation on worker machines, the driver process constructs a directed acyclic graph (DAG) representing computation and dependency according to the requested RDD’s lineage. Then the driver broadcasts this DAG to all involved workers requesting they execute their portion of the result RDD. When a requested RDD has a long lineage, as one would expect from iterative convergent or streaming applications [9, 15], constructing and broadcasting computational dependencies can become a significant bottleneck. For example, when solving matrix factorization using Gemulla’s iterative convergent algorithm [3], and taking tens of data passes to converge, each data pass is slowed down by 30-40% relative to the prior pass, so the eighth data pass is 8.5X slower than the first.

The current practice to avoid such performance penalty is to frequently checkpoint to durable storage device which truncates lineage size. Checkpointing as a performance speedup is difficult for a programmer to anticipate and fundamentally contradicts Spark’s philosophy that the working set should stay in memory and not be replicated across the network. Since Spark caches intermediate RDDs, one solution is to cache constructed DAGs and broadcast only new DAG elements. Our experiments show that with this optimization, per iteration execution time is almost independent of growing lineage size and comparable to the execution time provided by optimal checkpointing. On 10 machines using 240 cores in total, without checkpointing we wobserved a 3.4X speedup when solving matrix factorization and 10X speedup for a streaming application provided in the Spark distribution.

KEYWORDS: Apache Spark, Lineage

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