Book Image

Hadoop MapReduce v2 Cookbook - Second Edition: RAW

Book Image

Hadoop MapReduce v2 Cookbook - Second Edition: RAW

Overview of this book

Table of Contents (19 chapters)
Hadoop MapReduce v2 Cookbook Second Edition
Credits
About the Author
Acknowledgments
About the Author
About the Reviewers
www.PacktPub.com
Preface
Index

Speculative execution of straggling tasks


One of the main advantages of using Hadoop MapReduce is the framework-managed fault tolerance. When performing a large-scale distributed computation, parts of the computation can fail due to external causes such as network failures, disk failures, and node failures. When Hadoop detects an unresponsive task or a failed task, Hadoop will re-execute those tasks in a new node.

A Hadoop cluster may consist of heterogeneous nodes, and as a result there can be very slow nodes as well as fast nodes. Potentially, a few slow nodes and the tasks executing on those nodes can dominate the execution time of a computation. Hadoop introduces speculative execution optimization to avoid these slow-running tasks, which are called stragglers.

When most of the Map (or Reduce) tasks of a computation are completed, the Hadoop speculative execution feature will schedule duplicate executions of the remaining slow tasks in available alternate nodes. The slowness of a task is...