Amazon EMR now supports Auto Scaling and configurable scale down behavior

You can now configure policies to automatically add (scale out) and terminate (scale in) nodes in your Amazon EMR cluster. Amazon EMR can programmatically scale out applications like Apache Spark and Apache Hive to utilize additional nodes for increased performance and scale in the number of nodes in your cluster to save costs when utilization is low. Your cluster can scale based on Amazon CloudWatch metrics provided by Amazon EMR, including YARN utilization metrics.
Quelle: aws.amazon.com