mesos/spark — pull request #184 by rxin (2012-08-30 06:33:48+00:00)
Title: Disable running combiners on map tasks when mergeCombiners function is not specified by the user. 

I haven't done much testing yet beyond running sbt/sbt test. Want to get some early feedback.

Basically if the mergeCombiners field is not set (null), combiners are not applied on map side. It doesn't break any existing API.