The reason for killing a speculated task has to be logged. Currently, a speculated task is killed with a note of "Container killed by the ApplicationMaster. Container killed on request. Exit code is 137" which is not very useful. Better logging of this message stating the task was killed due to completion of its speculative task would be useful.
Also, this message is lost once the app is moved to history. All we are left with is a list of killed tasks without a reason being notified to the user.