Is there a way to kill reducer task in Hadoop?

You can kill the task-attempt by :

hadoop job -kill-task [task_attempt_id]

To get the task-attempt-id, you need to go one level deeper into the task(by clicking on task hyperlink on job tracker).

hadoop mapreduce reduce

First find the job ID:

hadoop job -list

Now, kill the job:

hadoop job -kill <job_ID_goes_here>

hadoop mapreduce reduce

hadoop job -kill-task [attempt-id] wherein the attempt-id can be obtained from the UI.

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