[MAPREDUCE-4867] reduces tasks won't start in certain circumstances

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 1.0.4
• Component/s: scheduler
• Assignee: Unassigned
• Reporter: Vincent Behar
• Created: 10/Dec/12 11:04
• Updated: 10/Dec/12 15:45

Description

Reduce tasks start are conditioned by the value of "mapred.reduce.slowstart.completed.maps". However, if the number of completed map tasks never reached the configured value (for example because "mapred.max.map.failures.percent" has been set to a high value, to permit a job to have a lot of failed tasks), then the reduce tasks won't start.
The job is still running, all map tasks are finished (either successful or not), and all reduce tasks are still pending. The only thing one can do is to kill the job.

There are 2 things that could be done :

• document the relation between "mapred.max.map.failures.percent" and "mapred.reduce.slowstart.completed.maps" : we can say that the rule to follow if you want to be sure that your reduce tasks will start is : "mapred.reduce.slowstart.completed.maps * 100 < 100 - mapred.max.map.failures.percent"

• fix JobInProgress.scheduleReduces() to return true if all map tasks are finished