[jira] Updated: (MAPREDUCE-936) Allow a load difference in fairshare scheduler

["dhruba borthakur (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/MAPREDUCE-936?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

dhruba borthakur updated MAPREDUCE-936:
---------------------------------------

    Assignee: Zheng Shao

hi zheng, would appreciate it a lot if u can provide a unit test for this one. Thanks.

> Allow a load difference in fairshare scheduler
> ----------------------------------------------
>
>                 Key: MAPREDUCE-936
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-936
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>          Components: contrib/fair-share
>            Reporter: Zheng Shao
>            Assignee: Zheng Shao
>         Attachments: MAPREDUCE-936.1.patch
>
>
> The problem we are facing: It takes a long time for all tasks of a job to get scheduled on the cluster, even if the cluster is almost empty.
> There are two reasons that together lead to this situation:
> 1. The load factor makes sure each TT runs the same number of tasks. (This is the part that this patch tries to change).
> 2. The scheduler tries to schedule map tasks locally (first node-local, then rack-local). There is a wait time (mapred.fairscheduler.localitywait.node and mapred.fairscheduler.localitywait.rack, both are around 10 sec in our conf), and accumulated wait time (JobInfo.localityWait). The accumulated wait time is reset to 0 whenever a non-local map task is scheduled. That means it takes N * wait_time to schedule N non-local map tasks.
> Because of 1, a lot of TT will not be able to take more tasks, even if they have free slots. As a result, a lot of the map tasks cannot be scheduled locally.
> Because of 2, it's really hard to schedule a non-local task.
> As a result, sometimes we are seeing that it takes more than 2 minutes to schedule all the mappers of a job.

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