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https://issues.apache.org/jira/browse/TEZ-2251?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14391741#comment-14391741
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Rajesh Balamohan commented on TEZ-2251:
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[~gopalv] - "Reducer 6" had 2 inputs "Reducer 5 / Map 11". In this case, Map
11 finished pretty fast and had lots of tasks. So based on this info, "Reducer
6" was able to auto-reduce very quickly once it received CONFIGURED signal from
"Reducer 5". The second patch is to certain extent similar along the lines you
mentioned as it waits for atleast 1 attempt to be finished from each source.
> Enabling auto reduce parallelism in certain jobs causes DAG to hang
> -------------------------------------------------------------------
>
> Key: TEZ-2251
> URL: https://issues.apache.org/jira/browse/TEZ-2251
> Project: Apache Tez
> Issue Type: Bug
> Reporter: Rajesh Balamohan
> Attachments: TEZ-2251.VertexImpl.patch,
> TEZ-2251.fix_but_slows_down.patch, hive_console.png, tez_2251_dag.png
>
>
> Scenario:
> - Run TPCH query20
> (https://github.com/cartershanklin/hive-testbench/blob/master/sample-queries-tpch/tpch_query20.sql)
> at 1 TB scale (tez-master branch, hive trunk)
> - Enable auto reduce parallelism
> - DAG didn't complete and got stuck in "Reducer 6"
> Vertex parallelism of "Reducer 5 & 6" happens within a span of 3
> milliseconds, and tasks of "reducer 5" ends up producing wrong partition
> details as it sees the updated task numbers of reducer 6 when scheduled.
> This causes, job to hang.
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