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https://issues.apache.org/jira/browse/TEZ-4130?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17051561#comment-17051561
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Jonathan Turner Eagles commented on TEZ-4130:
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The way pig does this is through global parameter _pig.exec.reducers.max_
(defaults to 999). This is applied only if its not a map stage and any operator
specific parallelism (PARALLEL key word) overrides the global parameter for
that vertex.
https://github.com/apache/pig/blob/trunk/src/org/apache/pig/backend/hadoop/executionengine/tez/plan/optimizer/TezOperDependencyParallelismEstimator.java#L87
> Config for max task parallelism in shuffle -
> tez.shuffle-vertex-manager.max-task-parallelism
> --------------------------------------------------------------------------------------------
>
> Key: TEZ-4130
> URL: https://issues.apache.org/jira/browse/TEZ-4130
> Project: Apache Tez
> Issue Type: Improvement
> Reporter: László Bodor
> Assignee: László Bodor
> Priority: Major
>
> During the investigation of a customer issue, I found that tez generated a
> dag plan containing >4k tasks. It failed for hive because of bucket number
> limitations (4k). It can be configured properly, e.g. bigger splits
> (tez.grouping.min-size), but maybe it would be more convenient for users to
> config a hard limit for shuffle vertex manager.
> However, I'm not really sure if it's correct to force changing the max task
> parallelism after split generation already happened (e.g.
> [HiveSplitGenerator|https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/exec/tez/HiveSplitGenerator.java#L192-L244]):
> https://github.com/apache/tez/blob/master/tez-runtime-library/src/main/java/org/apache/tez/dag/library/vertexmanager/ShuffleVertexManager.java#L477
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