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https://issues.apache.org/jira/browse/TEZ-3126?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15157440#comment-15157440
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Jonathan Eagles commented on TEZ-3126:
--------------------------------------
I'll use this ticket to log the reason parallelism was not reduced. As to
grouping, a better distribution may help. Empty partitions could be an
interesting case since it has 0 output size.
> Auto-Reduce Parallelism: Vertex not re-configured when reduced by less than
> half.
> ---------------------------------------------------------------------------------
>
> Key: TEZ-3126
> URL: https://issues.apache.org/jira/browse/TEZ-3126
> Project: Apache Tez
> Issue Type: Bug
> Reporter: Jonathan Eagles
> Assignee: Jonathan Eagles
> Priority: Critical
> Attachments: TEZ-3126.1.patch
>
>
> For example, when reducing parallelism from 36 to 22. The basePartitionRange
> will be 1 and will not re-configure the vertex.
> {code:java|title=ShuffleVertexManager#determineParallelismAndApply|borderStyle=dashed|bgColor=lightgrey}
> int desiredTaskParallelism =
> (int)(
> (expectedTotalSourceTasksOutputSize+desiredTaskInputDataSize-1)/
> desiredTaskInputDataSize);
> if(desiredTaskParallelism < minTaskParallelism) {
> desiredTaskParallelism = minTaskParallelism;
> }
>
> if(desiredTaskParallelism >= currentParallelism) {
> return true;
> }
>
> // most shufflers will be assigned this range
> basePartitionRange = currentParallelism/desiredTaskParallelism;
>
> if (basePartitionRange <= 1) {
> // nothing to do if range is equal 1 partition. shuffler does it by
> default
> return true;
> }
> {code}
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