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https://issues.apache.org/jira/browse/TEZ-1248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15371588#comment-15371588
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Zhiyuan Yang commented on TEZ-1248:
-----------------------------------
The patched uploaded rounds up the float when calculating number of tasks to
schedule, so that single reducer gets scheduled as long as source tasks
completion percentage becomes more than min slow start fraction.
> Reduce slow-start should special case 1 reducer runs
> ----------------------------------------------------
>
> Key: TEZ-1248
> URL: https://issues.apache.org/jira/browse/TEZ-1248
> Project: Apache Tez
> Issue Type: Improvement
> Affects Versions: 0.5.0
> Environment: 20 node cluster running tez
> Reporter: Gopal V
> Assignee: Zhiyuan Yang
> Priority: Critical
> Attachments: TEZ-1248.1.patch
>
>
> Reducer slow-start has a performance problem for the small cases where there
> is just 1 reducer for a case with a single wave.
> Tez knows the split count and wave count, being able to determine if the
> cluster has enough spare capacity to run the reducer earlier for lower
> latency in a N-mapper -> 1 reducer case.
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