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https://issues.apache.org/jira/browse/HADOOP-3961?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12623033#action_12623033
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Ari Rabkin commented on HADOOP-3961:
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Issue is a blocker, because in the case where maps have negligible output, but
there's only one reduce (e.g., sort validator), the reduce sometimes never gets
placed.
> resource estimation works badly in some cases
> ---------------------------------------------
>
> Key: HADOOP-3961
> URL: https://issues.apache.org/jira/browse/HADOOP-3961
> Project: Hadoop Core
> Issue Type: Bug
> Reporter: Ari Rabkin
> Assignee: Ari Rabkin
> Priority: Blocker
>
> The disk space estimation introduced in HADOOP-657 performs badly under some
> circumstances. In particular, if maps have very small output, the initial
> estimate of output size equals input size throws everything off.
> Fix is to use a smaller initial estimate, and to update it appropriately.
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